Expansion
Immediate tachycardia and hyperventilation, then polycythaemia and pulmonary hypertension
Expansion
Immediate, hours
- Peripheral chemoreceptor stimulation gives hyperventilation
- Tachycardia and increased cardiac output
- Respiratory alkalosis
Days to weeks
- Renal bicarbonate excretion corrects the alkalosis, allowing further hyperventilation
- Rising 2,3-diphosphoglycerate shifts the oxygen dissociation curve right, aiding tissue unloading
- Erythropoietin rises, producing polycythaemia and raised haematocrit
Chronic
- Hypoxic pulmonary vasoconstriction across all lung units raises pulmonary artery pressure, causing right ventricular hypertrophy and eventually cor pulmonale
- Increased capillary density and mitochondrial adaptation in tissue
- Cardiac output returns towards normal as oxygen carrying capacity improves
The polycythaemia is double-edged: it raises oxygen content but also viscosity, and by Poiseuille’s law increases resistance and cardiac work, which is the basis of chronic mountain sickness.