Physiology
Medical physiology for MBBS: cell and body fluids, blood, nerve and muscle, and the cardiovascular, respiratory, gastrointestinal, renal, endocrine, reproductive and nervous systems, special senses and integrative physiology.
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Definition
Gastric juice is the stomach’s secretion (~2–3 L/day) containing HCl, pepsinogen, mucus and intrinsic factor; its secretion occurs in three phases.
Components & Functions
- HCl (parietal cells) — activates pepsin, kills germs
- Pepsinogen (chief cells) → pepsin (protein digestion)
- Mucus (mucous cells) — protects the lining
- Intrinsic factor — for vitamin B12 absorption
Phases of Secretion
- Cephalic — sight/smell/thought → vagus
- Gastric — food distension → gastrin (largest phase)
- Intestinal — chyme in duodenum (first +, then −)
Secretion is turned up over three phases, then switched off in the intestine. Cell Secretion Parietal HCl, intrinsic factor Chief Pepsinogen Mucous Mucus G cell Gastrin Applied
- Peptic ulcer — ↑ acid, H. pylori, NSAIDs
- PPIs (omeprazole) block acid
- Pernicious anaemia — no intrinsic factor
🔑KEY POINTS TO REMEMBER- Gastric juice: HCl (parietal), pepsinogen (chief), mucus, intrinsic factor.
- Phases: cephalic (vagus), gastric (gastrin, largest), intestinal.
- Applied: ulcers (H. pylori/NSAIDs), PPIs, pernicious anaemia.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Pancreatic juice (enzymes + bicarbonate) and bile are digestive secretions poured into the duodenum, controlled by the hormones secretin and CCK.
Pancreatic Juice
- Enzymes — amylase, lipase, trypsin, chymotrypsin
- Bicarbonate — neutralises gastric acid
- Alkaline (pH ~8)
Hormonal Control
- Secretin (acid) → watery bicarbonate juice
- CCK (fat/protein) → enzyme-rich juice + gallbladder contraction
Bile
- Made by the liver; stored and concentrated in the gallbladder
- Bile salts → emulsify fat
- Bile pigments (bilirubin) — excreted
Secretin neutralises acid; CCK delivers enzymes and bile. Hormone Stimulus Action Secretin Acid Bicarbonate juice CCK Fat/protein Enzymes, bile Applied
- Pancreatitis — enzyme autodigestion
- Gallstones → obstructive jaundice
🔑KEY POINTS TO REMEMBER- Pancreatic juice = enzymes + bicarbonate (alkaline).
- Secretin → bicarbonate; CCK → enzymes + bile.
- Bile salts emulsify fat.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Digestion breaks food into absorbable units; absorption is their uptake into blood/lymph, mainly in the small intestine.
Digestion of Foods
- Carbohydrates → monosaccharides (glucose)
- Proteins → amino acids
- Fats → fatty acids + glycerol
Sites & Absorption
- Small intestine — main site (villi, microvilli)
- Glucose, amino acids → blood (portal vein)
- Fats → lymph (lacteals)
- Large intestine — water and electrolytes
Foods are cut to building blocks and absorbed — sugars/AAs to blood, fats to lymph. Food End product Absorbed into Carbohydrate Glucose Blood Protein Amino acids Blood Fat Fatty acids Lymph Applied
- Malabsorption — coeliac disease, pancreatic insufficiency
- Lactose intolerance
🔑KEY POINTS TO REMEMBER- Carbs → glucose, proteins → amino acids, fats → fatty acids.
- Small intestine = main absorptive site.
- Sugars/AAs → blood; fats → lymph; colon → water.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Gastrointestinal motility is the movement of the gut wall that mixes and propels food, controlled largely by the enteric nervous system.
Types of Movement
- Peristalsis — propels food forward
- Segmentation — mixes food with juices
- Migrating motor complex — housekeeper wave (between meals)
- Mass movement — colon
Control
- Enteric nervous system (the ‘gut brain’)
- Autonomic — parasympathetic ↑, sympathetic ↓
- Gut hormones
Peristalsis moves food along; segmentation mixes it. Movement Function Peristalsis Propulsion Segmentation Mixing MMC Clears residue Mass movement Colon emptying Applied
- ↑ Motility → diarrhoea
- ↓ Motility → constipation, paralytic ileus
🔑KEY POINTS TO REMEMBER- Movements: peristalsis (propel), segmentation (mix), MMC, mass movement.
- Controlled by the enteric nervous system + autonomic + hormones.
- ↑ motility → diarrhoea; ↓ → constipation.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The liver is the largest gland and the body’s metabolic factory, with metabolic, storage, synthetic, detoxifying and excretory functions.
Metabolic Functions
- Carbohydrate — glycogen storage, gluconeogenesis
- Protein — deamination, urea formation
- Fat — β-oxidation, cholesterol synthesis
Other Functions
- Storage — glycogen, iron, vitamins A, D, B12
- Synthesis — albumin, clotting factors, bile
- Detoxification — drugs, ammonia → urea
- Excretion — bilirubin in bile
The liver is a factory and warehouse: metabolises, stores, synthesises, detoxifies, excretes. Function Example Metabolic Glucose / glycogen Synthetic Albumin, clotting factors Detox Ammonia → urea Excretory Bilirubin Applied
- Liver failure → jaundice, bleeding, encephalopathy
- ↓ Albumin → oedema and ascites
🔑KEY POINTS TO REMEMBER- Liver: metabolic, storage, synthetic, detox, excretory.
- Makes albumin & clotting factors; converts ammonia → urea.
- Failure → jaundice, bleeding, encephalopathy.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Saliva is the secretion of the salivary glands (~1–1.5 L/day) — a watery fluid with mucus, salivary amylase, lysozyme and electrolytes.
Composition
- 99% water
- Salivary amylase (ptyalin), lingual lipase
- Mucus, lysozyme, IgA
- Electrolytes, bicarbonate
Functions
- Moistens food, forms a bolus
- Starts starch digestion (amylase)
- Protects the mouth (lysozyme, buffers)
- Aids taste and speech
Saliva begins digestion and protects the mouth and teeth. Component Role Amylase Starch digestion Lysozyme Antibacterial Mucus Lubrication Bicarbonate Buffer Applied
- Xerostomia (dry mouth) → dental caries
- Controlled by both autonomic divisions
🔑KEY POINTS TO REMEMBER- Saliva 1–1.5 L/day; 99% water.
- Amylase (starch), lysozyme (antibacterial), mucus, buffers.
- Dry mouth → caries.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Gastrointestinal hormones are secreted by the gut mucosa to coordinate the secretions and motility of digestion.
Main Hormones
- Gastrin (G cells) → ↑ acid
- Secretin (S cells) → bicarbonate
- CCK (I cells) → enzymes, bile, gallbladder contraction
- GIP → ↑ insulin, ↓ gastric activity
Each hormone readies the next step of digestion. Hormone Source Action Gastrin Stomach ↑ acid Secretin Duodenum Bicarbonate CCK Duodenum Enzymes, bile GIP Duodenum Insulin Applied
- Zollinger–Ellison — gastrin-secreting tumour → ulcers
- Incretins (GIP, GLP-1) → basis of diabetes therapy
🔑KEY POINTS TO REMEMBER- Gastrin → acid; secretin → bicarbonate.
- CCK → enzymes + bile; GIP → insulin.
- Zollinger–Ellison = gastrinoma.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Bile salts (made from cholesterol) emulsify fat; the enterohepatic circulation recycles them between liver and intestine.
Functions of Bile Salts
- Emulsify fat (↑ surface area for lipase)
- Form micelles → absorb fats & fat-soluble vitamins
- Aid cholesterol excretion
Enterohepatic Circulation
- Liver secretes bile salts
- Used in the intestine for fat digestion
- Reabsorbed in the terminal ileum
- Returned to the liver → reused (6–8×/day)
Bile salts are recycled through liver and gut, so a small pool serves the whole meal. Step Location Synthesis Liver Action Small intestine Reabsorption Terminal ileum Applied
- Terminal ileum disease → fat malabsorption, gallstones
- Bile-acid binders lower cholesterol
🔑KEY POINTS TO REMEMBER- Bile salts emulsify fat and form micelles.
- Enterohepatic: liver → gut → ileum → liver (recycled).
- Ileal disease → fat malabsorption.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Deglutition (swallowing) is the coordinated movement of food from mouth to stomach, in three phases, which also protects the airway.
Phases
- Oral (voluntary) — tongue pushes the bolus back
- Pharyngeal (reflex) — airway sealed, epiglottis closes
- Oesophageal — peristalsis carries it to the stomach
Swallowing moves through oral, pharyngeal and oesophageal phases, protecting the airway. Phase Control Event Oral Voluntary Bolus to pharynx Pharyngeal Reflex Airway protected Oesophageal Involuntary Peristalsis Applied
- Dysphagia — stroke, obstruction
- Aspiration risk if the reflex is impaired
🔑KEY POINTS TO REMEMBER- Three phases: oral (voluntary), pharyngeal (reflex), oesophageal.
- Pharyngeal phase protects the airway (epiglottis).
- Dysphagia → aspiration risk.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
A peptic ulcer is a break in the mucosa of the stomach or duodenum due to imbalance between acid-pepsin and mucosal defence.
Causes (Aggressive vs Defensive)
- Aggressive — acid, pepsin, H. pylori, NSAIDs
- Defensive — mucus, bicarbonate, blood flow, prostaglandins
- Ulcer forms when aggressive > defensive
Features & Complications
- Burning epigastric pain (related to meals)
- Bleeding, perforation, obstruction
An ulcer forms when acid attack outweighs mucosal defence. Type Pain with food Gastric ulcer Worse Duodenal ulcer Relieved Treatment
- PPIs, H2 blockers
- H. pylori eradication (triple therapy)
- Avoid NSAIDs
🔑KEY POINTS TO REMEMBER- Peptic ulcer = acid-pepsin vs defence imbalance.
- Main causes: H. pylori, NSAIDs.
- Gastric ulcer worse with food; duodenal relieved.
- Treat: PPI + H. pylori eradication.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Vomiting (emesis) is the forceful expulsion of gastric contents through the mouth — a protective reflex coordinated by the vomiting centre in the medulla.
Mechanism
- Triggers → vomiting centre (medulla)
- CTZ (area postrema) senses toxins/drugs
- Deep breath, glottis closes
- Diaphragm + abdominal muscles contract → expulsion
Triggers
- Gastric irritation
- Drugs/toxins (via CTZ)
- Motion (vestibular)
- Raised intracranial pressure, emotion
Several triggers feed one vomiting centre, which drives expulsion. Trigger Pathway Toxins/drugs CTZ Motion Vestibular Gut irritation Vagal Applied
- Anti-emetics target different receptors
- Prolonged vomiting → dehydration, metabolic alkalosis
🔑KEY POINTS TO REMEMBER- Vomiting centre in medulla; CTZ senses toxins.
- Triggers: gut, drugs, motion, raised ICP.
- Prolonged → dehydration + alkalosis.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Vitamin B12 absorption needs intrinsic factor (from gastric parietal cells) and occurs in the terminal ileum.
Steps
- B12 in food bound to R-protein
- Freed by pancreatic enzymes
- Binds intrinsic factor (parietal cells)
- B12–IF complex absorbed in the terminal ileum
Causes of Deficiency
- Pernicious anaemia (no intrinsic factor)
- Gastrectomy
- Ileal disease or resection
- Strict vegan diet
B12 needs intrinsic factor and an intact ileum — either failing causes deficiency. Requirement Source Intrinsic factor Stomach (parietal cells) Absorption site Terminal ileum Applied
- Deficiency → megaloblastic anaemia + neuropathy
🔑KEY POINTS TO REMEMBER- B12 needs intrinsic factor (parietal cells) + terminal ileum.
- Deficiency: pernicious anaemia, gastrectomy, ileal disease, vegan.
- → Megaloblastic anaemia + neuropathy.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).