SN7
SHORT NOTE
5M
★ High-yield
Describe the physiological changes that occur during acclimatisation to high altitude.
Definition
Acclimatisation = the adaptive physiological changes by which a person becomes able to tolerate the hypoxia of high altitude.
- At 5500 m, barometric pressure is half that at sea level → inspired PO2 halved
- Fractional O2 concentration remains 21% — only the pressure falls
Immediate Changes (minutes–hours)
- Hyperventilation — most important adaptation
– ↓ PaO2 < 60 mmHg → stimulates peripheral chemoreceptors
– Washes out CO2 → respiratory alkalosis → restrains further hyperventilation
- Tachycardia and ↑ cardiac output
Later Changes (days–weeks)
| Change | Mechanism | Time course |
|---|---|---|
| Renal compensation | HCO3− excreted in urine → corrects alkalosis → permits fuller hyperventilation | 2–3 days (rate-limiting step) |
| Polycythaemia | Hypoxia → renal erythropoietin → ↑ RBC and Hb → ↑ O2 capacity | Weeks |
| ↑ 2,3-BPG | Shifts O2 curve right → easier tissue unloading | Days |
| Tissue changes | ↑ capillary density, ↑ myoglobin, ↑ mitochondria and oxidative enzymes | Weeks–months |
| Pulmonary vasoconstriction | Alveolar hypoxia → ↑ pulmonary artery pressure → RV hypertrophy | Weeks |
Effects of Altitude on Exercise Capacity
- Maximum O2 consumption falls to 50% at 5500 m
- Even after full acclimatisation it recovers only partially
- Native highlanders show larger chest, ↑ lung volumes and ↑ diffusing capacity
Applied Aspects
Rapid ascent → Inadequate acclimatisation → Acute Mountain Sickness → HAPE / HACE
- AMS — headache, nausea, insomnia, lassitude
- HAPE — high-altitude pulmonary oedema
- HACE — high-altitude cerebral oedema
- Treatment of all three → immediate descent + O2
- Prophylaxis — acetazolamide (produces mild metabolic acidosis → stimulates ventilation)
- Chronic mountain sickness (Monge's disease) — excessive polycythaemia, pulmonary hypertension, cor pulmonale
All questions in Respiratory System
- Q1Describe the mechanics of breathing. Discuss the muscles involved and the pressure changes occurring during one normal respiratory cycle. Add a note on lung compliance. ★
- Q2Describe the transport of oxygen in the blood. Draw and explain the oxygen–haemoglobin dissociation curve and enumerate the factors that shift it. ★
- Q3Describe the nervous and chemical regulation of respiration. Add a note on the Hering–Breuer reflex. ★
- Q4Describe the transport of carbon dioxide in the blood. Explain the chloride shift and the Haldane effect. ★
- Q5Define the ventilation–perfusion ratio. Describe its regional variation in the upright lung and discuss the consequences of V/Q mismatch. ★
- SN1Write a short note on pulmonary surfactant. ★
- SN2Define lung volumes and capacities. Give their normal values and state which of them cannot be measured by spirometry. ★
- SN3Classify hypoxia. Give the causes and effects of each type. ★
- SN4Write a short note on cyanosis.
- SN5Write a short note on Cheyne–Stokes breathing.
- SN6Define dead space. Describe its types and explain its significance in alveolar ventilation. ★
- SN7Describe the physiological changes that occur during acclimatisation to high altitude. ★