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
Breathing (pulmonary ventilation) is the movement of air in and out of the lungs by changing thoracic volume and intrapulmonary pressure (Boyle’s law).
Inspiration (active)
- Diaphragm contracts and flattens
- External intercostals lift the ribs
- Thoracic volume ↑ → intrapulmonary pressure ↓
- Air flows IN
Expiration (passive at rest)
- Muscles relax
- Elastic recoil of the lungs
- Thoracic volume ↓ → pressure ↑
- Air flows OUT
Pressures
- Intrapleural pressure — always negative (−4 to −6 mmHg)
- Intrapulmonary pressure — fluctuates with breathing
- Negative intrapleural pressure prevents lung collapse
Enlarging the thorax lowers pressure so air flows in; recoil pushes it out. Phase Muscles Pressure Air Inspiration Diaphragm, ext. intercostals ↓ In Expiration Passive recoil ↑ Out Applied
- Pneumothorax → lung collapse (lost negative pressure)
- Forced expiration uses abdominal + internal intercostals
🔑KEY POINTS TO REMEMBER- Breathing by changing thoracic volume (Boyle’s law).
- Inspiration active (diaphragm); quiet expiration passive (recoil).
- Intrapleural pressure always negative → keeps lungs expanded.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Lung volumes are amounts of air moved during breathing; capacities are sums of two or more volumes, measured by spirometry.
Volumes
- Tidal volume (TV) ~500 mL
- Inspiratory reserve (IRV) ~3000 mL
- Expiratory reserve (ERV) ~1100 mL
- Residual volume (RV) ~1200 mL
Capacities
- Inspiratory capacity = TV + IRV
- Functional residual capacity = ERV + RV
- Vital capacity = TV + IRV + ERV (~4600 mL)
- Total lung capacity = VC + RV (~5800 mL)
A capacity is simply the sum of two or more volumes. Term Value (mL) Tidal volume 500 Vital capacity 4600 Residual volume 1200 Total lung capacity 5800 Applied (Obstructive vs Restrictive)
- Obstructive (asthma, COPD) → ↓ FEV1/FVC
- Restrictive (fibrosis) → ↓ all volumes, ratio preserved
- RV cannot be measured by spirometry
🔑KEY POINTS TO REMEMBER- Volumes: TV, IRV, ERV, RV; capacities are sums (IC, FRC, VC, TLC).
- VC = TV+IRV+ERV; TLC = VC+RV.
- Obstructive → ↓ FEV1/FVC; restrictive → ↓ volumes.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Oxygen and carbon dioxide are carried in combined and dissolved forms: O₂ mainly on haemoglobin, CO₂ mainly as bicarbonate.
Oxygen Transport
- ~98% bound to haemoglobin (oxyhaemoglobin)
- ~2% dissolved in plasma
- 1 g Hb carries 1.34 mL O₂
Carbon Dioxide Transport
- ~70% as bicarbonate (HCO₃⁻)
- ~23% as carbaminohaemoglobin
- ~7% dissolved
Effects
- Bohr effect — ↑CO₂/H⁺ → O₂ released in tissues
- Haldane effect — O₂ uptake → CO₂ released in lungs
Oxygen rides on haemoglobin; CO₂ travels mostly as bicarbonate. Gas Main form % O₂ Oxyhaemoglobin 98 CO₂ Bicarbonate 70 CO₂ Carbamino-Hb 23 Applied
- CO poisoning — CO binds Hb ~240× more than O₂
- Anaemia → ↓ O₂ carriage
🔑KEY POINTS TO REMEMBER- O₂: 98% on Hb, 2% dissolved (1 g Hb = 1.34 mL O₂).
- CO₂: 70% bicarbonate, 23% carbamino, 7% dissolved.
- Bohr (O₂ release in tissues) & Haldane (CO₂ release in lungs).
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Respiration is controlled by respiratory centres in the brainstem, adjusted by chemical and neural inputs to match ventilation to need.
Respiratory Centres
- Medullary — dorsal (inspiration) & ventral groups
- Pontine — pneumotaxic (limits inspiration), apneustic
Chemical Regulation
- Central chemoreceptors (medulla) — sense CO₂/H⁺ (main drive)
- Peripheral chemoreceptors (carotid, aortic) — sense low O₂
Neural / Reflex
- Hering–Breuer (stretch) reflex
- Voluntary control (cortex)
- Emotion, temperature, pain
Brainstem centres, driven mainly by CO₂, keep blood gases steady. Receptor Senses Role Central CO₂ / H⁺ Main drive Peripheral Low O₂ Backup Applied
- CO₂ is the main respiratory stimulant
- In COPD, the hypoxic drive may dominate
- Sleep apnoea, Cheyne–Stokes breathing
🔑KEY POINTS TO REMEMBER- Centres: medulla (rhythm) + pons (pneumotaxic/apneustic).
- Central chemoreceptors (CO₂/H⁺) = main drive; peripheral (O₂) = backup.
- CO₂ is the chief respiratory stimulant.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Gas exchange is the diffusion of O₂ and CO₂ across the respiratory membrane; efficiency depends on matching ventilation (V) and perfusion (Q).
Diffusion of Gases
- Across the alveolar–capillary (respiratory) membrane
- O₂: alveoli → blood; CO₂: blood → alveoli
- Driven by partial-pressure gradients
- CO₂ diffuses ~20× faster than O₂
Ventilation–Perfusion (V/Q)
- Normal V/Q ~0.8
- V/Q = 0 → shunt (perfusion, no ventilation)
- V/Q = very high → dead space (ventilation, no perfusion)
Exchange works only where ventilation and perfusion meet. V/Q Meaning Example ~0.8 Matched Healthy lung Low (0) Shunt Pneumonia High (very high) Dead space Pulmonary embolism Applied
- V/Q mismatch → hypoxaemia
- PE → dead space; pneumonia → shunt
🔑KEY POINTS TO REMEMBER- Gases diffuse across the respiratory membrane down partial-pressure gradients.
- Normal V/Q ~0.8; shunt (0) vs dead space (very high).
- V/Q mismatch → low blood oxygen.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The oxygen–haemoglobin dissociation curve is a sigmoid (S-shaped) graph relating the partial pressure of O₂ to haemoglobin O₂ saturation.
Significance of Shape
- Flat upper part — loading in lungs (safety margin)
- Steep lower part — unloading in tissues
- P50 ~27 mmHg (50% saturation)
Right Shift (↑ O₂ release)
- ↑ CO₂, ↑ H⁺ (↓ pH), ↑ temperature, ↑ 2,3-BPG
- = Bohr effect (active tissues)
Left Shift (↑ O₂ affinity)
- ↓ CO₂, ↑ pH, ↓ temperature
- Fetal haemoglobin, carbon monoxide
A right shift releases O₂ to active tissues; a left shift holds it. Shift Cause Effect Right ↑CO₂,↑H⁺,↑temp,↑2,3-BPG ↑ O₂ release Left ↓CO₂,↑pH,↓temp,fetal Hb ↑ O₂ affinity Applied
- Right shift helps exercising muscle
- Fetal Hb (left) draws O₂ from the mother
🔑KEY POINTS TO REMEMBER- Sigmoid curve: flat top (loading), steep lower (unloading); P50 ~27.
- Right shift (↑CO₂,↑H⁺,↑temp,↑2,3-BPG) → releases O₂ (Bohr).
- Left shift = fetal Hb, CO → holds O₂.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Surfactant is a phospholipid (mainly dipalmitoyl phosphatidylcholine) secreted by type II alveolar cells that lowers alveolar surface tension.
Functions
- ↓ Surface tension → prevents alveolar collapse
- ↑ Lung compliance (easier to inflate)
- Stabilises alveoli of different sizes (Laplace)
- Keeps alveoli dry
Development
- Appears ~24–28 weeks; adequate by ~35 weeks
- Deficiency → respiratory distress syndrome (RDS)
By lowering surface tension, surfactant keeps alveoli open; its lack causes RDS. Feature Detail Source Type II alveolar cells Main lipid DPPC (lecithin) Action ↓ surface tension Deficiency Neonatal RDS Applied
- L:S ratio >2 = mature fetal lungs
- RDS treated with exogenous surfactant + antenatal steroids
🔑KEY POINTS TO REMEMBER- Surfactant from type II cells; lowers surface tension.
- Prevents alveolar collapse, ↑ compliance.
- Deficiency → neonatal RDS; L:S >2 = mature.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Hypoxia is inadequate oxygen supply to the tissues for their metabolic needs.
Types
- Hypoxic — ↓ O₂ in blood (altitude, lung disease)
- Anaemic — ↓ Hb (anaemia, CO poisoning)
- Stagnant — ↓ blood flow (shock, heart failure)
- Histotoxic — tissue cannot use O₂ (cyanide)
Classify by the step that fails: getting, carrying, delivering or using O₂. Type Cause Blood O₂ Hypoxic Altitude, lung disease Low PO₂ Anaemic Anaemia, CO Low content Stagnant Shock Slow flow Histotoxic Cyanide Unused Features / Applied
- Cyanosis, dyspnoea, confusion
- Brain is most sensitive
- O₂ therapy helps the hypoxic type most
🔑KEY POINTS TO REMEMBER- Hypoxia = too little O₂ for tissue needs.
- Four types: hypoxic, anaemic, stagnant, histotoxic.
- O₂ therapy best for hypoxic type.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Cyanosis is a bluish discolouration of skin and mucous membranes, seen when reduced (deoxygenated) haemoglobin exceeds 5 g/dL.
Types
- Central — lung/heart disease; blue tongue and lips
- Peripheral — slow flow (cold, shock); blue extremities, pink tongue
Key Points
- Needs >5 g/dL reduced haemoglobin
- An anaemic patient may not show cyanosis
- Polycythaemia → cyanosis appears easily
Cyanosis appears once reduced haemoglobin exceeds ~5 g/dL. Feature Central Peripheral Cause Lung/heart Slow flow Tongue Blue Pink Warmth Warm Cold Applied
- Central — congenital heart disease, respiratory failure
- Peripheral — shock, cold exposure
🔑KEY POINTS TO REMEMBER- Cyanosis = bluish colour when reduced Hb >5 g/dL.
- Central (blue tongue) vs peripheral (pink tongue, cold).
- Anaemic patients may not show it.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
The chloride shift (Hamburger phenomenon) is the exchange of Cl⁻ into and HCO₃⁻ out of red cells, allowing CO₂ to be carried as bicarbonate.
Mechanism
- In tissues: CO₂ + H₂O → H₂CO₃ (carbonic anhydrase)
- H₂CO₃ → HCO₃⁻ + H⁺
- HCO₃⁻ diffuses out; Cl⁻ enters to balance charge
- The process reverses in the lungs
Bicarbonate leaves the red cell while chloride enters to keep charge balanced. Significance
- Enables ~70% of CO₂ to travel as bicarbonate
- Maintains electrical neutrality
- Carbonic anhydrase is the key enzyme
Applied
- Basis of CO₂ transport and acid-base balance
Location Ion movement Tissues HCO₃⁻ out, Cl⁻ in Lungs HCO₃⁻ in, Cl⁻ out Key enzyme Carbonic anhydrase 🔑KEY POINTS TO REMEMBER- Chloride shift = HCO₃⁻ out, Cl⁻ in (red cell).
- Lets ~70% of CO₂ travel as bicarbonate.
- Needs carbonic anhydrase; reverses in lungs.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Cheyne–Stokes breathing is a periodic pattern of gradually increasing then decreasing depth of breathing, alternating with apnoea.
Mechanism
- Unstable feedback control of breathing
- Apnoea → CO₂ rises → hyperpnoea
- Hyperpnoea → CO₂ falls → apnoea
- Cycle repeats (waxing–waning)
An over-correcting control system swings between apnoea and hyperpnoea. Causes
- Heart failure (delayed circulation)
- Brain damage / raised intracranial pressure
- High altitude
- Deep sleep (can be normal)
Phase CO₂ Breathing Apnoea Rising Stopped Hyperpnoea Falling Deep, fast Applied
- A poor prognostic sign in heart failure and stroke
🔑KEY POINTS TO REMEMBER- Cheyne–Stokes = cyclical waxing–waning breathing with apnoea.
- Caused by unstable CO₂ feedback.
- Seen in heart failure, brain damage, altitude.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).Definition
Dead space is the part of tidal air that does not take part in gas exchange.
Types
- Anatomical — conducting airways (~150 mL)
- Alveolar — ventilated but unperfused alveoli
- Physiological = anatomical + alveolar
Significance
- Normal physiological ≈ anatomical (~150 mL)
- ↑ in disease (e.g. pulmonary embolism)
- Alveolar ventilation = (TV − dead space) × respiratory rate
Air in the airways and unperfused alveoli does no exchange. Type Volume / cause Anatomical ~150 mL (airways) Alveolar Unperfused alveoli Physiological Anatomical + alveolar Applied
- Deep slow breathing is more efficient than rapid shallow
- Measured by Bohr’s method
🔑KEY POINTS TO REMEMBER- Dead space = ventilated air that does no exchange.
- Anatomical (~150 mL) + alveolar = physiological.
- Deep slow breaths waste less on dead space.
📚SOURCES: Guyton & Hall Textbook of Medical Physiology; Ganong’s Review of Medical Physiology; Textbook of Physiology (A.K. Jain).