Effects of Exposure to Mountain Altitudes on Haematocrit and Consequences on the Heart and Circulation

In the first days of exposure to moderate or high altitude, the circulating blood volume decreases by 11% due to leakage of liquids from the vascular bed, loss of water (increased ventilation and perspiration, increased urinary loss) and reduced water intake. In the same time, erythropoietin (EPO) production increases; it results in progressive production of reticulocytes (and erythrocytes) and increase of haematocrit, with changes that are parallel to the altitude of residence. Blood viscosity augments, leading to increasing peripheral and pulmonary resistances (partially counteracted by a certain degree of vasodilation) and possible increase in diastolic pressure. When persons reside chronically at high altitudes, a possible excessive increase of haemoglobin concentration may occur that is accompanied by a progressive reduction of Oxygen extraction, an altered distribution of pulmonary blood flow and possible appearance of signs of chronic mountain sickness. If high-altitude residents relocate at sea level for a relatively prolonged period, the abnormalities reduce progressively.