BSC NURSING SEM1 APPLIED PHYSIOLOGY UNIT 3 Digestive system
Functions of the organs of digestive tract
The digestive tract is a complex system that breaks down food, absorbs nutrients, and eliminates waste. Here are the key functions of each organ in the digestive system:
Mouth:
Mouth (Oral Cavity):
The mouth is the entry point of the digestive system where the initial stages of digestion occur. It performs both mechanical and chemical digestion, preparing food for further processing in the digestive tract.
Functions of the Mouth:
Ingestion:
The process of taking food and liquids into the mouth.
Mechanical Digestion:
Teeth: Chew and break food into smaller pieces (mastication), increasing the surface area for enzymes to act on.
Tongue: Assists in mixing food with saliva and helps move food into the pharynx for swallowing.
Chemical Digestion:
Saliva: Secreted by salivary glands, it contains:
Amylase: Begins the breakdown of carbohydrates into simpler sugars (e.g., maltose).
Lipase: Aids in the digestion of fats (minor role in the mouth).
Formation of Bolus:
The tongue and cheeks help mix food with saliva, forming a soft, cohesive mass called a bolus, which is easier to swallow.
Taste Perception:
The tongue contains taste buds that detect and differentiate flavors (sweet, salty, sour, bitter, umami), enhancing the eating experience.
Lubrication:
Saliva moistens food, making it easier to chew, swallow, and pass through the esophagus.
Speech:
Though not a digestive function, the mouth plays a critical role in articulation and producing sounds for communication.
Protection:
Saliva contains lysozymes and other antimicrobial agents that help protect against harmful microorganisms.
Function: The esophagus is a muscular tube that transports food from the mouth to the stomach. It uses wave-like muscle contractions called peristalsis to move the food.
Esophagus
Esophagus
The esophagus is a muscular tube approximately 25 cm (10 inches) long that connects the pharynx (throat) to the stomach. It plays a critical role in transporting food and liquids from the mouth to the stomach for further digestion.
Structure of the Esophagus:
Layers of the Esophagus:
Mucosa: Inner lining, secretes mucus to aid the passage of food.
Submucosa: Contains blood vessels, nerves, and glands for support and lubrication.
Muscularis: Made up of two muscle layers:
Circular muscles: Squeeze the food down.
Longitudinal muscles: Shorten the esophagus during swallowing.
Adventitia: Outermost connective tissue layer for structural support.
Upper and Lower Esophageal Sphincters:
Upper Esophageal Sphincter (UES): Prevents air from entering the esophagus during breathing.
Lower Esophageal Sphincter (LES): Prevents stomach contents and acid from refluxing back into the esophagus.
Functions of the Esophagus:
Food Transport:
The primary function of the esophagus is to move food, liquids, and saliva from the mouth to the stomach.
Peristalsis:
Wave-like muscle contractions in the esophageal walls push the bolus (chewed food) downward toward the stomach.
Protection:
The esophagus is lined with mucus-producing cells to lubricate food and protect its lining from abrasion by rough food particles.
Prevention of Reflux:
The Lower Esophageal Sphincter (LES) acts as a barrier, preventing the backflow of acidic gastric contents into the esophagus, protecting it from irritation and damage.
Common Disorders of the Esophagus:
Gastroesophageal Reflux Disease (GERD): Caused by a weak LES, leading to acid reflux.
Esophagitis: Inflammation of the esophagus due to infection, acid reflux, or other causes.
Hiatal Hernia: Part of the stomach pushes into the chest cavity through the diaphragm, affecting esophageal function.
Achalasia: Difficulty in food passage due to poor esophageal motility or sphincter issues.
Stomach
The stomach is a J-shaped, muscular organ located in the upper abdomen between the esophagus and the small intestine. It plays a vital role in digestion by mechanically breaking down food, mixing it with gastric secretions, and initiating the digestion of proteins and fats.
Structure of the Stomach:
Regions of the Stomach:
Cardia: Where the esophagus connects to the stomach; contains the lower esophageal sphincter.
Fundus: The dome-shaped upper portion, which stores undigested food and gases.
Body: The central and largest part of the stomach where most digestion occurs.
Antrum: The lower portion where food is mixed and moved toward the pylorus.
Pylorus: The passage connecting the stomach to the small intestine; controlled by the pyloric sphincter.
Layers of the Stomach Wall:
Mucosa: Secretes mucus, digestive enzymes, and gastric acid.
Submucosa: Contains blood vessels, nerves, and glands.
Muscularis: Three layers of smooth muscle (longitudinal, circular, oblique) aid in mechanical digestion.
Serosa: Outer protective layer.
Functions of the Stomach:
Temporary Storage:
The stomach stores ingested food and regulates its release into the small intestine in a controlled manner.
Mechanical Digestion:
Muscular contractions (churning) mix food with gastric juices, breaking it into smaller pieces to form a semi-liquid substance called chyme.
Chemical Digestion:
Hydrochloric Acid (HCl):
Creates an acidic environment (pH 1.5–3.5) to denature proteins and kill microorganisms.
Pepsin:
An enzyme activated by HCl that breaks down proteins into smaller peptides.
Gastric Lipase:
Initiates the digestion of fats.
Secretion of Gastric Juices:
Mucus: Protects the stomach lining from acidic gastric juices.
Intrinsic Factor: Essential for vitamin B12 absorption in the small intestine.
Absorption:
While the stomach primarily digests food, it absorbs small amounts of substances, such as water, alcohol, and some drugs (e.g., aspirin).
Regulation of Food Passage:
The pyloric sphincter controls the movement of chyme from the stomach to the duodenum, ensuring gradual digestion.
Common Disorders of the Stomach:
Gastritis: Inflammation of the stomach lining, often caused by infections (e.g., H. pylori) or irritants like alcohol and NSAIDs.
Peptic Ulcers: Sores in the stomach lining due to excessive acid or reduced mucus protection.
Gastroesophageal Reflux Disease (GERD): Acid from the stomach flows back into the esophagus, causing irritation.
Stomach Cancer: Abnormal cell growth in the stomach lining.
Small Intestine:
Small Intestine
The small intestine is the longest part of the digestive system, measuring approximately 6-7 meters (20-23 feet). It is a coiled tube located between the stomach and the large intestine and plays a critical role in digestion and nutrient absorption.
Structure of the Small Intestine:
The small intestine is divided into three regions:
Duodenum (25 cm or 10 inches):
The first section, which receives partially digested food (chyme) from the stomach.
It is the primary site for the chemical digestion of food, receiving secretions from:
The pancreas (digestive enzymes).
The liver and gallbladder (bile for fat digestion).
Jejunum (2.5 meters or ~8 feet):
The middle section where the majority of nutrient absorption occurs.
Its lining is highly folded with villi and microvilli to maximize the surface area for absorption.
Ileum (3.5 meters or ~11 feet):
The final section, which continues absorption, particularly of vitamin B12, bile salts, and any remaining nutrients.
It connects to the large intestine at the ileocecal valve.
Layers of the Small Intestine Wall:
Mucosa: Contains villi and microvilli to increase the surface area for absorption.
Submucosa: Contains blood vessels and lymphatics for nutrient transport.
Muscularis: Smooth muscle layers that assist in peristalsis and segmentation.
Serosa: Outer protective layer.
Functions of the Small Intestine:
Chemical Digestion:
Digestive enzymes from the pancreas and bile from the liver work in the duodenum to break down carbohydrates, proteins, and fats.
Enzymes include:
Amylase: Breaks down carbohydrates into sugars.
Proteases (e.g., trypsin): Break down proteins into peptides and amino acids.
Lipase: Breaks down fats into fatty acids and glycerol.
Absorption of Nutrients:
The small intestine is the primary site for nutrient absorption:
Carbohydrates: Absorbed as simple sugars (e.g., glucose).
Proteins: Absorbed as amino acids.
Fats: Absorbed as fatty acids and glycerol.
Vitamins and Minerals: Absorbed along with water.
Nutrients are transported into the bloodstream or lymphatic system via villi and microvilli.
Motility:
The small intestine moves chyme through peristalsis (wave-like contractions) and segmentation (mixing movements) to ensure proper digestion and absorption.
Hormone Secretion:
The small intestine produces hormones (e.g., secretin and cholecystokinin) that regulate digestive processes by signaling other organs like the pancreas, liver, and gallbladder.
Immune Defense:
Specialized cells (e.g., Peyer’s patches) in the ileum help protect the body from pathogens entering through the digestive system.
Common Disorders of the Small Intestine:
Celiac Disease: An autoimmune disorder triggered by gluten, damaging the villi and impairing nutrient absorption.
Crohn’s Disease: Inflammatory bowel disease affecting any part of the gastrointestinal tract, including the small intestine.
Small Intestinal Bacterial Overgrowth (SIBO): Overgrowth of bacteria in the small intestine, leading to bloating, diarrhea, and malabsorption.
Intestinal Obstruction: A blockage in the small intestine that prevents the passage of food or liquids.
Liver:
Liver
The liver is the largest internal organ in the human body and plays a vital role in metabolism, digestion, detoxification, and storage. It is located in the upper right abdomen, just beneath the diaphragm, and is divided into the right and left lobes.
Structure of the Liver:
Lobes:
The liver has two main lobes: the right lobe (larger) and the left lobe (smaller).
Each lobe is divided into smaller units called lobules, which are the functional units of the liver.
Blood Supply:
Hepatic artery: Supplies oxygenated blood to the liver.
Portal vein: Brings nutrient-rich blood from the digestive organs for processing.
Bile Production and Transport:
The liver produces bile, which is stored in the gallbladder and transported via bile ducts into the small intestine.
Functions of the Liver:
Production of Bile:
Bile is essential for the digestion and absorption of fats. It emulsifies fats into smaller droplets, making them easier to digest by lipase.
Metabolism:
Carbohydrate Metabolism:
Regulates blood glucose levels by storing excess glucose as glycogen (glycogenesis) and breaking it down into glucose when needed (glycogenolysis).
Converts ammonia (a toxic byproduct of protein metabolism) into urea, which is excreted in urine.
Lipid Metabolism:
Synthesizes cholesterol and lipoproteins (HDL and LDL).
Stores fats and converts them into energy when required.
Detoxification:
The liver neutralizes toxins and drugs by breaking them down into less harmful substances for excretion (e.g., alcohol, medications).
Filters and removes harmful substances, including old or damaged red blood cells.
Storage:
Vitamins: Stores fat-soluble vitamins (A, D, E, K) and water-soluble vitamin B12.
Minerals: Stores iron and copper.
Glycogen: Maintains energy reserves.
Synthesis of Blood Components:
Produces clotting factors and fibrinogen, essential for blood clotting.
Synthesizes albumin, which maintains oncotic pressure and transports hormones, drugs, and other substances in the blood.
Immune Function:
Contains Kupffer cells, which are specialized macrophages that remove bacteria and old red blood cells from the bloodstream.
Hormone Regulation:
Metabolizes and regulates hormones such as insulin, estrogen, and cortisol.
Excretion:
Converts waste products like bilirubin (from hemoglobin breakdown) into bile, which is excreted via the digestive tract.
Common Disorders of the Liver:
Hepatitis: Inflammation of the liver caused by viruses, alcohol, or toxins.
Cirrhosis: Chronic liver damage leading to scarring and impaired function.
Fatty Liver Disease: Accumulation of fat in liver cells, often linked to obesity or alcohol consumption.
Liver Cancer: Malignant tumors originating in the liver (e.g., hepatocellular carcinoma).
Jaundice: Yellowing of the skin and eyes due to high bilirubin levels, often caused by liver dysfunction.
Gallbladder:
Gallbladder
The gallbladder is a small, pear-shaped organ located beneath the liver in the upper right abdomen. It serves as a storage and concentration site for bile, a digestive fluid produced by the liver. Although not essential for survival, the gallbladder plays an important role in digestion, particularly in the breakdown of dietary fats.
Structure of the Gallbladder:
Parts:
Fundus: The rounded bottom portion.
Body: The main and largest section.
Neck: The narrow portion that connects to the cystic duct.
Biliary System:
The gallbladder is part of the biliary system, which includes the bile ducts, liver, and pancreas.
Cystic Duct: Connects the gallbladder to the common bile duct, which delivers bile to the small intestine.
Functions of the Gallbladder:
Storage of Bile:
The liver continuously produces bile, which is stored in the gallbladder until needed for digestion.
Concentration of Bile:
The gallbladder removes water and electrolytes from bile, concentrating it and making it more effective for fat digestion.
Release of Bile:
When fatty foods enter the small intestine, the hormone cholecystokinin (CCK) is released, signaling the gallbladder to contract and release bile through the cystic duct into the common bile duct.
Bile emulsifies fats, breaking them into smaller droplets to enhance digestion and absorption by enzymes like lipase.
Common Disorders of the Gallbladder:
Gallstones (Cholelithiasis):
Hard, crystalline deposits formed in the gallbladder due to imbalances in bile components (cholesterol, bile salts, or bilirubin).
Symptoms may include pain in the upper abdomen, nausea, and bloating.
Cholecystitis:
Inflammation of the gallbladder, often caused by gallstones blocking the cystic duct.
Symptoms include severe pain, fever, and nausea.
Biliary Dyskinesia:
A condition in which the gallbladder does not contract properly, leading to inadequate bile release.
Gallbladder Cancer:
A rare but serious condition that affects the gallbladder’s structure and function.
Choledocholithiasis:
Gallstones that migrate into the common bile duct, potentially causing blockages and complications such as jaundice or pancreatitis.
Gallbladder Removal (Cholecystectomy):
The gallbladder can be removed if it causes significant problems, such as recurrent gallstones or inflammation.
After removal, bile flows directly from the liver to the small intestine. Most people can digest fats adequately without a gallbladder, though some may experience mild digestive issues initially.
Pancreas:.
Pancreas
The pancreas is a vital organ in the digestive and endocrine systems, located in the upper abdomen behind the stomach. It is approximately 15–20 cm (6–8 inches) long and has a head, body, and tail. The pancreas performs both exocrine and endocrine functions, making it essential for digestion and blood sugar regulation.
Structure of the Pancreas:
Parts of the Pancreas:
Head: Located near the duodenum (the first part of the small intestine).
Body: The central portion.
Tail: The narrow end that extends toward the spleen.
Functional Units:
Exocrine Component:
Contains acinar cells that produce digestive enzymes.
Secretes these enzymes into the pancreatic duct, which joins the common bile duct and empties into the duodenum at the ampulla of Vater.
Endocrine Component:
Contains clusters of cells called the islets of Langerhans that release hormones directly into the bloodstream.
Functions of the Pancreas:
1. Exocrine Functions (Digestive Role):
The pancreas produces and secretes digestive enzymes and bicarbonate into the duodenum to aid in the breakdown of food.
Enzymes Produced:
Amylase: Breaks down carbohydrates into sugars.
Lipase: Breaks down fats into fatty acids and glycerol.
Proteases (e.g., trypsin, chymotrypsin): Break down proteins into amino acids.
Bicarbonate Secretion:
Neutralizes stomach acid in the chyme, creating an optimal pH for enzyme activity in the small intestine.
2. Endocrine Functions (Hormonal Role):
The pancreas regulates blood sugar levels by secreting hormones into the bloodstream.
Insulin:
Lowers blood glucose levels by promoting the uptake of glucose into cells for energy or storage as glycogen.
Glucagon:
Raises blood glucose levels by stimulating the liver to release stored glucose (glycogenolysis) and produce glucose (gluconeogenesis).
Somatostatin:
Regulates the secretion of insulin, glucagon, and other hormones.
Pancreatic Polypeptide:
Regulates pancreatic secretions and digestive functions.
Common Disorders of the Pancreas:
Pancreatitis:
Inflammation of the pancreas caused by gallstones, alcohol, infections, or certain medications.
Can be acute (short-term) or chronic (long-lasting).
Diabetes Mellitus:
Caused by insufficient insulin production (Type 1) or insulin resistance (Type 2), leading to high blood sugar levels.
Pancreatic Cancer:
A serious condition where malignant cells develop in the pancreatic tissue, often diagnosed at an advanced stage.
Cystic Fibrosis:
A genetic disorder that affects the exocrine function, leading to thick, sticky secretions that block the pancreatic ducts and impair digestion.
Pancreatic Insufficiency:
A condition where the pancreas fails to produce enough digestive enzymes, leading to malabsorption of nutrients.
Large Intestine (Colon):
Large Intestine (Colon)
The large intestine is the final part of the digestive system, measuring about 1.5 meters (5 feet) in length. Its primary functions include absorbing water and electrolytes, forming and storing feces, and housing beneficial gut bacteria.
Structure of the Large Intestine:
Regions of the Large Intestine:
Cecum: The pouch-like structure at the beginning, connected to the ileum (small intestine) via the ileocecal valve.
Colon: The main section, divided into four parts:
Ascending Colon: Runs upward on the right side of the abdomen.
Transverse Colon: Extends horizontally across the abdomen.
Descending Colon: Runs downward on the left side of the abdomen.
Sigmoid Colon: An S-shaped section leading to the rectum.
Rectum: The final part that stores feces before elimination.
Anus: The opening through which feces are excreted, controlled by the internal and external anal sphincters.
Layers of the Wall:
Mucosa: Inner lining that secretes mucus to ease the passage of feces.
Submucosa: Contains blood vessels and nerves.
Muscularis: Two layers of smooth muscle responsible for peristalsis.
Serosa: Outer connective tissue layer.
Functions of the Large Intestine:
Absorption:
Water: The large intestine absorbs water from undigested material, preventing dehydration.
Electrolytes: Absorbs essential minerals like sodium and potassium.
Formation of Feces:
The undigested material is compacted into solid waste (feces) by removing excess water.
Storage and Elimination:
Feces are temporarily stored in the rectum and expelled through the anus during defecation.
Gut Flora:
The colon houses trillions of beneficial bacteria that:
Produce vitamins (e.g., vitamin K and certain B vitamins).
Break down undigested carbohydrates through fermentation, producing gases (e.g., methane, hydrogen).
Help prevent the growth of harmful bacteria and maintain gut health.
Mucus Secretion:
The mucosa secretes mucus to lubricate the colon’s contents, facilitating smooth passage.
Immune Function:
The large intestine contains lymphoid tissue that helps protect against pathogens entering through the digestive system.
Common Disorders of the Large Intestine:
Constipation:
Difficulty passing stool due to slow movement through the colon or lack of fiber and water in the diet.
Diarrhea:
Frequent, watery stools caused by infections, irritants, or rapid movement of contents through the colon, leading to reduced water absorption.
Irritable Bowel Syndrome (IBS):
A functional disorder characterized by abdominal pain, bloating, constipation, and/or diarrhea.
Inflammatory Bowel Disease (IBD):
Includes conditions like Crohn’s disease and ulcerative colitis, which cause chronic inflammation of the colon.
Colon Cancer:
Abnormal cell growth in the colon, often beginning as benign polyps that can become malignant.
Diverticulitis:
Inflammation or infection of small pouches (diverticula) that form in the colon wall.
Rectum and Anus:
Rectum and Anus
The rectum and anus are the final parts of the digestive system. Their primary role is the storage and controlled elimination of feces, completing the process of digestion.
Rectum
Structure:
The rectum is a straight, muscular tube approximately 12–15 cm (5–6 inches) long, located between the sigmoid colon and the anus.
It is lined with a mucosa layer that secretes mucus to ease the passage of feces.
Functions:
Storage of Feces:
The rectum temporarily stores feces until it is ready to be excreted.
Sensation of Fullness:
The rectum contains stretch receptors that detect the presence of feces and send signals to the brain, creating the urge to defecate.
Preparation for Defecation:
The rectum contracts during defecation to push feces toward the anus.
Anus
Structure:
The anus is the opening at the end of the digestive tract through which feces are expelled.
It is about 2–3 cm long and surrounded by two muscular sphincters:
Internal Anal Sphincter:
Made of smooth muscle (involuntary control).
Maintains continence by staying contracted at rest.
External Anal Sphincter:
Made of skeletal muscle (voluntary control).
Allows conscious control over defecation.
Functions:
Control of Fecal Elimination:
The sphincters work together to control the release of feces and gases, ensuring continence.
Defecation:
During defecation, the internal sphincter relaxes involuntarily, and the external sphincter relaxes under voluntary control to allow the passage of feces.
Protection:
The anus has specialized skin that is sensitive and helps detect the consistency of stool (solid, liquid, or gas).
Process of Defecation:
Signal to Defecate:
When the rectum is full, stretch receptors in its walls send signals to the brain, creating the urge to defecate.
Sphincter Coordination:
The internal anal sphincter relaxes involuntarily, and the external anal sphincter relaxes under voluntary control.
Muscle Contraction:
The rectum contracts to push feces through the anus.
Excretion:
Feces are expelled from the body.
Common Disorders:
Hemorrhoids:
Swollen veins in the rectum or anus, often caused by straining during defecation, chronic constipation, or pregnancy.
Anal Fissures:
Small tears in the lining of the anus, causing pain and bleeding during bowel movements.
Rectal Prolapse:
A condition where the rectum protrudes through the anus, often due to weakened pelvic floor muscles.
Fecal Incontinence:
The inability to control bowel movements, which can result from nerve or muscle damage.
Anorectal Abscess:
A collection of pus in the tissues around the anus, often caused by infection.
Rectal Cancer:
Cancer that develops in the rectum, often associated with symptoms like bleeding, changes in bowel habits, and rectal pain.
Saliva-composition,
Saliva: Composition and Functions
Saliva is a clear, watery fluid secreted by the salivary glands into the mouth. It plays a crucial role in digestion, oral health, and maintaining the balance of the oral environment. Humans produce about 1 to 1.5 liters of saliva daily.
Composition of Saliva
Saliva is composed of 99% water and 1% solutes, which include organic and inorganic components.
1. Water (99%):
Acts as a solvent for the solutes and facilitates the movement of food and enzymes.
2. Organic Components:
Enzymes:
Amylase (Ptyalin):
Begins the digestion of carbohydrates by breaking down starch into maltose.
Lipase:
Plays a minor role in the digestion of fats, particularly in infants.
Lysozyme:
An antibacterial enzyme that helps protect the mouth by breaking down bacterial cell walls.
Lactoperoxidase:
Has antimicrobial properties, contributing to oral immunity.
Proteases:
Involved in the breakdown of proteins.
Mucins:
Glycoproteins that give saliva its viscous texture.
Help lubricate food for easier chewing and swallowing.
Immunoglobulins:
Mainly IgA, which helps prevent microbial infections in the mouth.
Other Organic Substances:
Urea and uric acid (byproducts of metabolism).
Epidermal growth factor (promotes tissue repair).
3. Inorganic Components:
Ions:
Sodium (Na⁺) and Potassium (K⁺):
Maintain the electrolyte balance of saliva.
Calcium (Ca²⁺) and Phosphate (PO₄³⁻):
Play a role in the remineralization of teeth.
Chloride (Cl⁻):
Assists in activating enzymes like amylase.
Bicarbonate (HCO₃⁻):
Buffers saliva and maintains a neutral pH (~6.5–7.5), protecting teeth from acid erosion.
Fluoride (F⁻):
Strengthens enamel and protects against dental caries (cavities).
Functions of Saliva
Digestive Functions:
Initiates Digestion: Amylase and lipase begin the breakdown of carbohydrates and fats.
Food Lubrication: Mucins and water facilitate chewing and swallowing by softening food and forming a bolus.
Protective Functions:
Antimicrobial Action: Enzymes like lysozyme and lactoperoxidase, as well as immunoglobulins, protect the mouth from infections.
Remineralization: Calcium, phosphate, and fluoride contribute to repairing early enamel damage.
Oral Health:
Moisturizes Oral Tissues: Prevents dryness and irritation of the oral mucosa.
Washes Away Debris: Cleanses the mouth by removing food particles and bacteria.
Taste Perception:
Dissolves food particles to enhance the detection of taste by the taste buds.
Speech:
Moistens the mouth, facilitating smooth movement of the tongue and lips for clear articulation.
regulation of secretion and functions of saliva
Regulation of Saliva Secretion and Its Functions
Regulation of Saliva Secretion
The secretion of saliva is a reflex activity primarily controlled by the autonomic nervous system (ANS). Both parasympathetic and sympathetic pathways regulate the secretion, but their effects differ in quality and quantity.
1. Nervous Regulation
Parasympathetic Control (Primary Mechanism):
Stimulates the production of a large volume of watery saliva rich in enzymes.
Activated during:
Eating or thinking about food.
The smell, sight, or taste of food.
Chewing and swallowing.
Pathway:
Salivary centers in the brainstem (medulla oblongata) are activated by sensory inputs (taste and touch).
Cranial nerves (Facial nerve – CN VII and Glossopharyngeal nerve – CN IX) stimulate salivary glands (parotid, submandibular, sublingual).
Sympathetic Control:
Produces a small volume of thick, mucus-rich saliva.
Activated during:
Stress or fear (fight-or-flight response).
Effect:
The saliva is more viscous and has reduced digestive enzyme content.
2. Reflex Mechanisms
Unconditioned Reflex:
Triggered by mechanical or chemical stimulation of the oral cavity.
Example: Chewing or tasting food.
Conditioned Reflex:
Triggered by external stimuli like the sight, smell, or thought of food (Pavlov’s experiment with dogs).
3. Hormonal Influence
Hormones like aldosterone can indirectly affect saliva composition by regulating the levels of sodium and potassium in saliva.
Functions of Saliva
Saliva performs critical roles in digestion, protection, and overall oral health. These functions can be categorized into digestive and non-digestive roles:
Digestive Functions
Food Lubrication:
Mucins in saliva coat food, forming a bolus that is easier to chew and swallow.
Chemical Digestion:
Amylase: Breaks down starch into maltose.
Lipase: Initiates the digestion of lipids, particularly in infants.
Taste Perception:
Dissolves food particles, allowing taste buds to detect flavors.
Non-Digestive Functions
Oral Health and Protection:
Antibacterial Properties: Enzymes like lysozyme and lactoperoxidase kill or inhibit bacterial growth.
Immunological Protection: IgA in saliva prevents microbial attachment and colonization.
Buffering Action: Bicarbonates neutralize acids, protecting teeth from erosion and caries.
Tooth Maintenance:
Provides calcium, phosphate, and fluoride for enamel remineralization.
Moistening and Lubrication:
Prevents dryness and irritation in the oral mucosa.
Facilitates smooth speech by reducing friction.
Wound Healing:
Growth factors in saliva promote tissue repair in the oral cavity.
Stimuli Affecting Saliva Secretion
Stimulatory Factors:
Food: Especially sour or spicy foods increase saliva production.
Chewing: Both food and non-food items like gum stimulate saliva.
External Stimuli: Sight, smell, and thought of food (conditioned response).
Inhibitory Factors:
Dehydration: Reduces saliva production due to decreased fluid availability.
Stress: Sympathetic activation leads to thick, reduced saliva secretion.
Diseases: Conditions like Sjögren’s syndrome or medications (anticholinergics) can decrease saliva production.
Composition and function of gastric juice,
Gastric Juice: Composition and Function
Gastric juice is a clear, acidic fluid secreted by the glands of the stomach. It is essential for digestion and defense against pathogens. On average, the stomach secretes about 1.5–2 liters of gastric juice daily.
Composition of Gastric Juice
Water (H₂O):
The primary component, making up most of the volume.
Acts as a solvent for the other components and facilitates mixing with food.
Hydrochloric Acid (HCl):
Produced by parietal cells in the stomach lining.
Maintains the stomach’s pH at approximately 1.5–3.5.
Functions:
Denatures proteins, making them easier for enzymes to digest.
Activates pepsinogen into pepsin (a protein-digesting enzyme).
Kills bacteria and other pathogens ingested with food.
Pepsinogen:
An inactive enzyme secreted by chief cells.
Activated into pepsin by HCl, which breaks down proteins into smaller peptides.
Mucus:
Secreted by mucous cells in the stomach lining.
Forms a protective layer over the stomach lining to prevent damage from acid and enzymes.
Intrinsic Factor:
Secreted by parietal cells.
Essential for the absorption of vitamin B12 in the small intestine.
Electrolytes:
Includes sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).
Help maintain the ionic balance of the gastric juice.
Gastric Lipase:
Secreted by chief cells.
Begins the digestion of fats into fatty acids and glycerol, though its role is relatively minor compared to pancreatic lipase.
Hormones (e.g., Gastrin):
Gastrin is secreted by G-cells in response to food.
Stimulates the secretion of HCl and gastric motility.
Functions of Gastric Juice
1. Digestive Functions
Protein Digestion:
Pepsin breaks down proteins into peptides, initiating the process of protein digestion.
Fat Digestion:
Gastric lipase starts the breakdown of triglycerides into fatty acids and glycerol.
Food Liquefaction:
Gastric juice mixes with food, forming a semi-liquid substance called chyme for easier passage into the small intestine.
2. Protective Functions
Acidic Barrier:
HCl kills most ingested microorganisms, reducing the risk of infection.
Mucus Protection:
Mucus protects the stomach lining from the corrosive effects of acid and digestive enzymes, preventing ulcers.
3. Activation of Enzymes
HCl converts pepsinogen to its active form, pepsin, enabling protein digestion.
4. Vitamin B12 Absorption
The intrinsic factor binds to vitamin B12, allowing its absorption in the ileum.
5. pH Regulation
Gastric juice helps maintain the acidic environment necessary for optimal enzyme activity.
Clinical Relevance: Disorders Related to Gastric Juice
Hypersecretion of Gastric Juice:
Leads to conditions like peptic ulcers or gastric reflux.
Often caused by stress, infection (H. pylori), or medications like NSAIDs.
Hyposecretion of Gastric Juice:
Results in poor digestion and nutrient malabsorption (e.g., vitamin B12 deficiency leading to pernicious anemia).
Common in conditions like atrophic gastritis.
Imbalance in Mucus Production:
Insufficient mucus can lead to gastric ulcers due to acid damage to the stomach lining.
mechanism and regulation of gastric secretion
Mechanism and Regulation of Gastric Secretion
Gastric secretion is a highly regulated process involving the interaction of neural, hormonal, and local mechanisms to ensure that gastric juice is secreted in response to the body’s digestive needs. The secretion occurs in three phases: cephalic, gastric, and intestinal.
Mechanism of Gastric Secretion
The process of gastric secretion involves the following key steps:
Secretion of Components:
Parietal cells:
Secrete hydrochloric acid (HCl) and intrinsic factor.
Chief cells:
Produce pepsinogen (inactive enzyme) and gastric lipase.
Mucous cells:
Secrete mucus and bicarbonate to protect the stomach lining.
Stimulation of Secretion:
Gastrin:
A hormone released by G-cells in the stomach stimulates HCl secretion by parietal cells.
Histamine:
Secreted by enterochromaffin-like (ECL) cells, histamine binds to H2 receptors on parietal cells, enhancing acid production.
Acetylcholine (ACh):
Released by the vagus nerve, ACh stimulates parietal cells, chief cells, and ECL cells.
Phases of Gastric Secretion
Cephalic Phase (Pre-ingestion Phase):
Stimulus:
Sight, smell, taste, thought, or chewing of food.
Mechanism:
Signals from the brain stimulate the vagus nerve, which releases acetylcholine.
Accounts for about 30% of total gastric secretion.
Gastric Phase (Food in Stomach):
Stimulus:
Distension of the stomach by food and the presence of partially digested proteins (peptides) and amino acids.
Mechanism:
Stretch receptors activate reflexes that stimulate gastrin and acetylcholine release.
Gastrin promotes acid secretion and stimulates ECL cells to release histamine, amplifying HCl production.
Contribution:
Accounts for about 60% of total gastric secretion.
Intestinal Phase (Food in Duodenum):
Stimulus:
Entry of chyme (partially digested food) into the duodenum.
Mechanism:
Initially, duodenal peptides and amino acids stimulate intestinal gastrin, which enhances gastric secretion briefly.
As digestion progresses, hormones like secretin, cholecystokinin (CCK), and gastric inhibitory peptide (GIP) inhibit further gastric secretion to prevent excessive acidity.
Contribution:
Accounts for about 10% of total gastric secretion.
Regulation of Gastric Secretion
1. Neural Regulation:
Parasympathetic Nervous System:
The vagus nerve stimulates gastric secretion via acetylcholine.
Local Reflexes:
Enteric nervous system reflexes regulate secretion in response to stomach distension.
2. Hormonal Regulation:
Gastrin:
Secreted by G-cells in response to stomach distension and protein presence.
Increases HCl secretion by parietal cells.
Histamine:
Released by ECL cells; binds to H2 receptors on parietal cells, enhancing acid production.
Somatostatin:
Secreted by D-cells; inhibits the secretion of gastrin, HCl, and histamine.
Secretin and CCK:
Released by the duodenum; inhibit gastric acid secretion to regulate pH and digestion rate.
3. Chemical Regulation:
pH Feedback:
Low pH (<3) in the stomach inhibits gastrin release and reduces acid secretion (negative feedback mechanism).
Inhibition of Gastric Secretion
Post-meal Feedback:
As the stomach empties, reduced distension and fewer proteins lead to a decrease in gastrin and acid secretion.
Hormonal Inhibition:
Secretin, CCK, and GIP from the small intestine suppress gastric activity.
Neural Inhibition:
Sympathetic activation (e.g., during stress) reduces gastric secretion.
Summary of Key Regulators
Regulator
Source
Function
Gastrin
G-cells in the stomach
Stimulates HCl secretion.
Histamine
ECL cells in the stomach
Enhances HCl production.
Acetylcholine
Vagus nerve
Stimulates parietal and chief cells.
Somatostatin
D-cells in the stomach
Inhibits gastrin and HCl secretion.
Secretin
Duodenum
Reduces gastric acid production.
CCK
Duodenum
Inhibits gastric motility and secretion.
Composition of pancreatic juice, function,
Pancreatic Juice: Composition and Function
Pancreatic juice is a clear, alkaline fluid secreted by the pancreas into the duodenum. It plays a critical role in the digestion of carbohydrates, proteins, and fats, as well as in neutralizing acidic chyme entering from the stomach.
Composition of Pancreatic Juice
Water (H₂O):
The primary component, constituting the majority of pancreatic juice.
Acts as a solvent and facilitates the transport of enzymes and ions.
Enzymes (Produced by acinar cells):
Amylase:
Breaks down carbohydrates (starch) into maltose and other simple sugars.
Lipase:
Digests triglycerides into fatty acids and glycerol.
Proteases:
Include trypsinogen, chymotrypsinogen, and procarboxypeptidase (inactive forms).
Activated in the small intestine (e.g., trypsinogen is converted to trypsin by enterokinase) and break proteins into peptides and amino acids.
Nucleases:
Include DNAase and RNAase, which digest DNA and RNA into nucleotides.
Bicarbonate Ions (HCO₃⁻) (Produced by ductal cells):
Neutralizes acidic chyme from the stomach.
Provides an alkaline pH (~7.5–8.0), optimal for pancreatic enzyme activity.
Electrolytes:
Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and calcium (Ca²⁺) maintain ionic balance and support enzyme activity.
Functions of Pancreatic Juice
1. Digestion:
Carbohydrate Digestion:
Amylase breaks down complex carbohydrates (starch) into maltose and smaller polysaccharides.
Protein Digestion:
Proteases digest proteins into smaller peptides and amino acids.
Fat Digestion:
Lipase breaks down triglycerides into fatty acids and glycerol, essential for fat absorption.
Nucleic Acid Digestion:
Nucleases digest DNA and RNA into nucleotides for absorption.
2. Neutralization of Acid:
Bicarbonate ions neutralize the acidic chyme from the stomach, protecting the intestinal lining and creating an optimal environment for enzyme activity in the small intestine.
3. Enzyme Activation:
The inactive zymogens (e.g., trypsinogen, chymotrypsinogen) prevent autodigestion of the pancreas.
Activation occurs in the duodenum, ensuring digestion occurs only where needed.
Regulation of Pancreatic Secretion
Neural Regulation:
The vagus nerve (parasympathetic system) stimulates pancreatic secretion in response to food or the anticipation of eating.
Hormonal Regulation:
Secretin:
Released by the duodenum in response to acidic chyme.
Stimulates the secretion of bicarbonate-rich pancreatic juice.
Cholecystokinin (CCK):
Released by the duodenum in response to fats and proteins.
Stimulates the secretion of enzyme-rich pancreatic juice.
Gastrin:
Stimulates pancreatic secretion indirectly by increasing gastric motility and acid production.
Clinical Relevance: Disorders Related to Pancreatic Juice
Pancreatitis:
Inflammation of the pancreas due to premature activation of pancreatic enzymes, leading to self-digestion.
Cystic Fibrosis:
Thick mucus blocks pancreatic ducts, reducing enzyme secretion and leading to malabsorption.
Pancreatic Insufficiency:
Inadequate enzyme production results in poor digestion and nutrient malabsorption.
Pancreatic Cancer:
Can obstruct the flow of pancreatic juice, impairing digestion and causing weight loss and jaundice.
regulation of pancreatic secretion
Regulation of Pancreatic Secretion
Pancreatic secretion is a carefully regulated process involving neural and hormonal mechanisms that ensure the appropriate quantity and composition of pancreatic juice is secreted in response to the digestive needs. This regulation is influenced by the presence of food in the stomach and duodenum.
Phases of Pancreatic Secretion
Pancreatic secretion occurs in three phases that correspond to the different stages of digestion:
Acinar cells to secrete enzyme-rich pancreatic juice.
Ductal cells to secrete bicarbonate.
Contribution:
Accounts for about 20% of pancreatic secretion.
2. Gastric Phase (Food in Stomach):
Stimulus:
Distension of the stomach and the presence of peptides and amino acids.
Mechanism:
Gastrin, released by G-cells in the stomach, stimulates the pancreas indirectly by enhancing acid secretion, which subsequently triggers duodenal secretin release.
The vagovagal reflex (stretch receptors in the stomach) stimulates enzyme secretion.
Contribution:
Accounts for about 10% of pancreatic secretion.
3. Intestinal Phase (Food in Duodenum):
Stimulus:
Acidic chyme, fats, and partially digested proteins entering the duodenum.
Mechanism:
This phase is the most important, contributing 70% of pancreatic secretion, and is mediated by two key hormones:
Secretin:
Released by the duodenum in response to acidic chyme.
Stimulates the ductal cells of the pancreas to secrete bicarbonate-rich fluid.
Cholecystokinin (CCK):
Released by the duodenum in response to fats and proteins.
Stimulates acinar cells to secrete enzyme-rich pancreatic juice.
Neural input from the vagus nerve amplifies the secretion.
Neural Regulation
Parasympathetic Stimulation:
The vagus nerve (cranial nerve X) releases acetylcholine, which:
Stimulates acinar cells to secrete digestive enzymes.
Enhances ductal cell activity for bicarbonate secretion.
Sympathetic Stimulation:
Inhibits pancreatic secretion during stress or fight-or-flight responses.
Hormonal Regulation
Secretin:
Source: S cells of the duodenum.
Stimulus: Acidic chyme entering the duodenum.
Function: Stimulates bicarbonate secretion from ductal cells to neutralize acidity and protect the intestinal lining.
Cholecystokinin (CCK):
Source: I cells of the duodenum.
Stimulus: Presence of fats and proteins in the chyme.
Function:
Stimulates acinar cells to release enzyme-rich pancreatic juice.
Enhances gallbladder contraction and bile release (for fat digestion).
Gastrin:
Source: G-cells in the stomach.
Stimulus: Stomach distension and protein presence.
Function: Indirectly stimulates pancreatic secretion by enhancing acid production, which prompts secretin release.
Somatostatin:
Source: D cells in the stomach and duodenum.
Function: Inhibits pancreatic secretion by suppressing the release of gastrin, secretin, and CCK.
Feedback Mechanisms
Positive Feedback:
Secretin and CCK continue stimulating pancreatic secretion as long as acidic chyme and nutrients are present in the duodenum.
Negative Feedback:
When digestion is complete and the duodenal pH normalizes, secretin and CCK secretion decline, reducing pancreatic output.
The liver is a vital organ responsible for numerous essential functions that support metabolism, digestion, detoxification, and immune defense. Below are the key functions of the liver:
1. Metabolic Functions
Carbohydrate Metabolism:
Glycogenesis: Converts excess glucose into glycogen for storage.
Glycogenolysis: Breaks down glycogen into glucose when blood sugar levels are low.
Gluconeogenesis: Synthesizes glucose from non-carbohydrate sources (e.g., amino acids, glycerol).
Protein Metabolism:
Deamination: Removes the amino group from amino acids, forming ammonia.
Urea Formation: Converts toxic ammonia into urea, which is excreted by the kidneys.
Plasma Protein Synthesis: Produces albumin (maintains blood oncotic pressure) and clotting factors (fibrinogen, prothrombin).
Lipid Metabolism:
Cholesterol Synthesis: Produces cholesterol for cell membranes and steroid hormones.
Lipogenesis: Converts excess carbohydrates into fats.
Lipoprotein Production: Synthesizes HDL and LDL for fat transport.
Ketogenesis: Produces ketone bodies during fasting for energy.
2. Digestive Functions
Bile Production:
The liver produces bile, a fluid essential for the digestion and absorption of fats.
Bile contains bile salts, cholesterol, and bilirubin, and it emulsifies fats into smaller droplets.
3. Detoxification and Filtration
Toxin Removal:
Detoxifies harmful substances like alcohol, drugs, and environmental toxins.
Hormone Metabolism:
Breaks down and inactivates excess hormones (e.g., estrogen, cortisol).
Kupffer Cells:
Specialized liver macrophages that filter bacteria, old red blood cells, and other debris from the blood.
4. Storage Functions
Vitamin Storage:
Stores fat-soluble vitamins (A, D, E, and K) and water-soluble vitamin B12.
Mineral Storage:
Stores iron (as ferritin) and copper.
Glycogen Storage:
Acts as a reservoir for glucose in the form of glycogen.
5. Blood Regulation
Clotting Factor Production:
Produces clotting proteins essential for blood coagulation.
Blood Filtration:
Removes damaged cells, pathogens, and toxins from the blood.
Blood Reservoir:
Acts as a storage site for blood and regulates its volume.
6. Immune Functions
Immune Surveillance:
Kupffer cells in the liver help remove pathogens and dead cells.
Acute-Phase Protein Production:
Produces proteins like C-reactive protein (CRP) during inflammation.
7. Hormonal Functions
Regulation of Hormones:
Metabolizes and inactivates hormones such as insulin and thyroid hormones.
Production of Angiotensinogen:
A precursor for angiotensin, which regulates blood pressure.
8. Bilirubin Metabolism
Processing Bilirubin:
The liver converts bilirubin (a byproduct of hemoglobin breakdown) into a water-soluble form for excretion in bile.
Prevents Jaundice:
Proper bilirubin processing prevents its accumulation, which could lead to jaundice.
9. Heat Production
The liver generates heat during its metabolic activities, contributing to body temperature regulation.
10. Energy Production
Fatty Acid Oxidation:
Breaks down fatty acids to generate ATP (energy).
Ketone Body Production:
Provides an energy source during fasting or starvation.
Summary Table of Liver Functions
Function
Key Role
Metabolism
Processes carbohydrates, proteins, and fats.
Bile Production
Aids fat digestion and absorption.
Detoxification
Neutralizes toxins and drugs.
Storage
Stores vitamins, minerals, and glycogen.
Blood Clotting
Produces essential clotting factors.
Immune Defense
Filters pathogens and activates immune responses.
Hormone Regulation
Metabolizes and regulates hormones.
Bilirubin Processing
Excretes bilirubin to prevent jaundice.
gall bladder and pancreas
Gallbladder and Pancreas
The gallbladder and pancreas are accessory organs of the digestive system that play crucial roles in digestion and metabolic regulation.
Gallbladder
Structure:
A small, pear-shaped organ located beneath the liver.
Connected to the liver and small intestine via the biliary system (cystic duct, common hepatic duct, and common bile duct).
Functions:
Bile Storage:
Stores bile produced by the liver until it is needed for digestion.
The gallbladder can hold about 30–50 mL of bile.
Bile Concentration:
Removes water and electrolytes from bile, making it more concentrated and effective for fat digestion.
Bile Release:
When fatty foods enter the small intestine, the hormone cholecystokinin (CCK) signals the gallbladder to contract.
Bile is released through the cystic duct and common bile duct into the duodenum.
Bile Composition:
Bile Salts: Emulsify fats, breaking them into smaller droplets.
Bilirubin: A byproduct of hemoglobin breakdown, giving bile its yellow-green color.
Cholesterol: Helps with bile salt formation.
Common Disorders:
Gallstones (Cholelithiasis): Hardened deposits of bile components.
Cholecystitis: Inflammation of the gallbladder, often due to gallstones.
Gallbladder Cancer: A rare but serious condition.
Pancreas
Structure:
A gland located behind the stomach.
Divided into head, body, and tail.
Has both exocrine and endocrine functions.
Functions:
1. Exocrine Functions (Digestive Role):
Produces and secretes pancreatic juice containing:
Enzymes:
Amylase: Breaks down carbohydrates into simple sugars.
Lipase: Breaks down fats into fatty acids and glycerol.
Proteases: (Trypsin, chymotrypsin) Digest proteins into amino acids.
Nucleases: Digest DNA and RNA.
Bicarbonate Ions:
Neutralize acidic chyme from the stomach, creating an alkaline environment for enzyme activity.
The juice is released into the duodenum via the pancreatic duct, often joining the common bile duct.
2. Endocrine Functions (Hormonal Role):
Islets of Langerhans secrete hormones into the bloodstream:
Insulin:
Lowers blood glucose by promoting glucose uptake into cells.
Glucagon:
Raises blood glucose by stimulating glycogen breakdown in the liver.
Somatostatin:
Regulates and inhibits the secretion of insulin, glucagon, and digestive enzymes.
Common Disorders:
Pancreatitis:
Inflammation of the pancreas caused by enzyme activation within the pancreas itself.
Diabetes Mellitus:
A condition caused by insufficient insulin (Type 1) or insulin resistance (Type 2).
Pancreatic Cancer:
Malignant tumors affecting the pancreas, often diagnosed at an advanced stage.
Cystic Fibrosis:
A genetic disorder affecting exocrine pancreatic secretions, leading to malabsorption.
Comparison of Gallbladder and Pancreas
Feature
Gallbladder
Pancreas
Primary Role
Stores and concentrates bile.
Produces digestive enzymes and hormones.
Digestive Function
Aids in fat digestion.
Breaks down carbohydrates, proteins, and fats.
Hormonal Role
None
Regulates blood glucose (insulin, glucagon).
Secretion
Bile into the small intestine.
Pancreatic juice into the small intestine.
Disorders
Gallstones, cholecystitis, cancer.
Pancreatitis, diabetes, cancer.
Composition of bile and function
Composition of Bile and Its Functions
Bile is a yellow-green fluid produced by the liver and stored in the gallbladder. It is essential for the digestion and absorption of fats, as well as the elimination of waste products.
Composition of Bile
Bile is composed of water, organic substances, and inorganic ions.
1. Water (97-98%):
The primary component, acting as a solvent to carry bile salts and other substances.
2. Organic Components:
Bile Salts (Conjugated bile acids):
Derived from cholesterol (e.g., cholic acid and chenodeoxycholic acid).
Conjugated with amino acids (glycine or taurine) to form more water-soluble compounds.
Function: Emulsify fats, aiding in their digestion and absorption.
Cholesterol:
Excess cholesterol is excreted in bile.
Function: A precursor for bile salt synthesis.
Phospholipids:
Primarily lecithin.
Function: Helps solubilize cholesterol and emulsify fats.
Bilirubin:
A yellow pigment formed from the breakdown of hemoglobin in red blood cells.
Function: Excreted as a waste product; gives bile its color.
Bile Pigments:
Bilirubin and biliverdin are waste products of hemoglobin metabolism.
Function: Excretory role.
Proteins:
Small amounts, with minimal contribution to function.
3. Inorganic Components:
Electrolytes:
Sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), and bicarbonate (HCO₃⁻).
Function: Maintain the pH of bile (~7.6–8.6) and neutralize stomach acid in the small intestine.
Functions of Bile
1. Digestion and Absorption of Fats:
Emulsification:
Bile salts reduce the surface tension of fat droplets, breaking them into smaller droplets for easier digestion by pancreatic lipase.
Micelle Formation:
Bile salts and phospholipids form micelles, which transport digested fats (fatty acids, monoglycerides) and fat-soluble vitamins (A, D, E, K) to the intestinal lining for absorption.
2. Neutralization of Stomach Acid:
Bicarbonate ions in bile help neutralize the acidic chyme from the stomach, providing an optimal pH for digestive enzymes in the duodenum.
3. Excretion of Waste Products:
Cholesterol:
Bile is the primary route for cholesterol excretion.
Bilirubin:
A waste product from hemoglobin breakdown is eliminated through bile.
Drug and Toxin Elimination:
The liver excretes drugs, toxins, and metabolites into bile for removal from the body.
4. Antimicrobial Function:
Bile has a detergent-like effect that can disrupt bacterial membranes, reducing microbial growth in the intestine.
Summary Table
Component
Source/Role
Function
Bile Salts
Derived from cholesterol; conjugated with glycine/taurine
Emulsify fats and aid absorption.
Cholesterol
Excess excreted in bile
Precursor for bile salts; fat excretion.
Phospholipids
Lecithin
Solubilize cholesterol; emulsify fats.
Bilirubin
Hemoglobin breakdown product
Excretion of waste; gives bile its color.
Electrolytes
Sodium, potassium, chloride, bicarbonate
Neutralize acidic chyme; maintain bile pH.
Water
Major component
Solvent for bile components.
Clinical Relevance: Disorders Related to Bile
Gallstones:
Excess cholesterol or bilirubin can form solid stones in the gallbladder.
Cholestasis:
A condition where bile flow is reduced or blocked, leading to fat malabsorption and jaundice.
Bile Acid Deficiency:
Results in impaired fat digestion and vitamin deficiencies.
Jaundice:
Accumulation of bilirubin due to impaired bile excretion.
Secretion and Function of small and large intestine
Secretion and Function of Small and Large Intestines
The small intestine and large intestine are critical parts of the digestive system. While the small intestine is primarily involved in digestion and nutrient absorption, the large intestine focuses on water absorption and waste elimination.
Small Intestine
Secretions of the Small Intestine
The small intestine secretes fluids and enzymes that aid in digestion and absorption.
Mucus:
Secreted by goblet cells in the intestinal lining.
Protects the mucosa and lubricates food for smooth passage.
Digestive Enzymes (Produced by the epithelium of the small intestine):
Peptidases: Break peptides into amino acids.
Disaccharidases (e.g., maltase, sucrase, lactase): Break down disaccharides into monosaccharides.
Lipase: Breaks down fats into fatty acids and glycerol.
Intestinal Juice (Succus Entericus):
A mixture of water, mucus, and enzymes secreted by intestinal glands.
Facilitates digestion by providing a medium for enzymatic activity.
Bicarbonate Ions:
Secreted by Brunner’s glands in the duodenum.
Neutralizes acidic chyme from the stomach.
Hormones:
Secretin: Stimulates the pancreas to release bicarbonate.
Cholecystokinin (CCK): Stimulates bile secretion and pancreatic enzyme release.
Functions of the Small Intestine
Digestion:
Enzymes from the pancreas and intestinal lining digest carbohydrates, proteins, and fats.
Completes the breakdown of nutrients into absorbable forms.
Absorption:
The small intestine is the primary site for nutrient absorption:
Carbohydrates: Absorbed as monosaccharides (e.g., glucose).
Proteins: Absorbed as amino acids.
Fats: Absorbed as fatty acids and glycerol in micelles.
Vitamins and Minerals: Absorbed along with water.
Villi and microvilli increase the surface area for absorption.
Neutralization:
Bicarbonate neutralizes the acidic chyme, creating an optimal pH for enzyme activity.
Hormone Secretion:
Regulates pancreatic and bile secretions for digestion.
Immune Defense:
Peyer’s patches in the ileum provide protection against pathogens.
Large Intestine
Secretions of the Large Intestine
Mucus:
Secreted by goblet cells to lubricate the passage of feces and protect the mucosa.
Bicarbonate and Potassium:
Secreted in small amounts to maintain pH balance and electrolyte levels.
Minimal Enzyme Secretion:
Unlike the small intestine, the large intestine does not secrete digestive enzymes.
Functions of the Large Intestine
Absorption:
Water and Electrolytes:
Reabsorbs water, sodium, potassium, and chloride from undigested material.
Maintains fluid and electrolyte balance.
Vitamins:
Absorbs vitamins (e.g., vitamin K, B12, riboflavin, thiamine) produced by gut bacteria.
Formation and Storage of Feces:
Compacts undigested material, dead cells, and bacteria into solid feces.
Stores feces in the rectum until excretion.
Microbial Activity:
Houses a large population of beneficial bacteria that:
Ferment undigested carbohydrates, producing short-chain fatty acids and gases.
Synthesize vitamins like vitamin K and some B vitamins.
Defecation:
Expels feces via the rectum and anus through voluntary and involuntary muscle contractions.
Immune Function:
Gut-associated lymphoid tissue (GALT) in the colon provides immune defense.
pH Regulation:
Bicarbonate secretion helps maintain a slightly alkaline environment.
Comparison of Functions
Function
Small Intestine
Large Intestine
Primary Role
Digestion and nutrient absorption
Water and electrolyte absorption, feces formation
Digestive Enzymes
Produces enzymes for digestion
Does not produce enzymes
Absorption
Absorbs nutrients, vitamins, and minerals
Absorbs water, electrolytes, and some vitamins
Bacterial Activity
Minimal bacterial role
Significant role in vitamin synthesis and fermentation
Feces Formation
Does not form feces
Forms and stores feces
Movements of alimentary tract
Movements of the Alimentary Tract
The alimentary tract exhibits various types of movements to facilitate the processes of digestion, absorption, and excretion. These movements are coordinated by the enteric nervous system, autonomic nervous system, and local reflexes.
Types of Movements in the Alimentary Tract
1. Peristalsis
Definition: A wave-like, rhythmic contraction of the circular and longitudinal muscles in the wall of the alimentary tract.
Purpose:
Moves food (bolus, chyme, or feces) forward through the digestive tract.
Location:
Found throughout the digestive system, including the esophagus, stomach, small intestine, and large intestine.
Mechanism:
Circular muscles contract behind the food bolus while longitudinal muscles relax, propelling the food forward.
2. Segmentation
Definition: Non-propulsive, rhythmic contractions of the circular muscles in the intestine.
Purpose:
Mixes food with digestive juices to enhance digestion.
Promotes contact between nutrients and the intestinal mucosa for absorption.
Location:
Small intestine and, to a lesser extent, the large intestine.
3. Pendular Movements
Definition: Oscillatory movements caused by contractions of longitudinal muscles.
Purpose:
Mixes intestinal contents and brings them into closer contact with the intestinal wall.
Location:
Small intestine.
4. Mass Movements
Definition: Strong, prolonged peristaltic contractions that move fecal matter over long distances in the large intestine.
Purpose:
Pushes feces toward the rectum in preparation for defecation.
Location:
Large intestine (colon).
5. Swallowing (Deglutition)
Definition: The movement of food from the mouth to the stomach via the esophagus.
Phases:
Oral Phase: Voluntary movement of the bolus into the pharynx.
Pharyngeal Phase: Involuntary movement through the pharynx.
Esophageal Phase: Involuntary peristalsis moves the bolus to the stomach.
6. Tonic Contractions
Definition: Sustained contractions of smooth muscle.
Purpose:
Maintain sphincter tone (e.g., lower esophageal sphincter, pyloric sphincter) to regulate food passage and prevent backflow.
Location:
Sphincters throughout the digestive tract.
7. Migrating Motor Complex (MMC)
Definition: Cyclic, wave-like contractions during fasting that sweep undigested material through the small intestine.
Purpose:
Clears the intestine of residual food and bacteria.
Location:
Small intestine.
8. Churning
Definition: Vigorous mixing movements that occur in the stomach.
Purpose:
Mixes food with gastric juices to form chyme.
Location:
Stomach.
9. Haustral Contractions
Definition: Slow, segmental contractions in the large intestine that form pouch-like structures called haustra.
Purpose:
Facilitate water and electrolyte absorption.
Slowly propel feces toward the rectum.
Location:
Large intestine.
10. Defecation
Definition: The voluntary and involuntary movements involved in expelling feces from the rectum.
Phases:
Internal Anal Sphincter Relaxation: Involuntary.
External Anal Sphincter Relaxation: Voluntary.
Summary Table of Movements
Movement
Location
Purpose
Peristalsis
Throughout the tract
Propels food forward.
Segmentation
Small and large intestine
Mixes food with digestive juices.
Pendular Movements
Small intestine
Mixes and spreads chyme.
Mass Movements
Large intestine
Moves feces toward the rectum.
Swallowing
Mouth to stomach
Transfers food to the stomach.
Tonic Contractions
Sphincters
Regulates passage and prevents backflow.
MMC
Small intestine
Cleans residual material during fasting.
Churning
Stomach
Mixes food with gastric juices.
Haustral Contractions
Large intestine
Facilitates absorption and feces movement.
Defecation
Rectum and anus
Eliminates waste.
Clinical Significance
Disorders of Motility:
Achalasia: Impaired relaxation of the lower esophageal sphincter.
Irritable Bowel Syndrome (IBS): Abnormal intestinal motility leading to diarrhea or constipation.
Gastroparesis: Delayed gastric emptying due to reduced peristalsis.
Hyperactive Movements:
Diarrhea: Increased peristalsis reduces water absorption in the intestine.
Hypoactive Movements:
Constipation: Reduced motility in the large intestine leads to excessive water absorption.
Digestion in mouth,
Digestion in the Mouth
The process of digestion begins in the mouth, where both mechanical and chemical digestion take place. These processes prepare food for further digestion in the stomach and intestines.
1. Mechanical Digestion
Chewing (Mastication):
Teeth break food into smaller pieces, increasing the surface area for enzyme action.
The tongue and cheeks help position food for effective chewing and mix it with saliva.
Formation of Bolus:
The broken-down food is mixed with saliva, forming a soft, cohesive mass (bolus) that is easier to swallow.
2. Chemical Digestion
Saliva, produced by the salivary glands (parotid, submandibular, and sublingual glands), contains enzymes and other substances essential for chemical digestion.
Components of Saliva Involved in Digestion:
Salivary Amylase (Ptyalin):
Function: Begins the digestion of carbohydrates by breaking down starch into maltose (a disaccharide) and smaller polysaccharides.
Optimal pH: ~6.7–7.0 (neutral to slightly acidic).
Lingual Lipase:
Function: Initiates the breakdown of triglycerides into fatty acids and glycerol.
Role: Limited in adults but significant in infants, where milk fats are a primary energy source.
Optimal pH: Functions best in the acidic environment of the stomach.
Mucus:
Function: Lubricates food, making it easier to chew, swallow, and pass through the esophagus.
Water:
Function: Dissolves food particles, aiding the perception of taste and enzyme action.
Lysozyme and Immunoglobulin A (IgA):
Function: Provide antimicrobial protection, ensuring a clean environment for digestion.
3. Process of Digestion in the Mouth
Ingestion:
Food enters the mouth and is sensed by taste buds, triggering salivation.
Chewing and Mixing:
Teeth grind the food while saliva mixes with it to start chemical digestion.
Initial Carbohydrate Digestion:
Salivary amylase breaks down starch into simpler sugars.
Digestion continues until the bolus reaches the acidic environment of the stomach, which inactivates salivary amylase.
Minimal Fat Digestion:
Lingual lipase starts breaking down triglycerides but becomes more active in the stomach.
4. Functions of Digestion in the Mouth
Breakdown of Food:
Reduces food size for easier swallowing and digestion.
Facilitation of Swallowing:
Saliva moistens and lubricates food, aiding in the formation of a bolus for swallowing.
Taste Sensation:
Dissolved food particles interact with taste buds, enhancing the sensory experience of eating.
Initiation of Digestion:
Begins carbohydrate and fat digestion.
Protection:
Antimicrobial components in saliva protect the oral cavity from infection.
5. Summary Table
Aspect
Description
Mechanical Digestion
Chewing by teeth; bolus formation by tongue and saliva.
Mucus (lubrication), water (dissolution), lysozyme, IgA (protection).
Key Outcomes
Formation of bolus; partial carbohydrate digestion; antimicrobial defense.
Clinical Relevance
Xerostomia (Dry Mouth):
Reduced saliva production can impair digestion and increase the risk of oral infections.
Dental Issues:
Poor mastication due to dental problems can hinder mechanical digestion.
Salivary Amylase Deficiency:
May affect the initial digestion of carbohydrates, leading to incomplete digestion.
stomach,
Digestion in the Stomach
The stomach is a J-shaped muscular organ located in the upper abdomen. It plays a critical role in the digestion of food by performing mechanical and chemical digestion. Food is mixed with gastric secretions to form a semi-liquid mixture called chyme, which is gradually released into the small intestine.
1. Structure of the Stomach
The stomach is divided into four main regions:
Cardia: Connects the esophagus to the stomach.
Fundus: Dome-shaped upper portion that stores undigested food and gases.
Body: The largest part, where most digestion occurs.
Pylorus: The lower part that connects to the duodenum via the pyloric sphincter.
2. Gastric Secretions
The stomach secretes gastric juice, which contains the following components:
Hydrochloric Acid (HCl):
Produced by parietal cells.
Functions:
Lowers the pH (1.5–3.5) to activate enzymes.
Denatures proteins, making them easier to digest.
Kills bacteria and other pathogens.
Pepsinogen:
An inactive enzyme secreted by chief cells.
Activated to pepsin by HCl.
Function: Breaks down proteins into smaller peptides.
Gastric Lipase:
Secreted by chief cells.
Function: Begins the digestion of triglycerides into fatty acids and glycerol (minor role compared to pancreatic lipase).
Intrinsic Factor:
Produced by parietal cells.
Function: Essential for the absorption of vitamin B12 in the small intestine.
Mucus:
Secreted by mucous cells.
Function: Protects the stomach lining from acidic gastric juice and enzymatic digestion.
Water:
Function: Provides a medium for mixing and dissolving food and enzymes.
3. Digestive Processes in the Stomach
1. Mechanical Digestion:
Churning and Mixing:
The stomach muscles contract to mix food with gastric juice.
These movements help break food into smaller particles and form chyme.
2. Chemical Digestion:
Protein Digestion:
HCl denatures proteins, and pepsin breaks them into peptides.
Fat Digestion:
Gastric lipase initiates the breakdown of fats (limited compared to small intestine).
Carbohydrate Digestion:
Minimal carbohydrate digestion occurs, as salivary amylase is inactivated by the acidic environment.
3. Absorption:
The stomach absorbs small amounts of:
Water.
Alcohol.
Certain drugs (e.g., aspirin).
4. Regulation of Gastric Activity
Phases of Gastric Secretion:
Cephalic Phase:
Triggered by the sight, smell, or thought of food.
The vagus nerve stimulates the release of gastric juice.
Gastric Phase:
Triggered by food entering the stomach.
Stomach distension and the presence of peptides stimulate gastrin release, increasing HCl and pepsinogen secretion.
Intestinal Phase:
Triggered by chyme entering the duodenum.
Hormones like secretin and cholecystokinin (CCK) inhibit gastric activity to regulate chyme release.
5. Functions of the Stomach
Food Storage:
Stores food temporarily and releases it gradually into the small intestine.
Protein Digestion:
Pepsin and HCl initiate protein breakdown.
Chyme Formation:
Mixes food with gastric juices to create a semi-liquid mixture.
Defense:
HCl destroys harmful microorganisms ingested with food.
Absorption:
Absorbs small amounts of water, alcohol, and certain medications.
Intrinsic Factor Secretion:
Facilitates vitamin B12 absorption, essential for red blood cell production.
6. Common Disorders of the Stomach
Gastritis:
Inflammation of the stomach lining caused by infection (e.g., H. pylori), alcohol, or NSAIDs.
Peptic Ulcers:
Erosion of the stomach lining due to excessive acid or reduced mucus protection.
Gastroesophageal Reflux Disease (GERD):
Backflow of stomach acid into the esophagus, causing irritation.
The small intestine is the primary site for digestion and absorption of nutrients. It is a long, coiled tube measuring about 6–7 meters (20–23 feet) in length, extending from the stomach to the large intestine.
Structure of the Small Intestine
Duodenum (~25 cm):
First portion, connected to the stomach.
Receives chyme from the stomach, bile from the gallbladder, and digestive enzymes from the pancreas.
Neutralizes acidic chyme with bicarbonate.
Jejunum (~2.5 meters):
Middle portion.
Major site for nutrient absorption.
Ileum (~3.5 meters):
Final portion.
Absorbs vitamin B12, bile salts, and remaining nutrients.
Ends at the ileocecal valve, connecting to the large intestine.
Functions of the Small Intestine
Digestion:
Enzymes from the pancreas (e.g., amylase, lipase, proteases) and the intestinal lining break down carbohydrates, proteins, and fats into absorbable units.
Absorption:
Nutrients (glucose, amino acids, fatty acids, vitamins, minerals) are absorbed through villi and microvilli into the bloodstream or lymphatic system.
Neutralization:
Bicarbonate from the pancreas neutralizes stomach acid to create an optimal pH for enzyme activity.
Hormone Secretion:
Hormones like secretin and cholecystokinin (CCK) regulate bile and enzyme secretion.
Immune Defense:
Peyer’s patches (lymphatic tissue) provide immune protection.
Large Intestine
The large intestine is the final part of the digestive system, measuring about 1.5 meters (5 feet) in length. Its primary role is water and electrolyte absorption and waste elimination.
Structure of the Large Intestine
Cecum:
A pouch connected to the ileum.
Houses the appendix, which contains immune tissue.
Colon:
Divided into four parts:
Ascending Colon (right side of the abdomen).
Transverse Colon (crosses the abdomen).
Descending Colon (left side of the abdomen).
Sigmoid Colon (S-shaped, leading to the rectum).
Rectum:
Stores feces until defecation.
Anus:
The opening through which feces are expelled, controlled by internal and external anal sphincters.
Functions of the Large Intestine
Water and Electrolyte Absorption:
Reabsorbs water and minerals (sodium, potassium) from indigestible food residues.
Formation and Storage of Feces:
Compacts undigested material, dead cells, and bacteria into solid waste (feces).
Vitamin Production:
Gut bacteria produce vitamins (e.g., vitamin K, biotin), which are absorbed.
Microbial Fermentation:
Ferments undigested carbohydrates, producing gases and short-chain fatty acids.
Defecation:
Expels waste through the rectum and anus.
Comparison of Small and Large Intestines
Aspect
Small Intestine
Large Intestine
Length
~6–7 meters
~1.5 meters
Diameter
Narrower
Wider
Primary Function
Digestion and absorption of nutrients
Water absorption and feces formation
Digestive Enzymes
Yes
No
Absorption
Nutrients, vitamins, minerals
Water, electrolytes, vitamins (from bacteria)
Bacteria Role
Minimal
Significant (fermentation, vitamin production)
Hormones
Secretin, CCK
None
Key Points
Small Intestine: Critical for digestion and absorption, using enzymes and villi.
Large Intestine: Focuses on water absorption, feces formation, and hosting gut bacteria.
Absorption of food
Absorption of Food
Absorption is the process by which digested nutrients are taken up into the bloodstream or lymph from the digestive tract. Most of the absorption occurs in the small intestine, while the large intestine absorbs water and electrolytes.
Sites of Absorption in the Digestive Tract
1. Mouth
Minimal absorption occurs in the mouth.
Substances Absorbed:
Some drugs (e.g., nitroglycerin) and small amounts of glucose.
2. Stomach
Limited absorption due to the thick mucus lining and acidic pH.
Substances Absorbed:
Alcohol.
Water (small amounts).
Certain drugs (e.g., aspirin).
3. Small Intestine
The primary site for absorption.
Mechanisms of Absorption:
Passive Diffusion: Substances move along the concentration gradient.
Facilitated Diffusion: Substances require carrier proteins but no energy.
Active Transport: Substances move against the concentration gradient using energy (ATP).
Endocytosis: Small particles are engulfed by the cell membrane.
Key Areas of Absorption:
Duodenum:
Iron, calcium, magnesium, and some vitamins.
Jejunum:
Majority of macronutrients (carbohydrates, proteins, fats) and water-soluble vitamins.
Ileum:
Vitamin B12 and bile salts.
4. Large Intestine
Absorbs remaining water and electrolytes.
Substances Absorbed:
Water (important for fecal consistency).
Sodium, potassium, and chloride.
Vitamins produced by gut bacteria (e.g., vitamin K, biotin).
Site of Absorption: Small intestine (duodenum and jejunum).
Mechanism:
Amino Acids: Absorbed via active transport linked to sodium.
Dipeptides and Tripeptides: Absorbed via peptide transporters and further broken down into amino acids inside the enterocytes.
3. Fats
Digestion Products: Fatty acids, monoglycerides, and glycerol.
Site of Absorption: Small intestine (jejunum and ileum).
Mechanism:
Emulsified by bile salts into micelles.
Micelles transport fats to the intestinal epithelium.
Inside enterocytes:
Reassembled into triglycerides.
Packaged into chylomicrons and released into the lymphatic system via lacteals.
4. Vitamins
Fat-Soluble Vitamins (A, D, E, K):
Absorbed with dietary fats into the lymph.
Water-Soluble Vitamins (B-complex, C):
Absorbed via active transport or diffusion in the small intestine.
Vitamin B12 requires intrinsic factor (secreted by parietal cells in the stomach) and is absorbed in the ileum.
5. Minerals
Calcium:
Absorbed in the duodenum; requires vitamin D for optimal absorption.
Iron:
Absorbed in the duodenum; requires an acidic environment and is enhanced by vitamin C.
Sodium and Potassium:
Absorbed throughout the small and large intestines via active transport.
6. Water
Absorbed in both the small and large intestines.
About 90% of water is absorbed in the small intestine through osmosis, with the remainder absorbed in the large intestine.
Factors Influencing Absorption
Surface Area:
The presence of villi and microvilli in the small intestine increases the surface area for absorption.
Transit Time:
Longer time in the small intestine enhances nutrient absorption.
pH:
Optimal pH levels in different sections of the digestive tract aid specific nutrient absorption.
Presence of Enzymes and Bile:
Digestive enzymes and bile salts are essential for breaking down and emulsifying nutrients for absorption.
Summary Table
Nutrient
End Product
Absorption Site
Mechanism
Carbohydrates
Monosaccharides
Duodenum and jejunum
Active transport and facilitated diffusion
Proteins
Amino acids, dipeptides
Duodenum and jejunum
Active transport
Fats
Fatty acids, monoglycerides
Jejunum and ileum
Micelle transport; passive diffusion
Vitamins (Fat-Soluble)
A, D, E, K
Jejunum and ileum
With fats into lymph
Vitamins (Water-Soluble)
B-complex, C
Duodenum and jejunum
Diffusion or active transport
Minerals
Iron, calcium, sodium, etc.
Duodenum (iron, calcium)
Active transport
Water
Water
Small and large intestine
Osmosis
Metabolism of CHO
Metabolism of Carbohydrates (CHO)
Carbohydrate metabolism refers to the biochemical processes by which the body processes and utilizes carbohydrates to generate energy and maintain cellular functions. The primary goal of carbohydrate metabolism is to provide glucose, which is a major energy source.
Key Steps in Carbohydrate Metabolism
Digestion and Absorption:
Carbohydrates (starch, glycogen, disaccharides) are broken down into monosaccharides (glucose, fructose, galactose) during digestion.
These monosaccharides are absorbed in the small intestine and transported to the liver via the hepatic portal vein.
1. Glycolysis
Definition: The breakdown of glucose (6-carbon molecule) into two molecules of pyruvate (3-carbon molecules).
Location: Cytoplasm of the cell.
Aerobic or Anaerobic: Can occur with or without oxygen.
Steps:
Glucose is phosphorylated to glucose-6-phosphate.
Glucose-6-phosphate undergoes a series of reactions to form pyruvate.
Net ATP Yield: 2 ATP molecules (4 produced, 2 consumed).
Other Products: 2 NADH molecules.
2. Pyruvate Metabolism
The fate of pyruvate depends on oxygen availability:
Aerobic Conditions:
Pyruvate is converted into acetyl-CoA in the mitochondria, entering the citric acid cycle.
Anaerobic Conditions:
Pyruvate is reduced to lactate in the cytoplasm (e.g., during intense exercise).
3. Citric Acid Cycle (Krebs Cycle)
Definition: The cyclic oxidation of acetyl-CoA to produce energy intermediates.
Location: Mitochondria.
Steps:
Acetyl-CoA combines with oxaloacetate to form citrate.
Citrate undergoes a series of reactions, regenerating oxaloacetate.
Products per Acetyl-CoA:
1 ATP (or GTP).
3 NADH.
1 FADH₂.
CO₂ as a byproduct.
4. Electron Transport Chain (ETC) and Oxidative Phosphorylation
Definition: The transfer of electrons from NADH and FADH₂ to oxygen to generate ATP.
Location: Inner mitochondrial membrane.
Steps:
Electrons from NADH and FADH₂ pass through protein complexes.
The energy released pumps protons (H⁺) into the intermembrane space, creating a proton gradient.
ATP synthase uses this gradient to synthesize ATP.
ATP Yield:
1 NADH = ~2.5 ATP.
1 FADH₂ = ~1.5 ATP.
Final Electron Acceptor: Oxygen, which forms water.
5. Glycogenesis
Definition: The conversion of glucose into glycogen for storage.
Location: Liver and muscle cells.
Enzyme: Glycogen synthase.
Purpose: To store excess glucose for later use.
6. Glycogenolysis
Definition: The breakdown of glycogen into glucose.
Location: Liver and muscle cells.
Enzymes: Glycogen phosphorylase and debranching enzymes.
Purpose: To provide glucose during fasting or energy demand.
7. Gluconeogenesis
Definition: The synthesis of glucose from non-carbohydrate sources like amino acids, glycerol, and lactate.
Location: Liver (primary site) and kidney (minor site).
Enzymes: Pyruvate carboxylase, phosphoenolpyruvate carboxykinase (PEPCK), etc.
Purpose: Maintains blood glucose levels during prolonged fasting or starvation.
8. Pentose Phosphate Pathway (PPP)
Definition: A parallel pathway to glycolysis that produces NADPH and ribose-5-phosphate.
Location: Cytoplasm.
Purpose:
NADPH: Used in fatty acid synthesis and antioxidant defense.
Ribose-5-phosphate: Used for nucleotide synthesis.
Summary of Key Pathways
Pathway
Purpose
Location
Glycolysis
Breaks down glucose into pyruvate
Cytoplasm
Krebs Cycle
Generates energy intermediates
Mitochondria
ETC
Produces ATP
Inner mitochondrial membrane
Glycogenesis
Glucose storage as glycogen
Liver and muscle
Glycogenolysis
Glycogen breakdown to glucose
Liver and muscle
Gluconeogenesis
Glucose synthesis
Liver, kidney
PPP
Produces NADPH and ribose
Cytoplasm
Energy Yield from Glucose
Step
ATP Yield
Glycolysis
2 ATP (net) + 2 NADH (~5 ATP)
Pyruvate to Acetyl-CoA
2 NADH (~5 ATP)
Citric Acid Cycle
2 ATP/GTP + 6 NADH (~15 ATP) + 2 FADH₂ (~3 ATP)
Total
~30–32 ATP per glucose molecule
Clinical Relevance
Diabetes Mellitus:
Impaired glucose uptake leads to high blood sugar and reliance on alternative energy sources.
Excessive lactate production due to anaerobic glycolysis.
Hypoglycemia:
Low blood glucose levels caused by impaired gluconeogenesis or glycogenolysis.
fat and proteins
Metabolism of Fats and Proteins
The metabolism of fats (lipids) and proteins involves complex biochemical processes that provide energy, build structural components, and maintain various bodily functions.
Fat Metabolism
Fats are a major source of energy, especially during fasting or prolonged exercise. They are stored as triglycerides in adipose tissue and are broken down when energy is needed.
Key Steps in Fat Metabolism
Digestion and Absorption:
Fats are emulsified by bile salts in the small intestine.
Lipase enzymes break triglycerides into fatty acids and monoglycerides.
These products are absorbed into intestinal cells, reassembled into triglycerides, and packaged into chylomicrons for transport via the lymphatic system.
Lipolysis:
Definition: Breakdown of triglycerides into glycerol and free fatty acids.
Location: Adipose tissue.
Enzyme: Hormone-sensitive lipase (HSL), activated by hormones like glucagon and epinephrine.
Beta-Oxidation:
Definition: The breakdown of fatty acids into acetyl-CoA.
Location: Mitochondria.
Steps:
Fatty acids are transported into mitochondria by the carnitine shuttle.
They are oxidized in cycles, producing acetyl-CoA, NADH, and FADH₂.
Energy Yield:
1 molecule of palmitic acid (16 carbons) generates ~106 ATP.
Ketogenesis:
Definition: Conversion of acetyl-CoA into ketone bodies (acetoacetate, beta-hydroxybutyrate, acetone).
Location: Liver.
Purpose: Provides an alternative energy source during fasting or carbohydrate depletion.
Lipogenesis:
Definition: Synthesis of fatty acids from acetyl-CoA.
Location: Liver and adipose tissue.
Purpose: Stores excess energy as fat.
Functions of Fats:
Energy Source: Fats provide ~9 kcal/g, the most energy-dense macronutrient.
Structural Role: Essential components of cell membranes (phospholipids).
Hormone Synthesis: Precursors for steroid hormones.
Insulation and Protection: Adipose tissue cushions organs and regulates body temperature.
Protein Metabolism
Proteins are primarily used for growth, repair, and enzyme production. They serve as an energy source only during prolonged fasting or starvation.
Key Steps in Protein Metabolism
Digestion and Absorption:
Proteins are broken down into amino acids by:
Pepsin in the stomach.
Proteases (trypsin, chymotrypsin) in the small intestine.
Amino acids are absorbed into the bloodstream and transported to the liver.
Amino Acid Utilization:
Protein Synthesis:
Amino acids are used to build structural proteins, enzymes, hormones, and other molecules.
Energy Production:
During starvation, amino acids are deaminated to produce energy.
Deamination:
Definition: Removal of the amino group (-NH₂) from amino acids.
Location: Liver.
Byproduct: Ammonia, which is converted to urea in the urea cycle for excretion by the kidneys.
Gluconeogenesis:
Amino acids, especially alanine and glutamine, are converted to glucose during fasting.
Transamination:
Definition: Transfer of an amino group to a keto acid to form a new amino acid.
Purpose: Allows the synthesis of non-essential amino acids.
Ketogenic and Glucogenic Amino Acids:
Ketogenic Amino Acids: Converted into acetyl-CoA or ketone bodies (e.g., leucine, lysine).
Glucogenic Amino Acids: Converted into glucose (e.g., alanine, serine).