Theory 06/Altitude & Hypoxia Physiology
Adaptation Is a Vascular Ratio
The claim
High-altitude adaptation is set by the ratio of nitric oxide availability to endothelin tone rather than by haemoglobin concentration, so populations with similar haemoglobin can differ in adaptation.
01The mechanism
Hypoxia raises endothelin, which constricts pulmonary vessels, while nitric oxide dilates them. The balance sets pulmonary pressure and therefore exercise capacity.
Haemoglobin raises oxygen content but viscosity rises with it, so the vascular ratio determines whether the gain is realised.
02What this is not
Haemoglobin-centric models
Those focus on oxygen carriage. This focuses on vascular tone, which explains variation at equal haemoglobin.
03Predictions
- P1Capacity should track the vasoactive ratio.
- P2Populations matched on haemoglobin should differ by ratio.
04Refutation
Capacity tracking haemoglobin at controlled ratio would favour the carriage model.