Hypoxia is the main physiological complication when ascending to high altitudes. The percentage of oxygen in the atmosphere stays the same, but the partial pressure and barometric pressure begin to drop on rising to a higher altitude. The level of altitude, rate of ascent, and duration of exposure all determine the body’s response to hypoxia. The physiological responses to high altitude hypoxia are divided into two categories. First there are acute responses known as accommodation, which refers to the immediate reflex adjustments of the respiratory and cardiovascular systems to hypoxia.
Second there are long term responses called acclimatization, which refers to the changes in body tissues in response to long term exposure to hypoxia. While long term responses occur within a cell, the acute physiologic responses are necessary for letting the cells adjust. The most significant acute response to hypoxia is sustained hyperventilation triggered by chemoreceptors in the carotid and aortic bodies. With this prolonged hyperventilation a high alveolar P02 is produced, but with that there is a lowered alveolar PCO2 that results in respiratory alkalosis.
This wants to decrease the ventilation, but renal compensation is activated, secreting bicarbonate and reabsorbing hydrogen which gradually brings the blood PH back to normal and allows further increase in ventilation. This acute hypoxic ventilatory response takes approximately 4 days. In addition to changes in ventilation, cardiovascular or circulatory changes take place that increase oxygen delivery to the tissues. When ascending to high altitude, the sympathetic nervous system is stimulated, leading to an increased resting heart rate, cardiac output, and a mild increase in blood pressure.
This happens by hypoxic pulmonary vasoconstriction, the pulmonary arteries constrict, and redirect the blood flow to alveoli with a higher oxygen capacity. This in turn improves ventilation/perfusion matching and arteriole blood flow, but can be harmful in long term hypoxia. On ascent to high altitude, cerebral blood flow increases instantly, and returns to normal after approximately one week. Upon ascent to high altitude, there are many hematological adjustments to increase the PO2. Within 24 hours of exposure to altitude, the hemoglobin and hematocrit have been shown to increase.
When the kidneys detect hypoxia, the production of erythropoietin stimulates the production of red blood cells, therefore increasing the amount of oxygen that can be carried by blood. Another change occurs within the red blood cell after ascending to a high altitude. There is an increase in 2,3-DPG that is activated by a rise in PH. 2,3-DPG controls the movement of oxygen from red blood cells to tissues. If there is an increase in 2,3-DPG, there is going to be a lower affinity of hemoglobin for oxygen, resulting in oxygen being released by hemoglobin to the tissues alleviating the problems associated with poor oxygenation.
Several systems chronically adapt to altitude changes, known as acclimatization, to allow for improved tolerance and endurance. This process may take anywhere from 2 weeks to several months. Prolonged exposure to altitude will increase pulmonary blood pressure and vascularity resulting in improved pulmonary perfusion. Eventually the resting heart rate returns to normal and there is a decrease in stroke volume because of minimized cardiac filling pressure. Even with adaption to altitude, the hemoglobin, hematocrit, red blood cells, and 2,3-DPG all stay raised, which is probably the most beneficial effect of a prolonged stay at high altitude.
Physiological acute reactions begin with the activation of chemoreceptors sensitive to hypoxia, leading to hyperventilation. The kidney is also involved by detecting low oxygen levels secreting erythropoietin in order to produce more red blood cells. Within these red blood cells there is 2,3-DPG that can increase the amount of oxygen delivered to the tissues. All of these changes allow for adaptation within weeks or months of being exposed to high altitude. It is evident that the body acclimates in a positive way to acute and chronic exposure to altitude, in order to return the body to homeostasis.