Opinions differ as to whether cardiac output is initially elevated or unchanged. The second phase of adaptation deals more with the body's adaptations to increase the oxygen transport capacity of the blood. Changes in ventilatory, cardiovascular, and hematological variables occur. Cardiac output is reduced when compared to sea-level variables several days to weeks after exposure primarily through a reduced stroke volume. Once depressed, cardiovascular function remains depressed and does not recover with increasing length of exposure.
Hematological adjustments occur to augment the oxygen content of the blood.
Hemoglobin concentration increases as the plasma volume decreases. RBCs increase due to a hypoxic-sensitive erythropoietic mechanism that alters HB and hematocrit values. The greater circulating volume of Hb increases the oxygen-carrying capacity of the blood. The pH changes, and other biochemical changes occur as an adjustment to altered internal conditions. The changes that occur raise the question of whether the altered conditions would increase sea-level performance because of the supranormal condition.