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
The nephron is the functional unit of the kidney (~1 million per kidney); glomerular filtration is the first step of urine formation, forming a protein-free filtrate.
Parts of the Nephron
- Glomerulus + Bowman’s capsule — filtration
- Proximal convoluted tubule — bulk reabsorption
- Loop of Henle — concentration
- Distal tubule & collecting duct — fine adjustment
Glomerular Filtration
- Filtration across the glomerular membrane
- Driven by glomerular hydrostatic pressure
- Opposed by colloid osmotic + capsular pressure
- Net filtration pressure ~10 mmHg
- GFR ~125 mL/min (~180 L/day)
Filtration Membrane (3 layers)
- Capillary endothelium (fenestrated)
- Basement membrane
- Podocytes (filtration slits)
Filtrate flows from glomerulus through the tubule, being modified into urine. Pressure Value Effect Glomerular hydrostatic ~60 mmHg Favours filtration Colloid osmotic ~32 mmHg Opposes Capsular ~18 mmHg Opposes Net ~10 mmHg Filtration Applied
- GFR measured by inulin / creatinine clearance
- ↓ GFR in renal failure
- Proteinuria → filtration membrane damage
🔑KEY POINTS TO REMEMBER- Nephron = functional unit; filtration → reabsorption → secretion.
- GFR ~125 mL/min; net filtration pressure ~10 mmHg.
- Membrane: endothelium, basement membrane, podocytes.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Tubular reabsorption returns useful substances from filtrate to blood; tubular secretion adds wastes from blood into the filtrate.
Reabsorption — Sites
- PCT — 65% (glucose, amino acids, Na⁺, water)
- Loop of Henle — Na⁺, Cl⁻, water
- DCT — Na⁺ (aldosterone), Ca²⁺ (PTH)
- Collecting duct — water (ADH)
Mechanisms
- Active — Na⁺–K⁺ pump, glucose cotransport
- Passive — water, urea, Cl⁻
- Glucose fully reabsorbed unless > renal threshold
Tubular Secretion
- H⁺, K⁺, ammonia, drugs (e.g. penicillin)
- Regulates pH and K⁺
- Removes substances not filtered
The tubule reclaims useful solutes and adds wastes, refining the filtrate into urine. Segment Reabsorbs Regulated by PCT 65%, glucose, AA — Loop Na, Cl, water — DCT Na, Ca Aldosterone, PTH Collecting Water ADH Applied
- Diuretics act on specific tubular segments
- Glycosuria if plasma glucose > 180 mg/dL
🔑KEY POINTS TO REMEMBER- PCT reabsorbs 65% (glucose, AA, Na⁺, water).
- DCT/collecting duct fine-tuned by aldosterone & ADH.
- Secretion handles H⁺, K⁺, drugs.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The counter-current mechanism concentrates urine using the loop of Henle (multiplier) and vasa recta (exchanger) to build a medullary osmotic gradient.
Counter-current Multiplier (Loop of Henle)
- Descending limb — permeable to water (water out)
- Ascending limb — pumps out Na⁺/Cl⁻, impermeable to water
- Creates a hyperosmotic medulla (up to 1200 mOsm/L)
Counter-current Exchanger (Vasa Recta)
- Maintains the medullary gradient
- Prevents washout of solute
Role of ADH
- Makes the collecting duct permeable to water
- Water reabsorbed → concentrated urine
Salt pumped from the ascending limb makes the medulla salty, so ADH can concentrate urine. Limb Permeable to Effect Descending Water Water out Ascending Na/Cl (active) Salt out → hyperosmotic medulla Applied
- Loop diuretics (furosemide) block the ascending limb
- Diabetes insipidus — no ADH → dilute urine
🔑KEY POINTS TO REMEMBER- Loop of Henle = counter-current multiplier; vasa recta = exchanger.
- Ascending limb pumps salt → hyperosmotic medulla (1200 mOsm/L).
- ADH lets the collecting duct concentrate urine.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The kidney regulates body fluid volume, osmolarity and acid-base balance by adjusting water, electrolyte and H⁺/HCO₃⁻ handling.
Fluid & Osmolarity
- ADH — controls water reabsorption
- Aldosterone — Na⁺ (and water) retention
- RAAS — volume and BP control
- Thirst mechanism
Acid-Base Regulation
- Reabsorbs filtered bicarbonate
- Secretes H⁺ (excretes acid)
- Forms new HCO₃⁻
- Buffers: phosphate and ammonia
Role in pH
- Kidney = slow but powerful regulator
- Lungs handle CO₂ (fast); kidney handles fixed acids (slow)
The kidney excretes acid and reclaims bicarbonate to keep blood pH normal. Regulator Controls ADH Water Aldosterone Na⁺ H⁺ secretion Acid excretion HCO₃⁻ Buffer base Applied
- Renal failure → metabolic acidosis
- Renal compensation in respiratory acid-base disorders
🔑KEY POINTS TO REMEMBER- Kidney controls fluid (ADH, aldosterone, RAAS) and pH.
- Acid-base: reabsorb HCO₃⁻, secrete H⁺, buffer with NH₃/phosphate.
- Slow but powerful pH regulator (vs fast lungs).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Micturition is the reflex emptying of the urinary bladder; the kidney also performs excretory, regulatory and endocrine functions.
Functions of the Kidney
- Excretion of wastes (urea, creatinine, drugs)
- Regulation of fluid, electrolytes, pH, BP
- Endocrine — erythropoietin, renin, calcitriol
Micturition Reflex
- Bladder fills → stretch receptors activated
- Afferent → sacral spinal cord (S2–S4)
- Parasympathetic → detrusor contracts, sphincter relaxes
- Voluntary control via the cortex
Bladder filling triggers a sacral reflex that contracts the detrusor to void. Function Example Excretory Urea, creatinine Regulatory Fluid, pH, BP Endocrine EPO, renin, calcitriol Applied
- Spinal injury → automatic (reflex) bladder
- BPH → urinary retention
🔑KEY POINTS TO REMEMBER- Kidney: excretory, regulatory, endocrine.
- Micturition = sacral (S2–S4) parasympathetic reflex + cortical control.
- Detrusor contracts, sphincter relaxes to void.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The renin–angiotensin–aldosterone system (RAAS) is a hormonal system that regulates blood pressure and Na⁺/water balance.
Steps
- ↓ BP/Na⁺ → JG cells release renin
- Renin: angiotensinogen → angiotensin I
- ACE (lungs): angiotensin I → angiotensin II
- Angiotensin II → vasoconstriction + aldosterone
Actions of Angiotensin II
- Vasoconstriction → ↑ BP
- Aldosterone → Na⁺/water retention
- ADH release and thirst
Low BP triggers renin → angiotensin II → vasoconstriction and salt retention → BP rises. Component Action Renin Starts the cascade Angiotensin II Vasoconstriction Aldosterone Na⁺/water retention Applied
- ACE inhibitors / ARBs treat hypertension & heart failure
- Overactivity → hypertension
🔑KEY POINTS TO REMEMBER- RAAS: renin → angiotensin I → (ACE) angiotensin II → aldosterone.
- Angiotensin II: vasoconstriction + Na⁺/water retention → ↑ BP.
- Blocked by ACE inhibitors and ARBs.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Renal clearance is the volume of plasma completely cleared of a substance per minute: Clearance = (U × V) / P.
Formula & Uses
- C = (urine conc × urine flow) / plasma conc
- Inulin clearance = GFR (freely filtered, not reabsorbed/secreted)
- Creatinine clearance ≈ GFR (used clinically)
- PAH clearance = renal plasma flow
Clearance uses urine and plasma concentrations to measure kidney function. Substance Measures Inulin GFR Creatinine GFR (clinical) PAH Renal plasma flow Glucose 0 (fully reabsorbed) Applied
- Assess kidney function
- ↓ Creatinine clearance → renal impairment
🔑KEY POINTS TO REMEMBER- Clearance = (U × V) / P.
- Inulin = GFR; PAH = renal plasma flow; creatinine ≈ GFR clinically.
- Glucose clearance = 0 (fully reabsorbed).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The juxtaglomerular apparatus (JGA) is a specialised structure at the vascular pole of the glomerulus that regulates GFR and renin secretion.
Components
- JG cells (afferent arteriole) — secrete renin
- Macula densa (DCT) — sense Na⁺/Cl⁻
- Mesangial cells — support
Functions
- Secretes renin (drives RAAS)
- Tubuloglomerular feedback — autoregulates GFR
The JGA senses salt and pressure, then releases renin to defend BP and GFR. Component Function JG cells Secrete renin Macula densa Sense Na⁺/Cl⁻ Mesangial cells Support Applied
- Central to blood-pressure regulation
🔑KEY POINTS TO REMEMBER- JGA = JG cells + macula densa + mesangial cells.
- JG cells secrete renin; macula densa senses Na⁺/Cl⁻.
- Provides tubuloglomerular feedback (autoregulates GFR).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Antidiuretic hormone (ADH, vasopressin) is made in the hypothalamus, released from the posterior pituitary, and controls water reabsorption.
Actions
- ↑ Water permeability of the collecting duct (aquaporins)
- → water reabsorbed → concentrated urine
- Vasoconstriction at high concentration
Control of Release
- ↑ Plasma osmolarity (osmoreceptors) — main stimulus
- ↓ Blood volume / BP
- Stimulated by nicotine; inhibited by alcohol
Rising osmolarity triggers ADH, which conserves water and concentrates urine. State ADH Urine High osmolarity ↑ ADH Concentrated Low osmolarity ↓ ADH Dilute Applied
- Diabetes insipidus — ↓ ADH → polyuria, dilute urine
- SIADH — excess ADH → water retention
🔑KEY POINTS TO REMEMBER- ADH from hypothalamus → posterior pituitary.
- Acts on collecting duct (aquaporins) → water reabsorption.
- ↓ ADH = diabetes insipidus; excess = SIADH.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Micturition is the reflex process of emptying the urinary bladder, controlled by a spinal reflex with voluntary override.
Mechanism
- Bladder fills (~300–400 mL) → stretch receptors
- Afferent → sacral cord (S2–S4)
- Parasympathetic → detrusor contracts
- Internal + external sphincters relax
- Voluntary control via cortex
Filling triggers a sacral reflex that empties the bladder, under cortical control. Nerve Action Parasympathetic (S2–S4) Detrusor contracts Sympathetic Storage (relax detrusor) Pudendal (somatic) External sphincter Applied
- Spinal injury → automatic or atonic bladder
- Incontinence and retention
🔑KEY POINTS TO REMEMBER- Micturition = sacral (S2–S4) parasympathetic reflex.
- Detrusor contracts, sphincters relax; cortex gives voluntary control.
- Spinal injury → automatic bladder.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Renal threshold is the plasma concentration above which a substance appears in urine; for glucose it is ~180 mg/dL, above which glycosuria occurs.
Mechanism
- Glucose is normally fully reabsorbed in the PCT
- Transport maximum (Tm) ~375 mg/min
- Plasma glucose > 180 mg/dL → transporters saturated
- Excess glucose spills into urine (glycosuria)
Once glucose transporters saturate, extra glucose is lost in the urine. Term Value Renal threshold (glucose) ~180 mg/dL Tm glucose ~375 mg/min Applied
- Diabetes mellitus → glycosuria
- Renal glycosuria — low threshold with normal blood glucose
🔑KEY POINTS TO REMEMBER- Renal threshold (glucose) ~180 mg/dL; Tm ~375 mg/min.
- Above threshold → glycosuria.
- Seen in diabetes mellitus.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Besides excretion, the kidney is an endocrine organ, producing hormones that regulate blood, blood pressure and calcium.
Hormones Produced
- Erythropoietin → RBC production (in hypoxia)
- Renin → RAAS, BP control
- Calcitriol (active vitamin D) → Ca²⁺ absorption
- Prostaglandins → renal blood flow
The kidney secretes hormones controlling red cells, blood pressure and calcium. Hormone Function Erythropoietin RBC production Renin BP (RAAS) Calcitriol Ca²⁺ absorption Applied
- Renal failure → anaemia (↓ EPO) and renal bone disease (↓ calcitriol)
🔑KEY POINTS TO REMEMBER- Kidney hormones: erythropoietin, renin, calcitriol.
- EPO → RBCs; renin → BP; calcitriol → calcium.
- Renal failure → anaemia + bone disease.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).