a few hours up to two or three days after ascension to a high altitude. There exist two cases: acute cerebral edema and acute pulmonary edema. The first one is caused by the vasodilatation of the cerebral blood vessels produced by the hypoxia; the second one is caused by the vasoconstriction of the pulmonary arterioles, caused by the hypoxia. Adaptation to low oxygen environments Hypoxia is the principal stimulus that increases the number of erythrocytes, increasing the hematocrit from 40 up to 60%, with an increase of the hemoglobin concentration in blood from 15 g/dl up to 20–21 g/dl. Also the blood volume increases 20% producing an increase of the corporal hemoglobin up 15% or more. A person that stays for a period of time at higher altitudes acclimates, producing fewer effects over the human body. There are several mechanisms that help with acclimation, which are an increase of pulmonary ventilation, higher erythrocytes levels, increase of the pulmonary diffusion capacity and increase of the vascularization of the peripheral tissues. Arterial chemical receptors are stimulated by exposure to a low partial pressure and hence increase alveolar ventilation, up to a maximum of 1.65 times. Almost immediately, compensation for the higher altitude begins with an increase of pulmonary ventilation eliminating a large amount CO2. Carbon dioxide partial pressure reduces and corporal fluids pH increase. These actions inhibit the respiratory center of the encephalic trunk, but later this inhibition disappears and the respiratory center responds to the stimulation of the peripheral chemical receptors because of the hypoxia increasing ventilation up to six times. Cardiac output increases up to 30% after a person rises to a high altitude, but it will decrease back to normal levels, depending on the increase of the hematocrit. The quantity of oxygen that goes to the peripheral tissues its relatively normal. Also a disease called "angiogenia" appears. The kidneys respond to low carbon dioxide partial pressure by decreasing the secretion of hydrogen ions, and increasing the excretion of bicarbonate. This respiratory alkalosis reduces the concentration of HCO3 and return plasma pH to normal levels. The respiratory center responds to the stimulation of the peripheral chemical receptors produced by the hypoxia after the kidneys have recover the alkalosis. References External links Airman Education Programs: Aerospace Physiology Training FAA: Aircrew Health and Safety Videos Aviation medicine Physiology Physiology