Acclimatization to Low PO2
A person remaining at high altitudes for days, weeks, or years becomes more and more acclimatized to the low PO2, so that it causes fewer deleterious effects on the body. And it becomes possible for the person to work harder without hypoxic effects or to ascend to still higher altitudes.
The principal means by which acclimatization comes about are (1) a great increase in pulmonary ventila-tion, (2) increased numbers of red blood cells, (3) increased diffusing capacity of the lungs, (4) increased vascularity of the peripheral tissues, and (5) increased ability of the tissue cells to use oxygen despite low PO2.
Increased Pulmonary Ventilation—Role of Arterial Chemore- ceptors. Immediate exposure to low PO2stimulates thearterial chemoreceptors, and this increases alveolar ventilation to a maximum of about 1.65 times normal. Therefore, compensation occurs within seconds for the high altitude, and it alone allows the person to rise several thousand feet higher than would be possible without the increased ventilation. Then, if the person remains at very high altitude for several days, the chemoreceptors increase ventilation still more, up to about five times normal.
The immediate increase in pulmonary ventilation on rising to a high altitude blows off large quantities of carbon dioxide, reducing the PCO2 and increasing the pH of the body fluids. These changes inhibit the brain stem respiratory center and thereby oppose the effectof low PO2 to stimulate respiration by way of the peripheral arterial chemoreceptors in the carotid and aortic bodies. But during the ensuing 2 to 5 days, thisinhibition fades away, allowing the respiratory center to respond with full force to the peripheral chemore-ceptor stimulus from hypoxia, and ventilation in-creases to about five times normal.
The cause of this fading inhibition is believed to be mainly a reduction of bicarbonate ion concentration in the cerebrospinal fluid as well as in the brain tissues. This in turn decreases the pH in the fluids surround-ing the chemosensitive neurons of the respiratory center, thus increasing the respiratory stimulatory activity of the center.
An important mechanism for the gradual decrease in bicarbonate concentration is compensation by the kidneys for the respiratory alkalosis. The kidneys respond to decreased PCO2 by reducing hydrogen ion secretion and increasing bicarbonate excretion. This metabolic compensation for the respiratory alkalosis gradually reduces plasma and cerebrospinal fluid bicarbonate concentration and pH toward normal and removes part of the inhibitory effect on respiration of low hydrogen ion concentra-tion. Thus, the respiratory centers are much more responsive to the peripheral chemoreceptor stimulus caused by the hypoxia after the kidneys compensate for the alkalosis.
Increase in Red Blood Cells and Hemoglobin Concentration During Acclimatization. As discussed, hypoxia is the principal stimulus for causing an increase in red blood cell production. Ordinarily, when a person remains exposed to low oxygen for weeks at a time, the hematocrit rises slowly from a normal value of 40 to 45 to an average of about 60, with an average increase in whole blood hemoglobin concentration from normal of 15 g/dl to about 20 g/dl.
In addition, the blood volume also increases, often by 20 to 30 per cent, and this increase times the increased blood hemoglobin concentration gives an increase in total body hemoglobin of 50 or more per cent.
Increased Diffusing Capacity After Acclimatization. It willbe recalled that the normal diffusing capacity for oxygen through the pulmonary membrane is about 21 ml/mm Hg/min, and this diffusing capacity can increase as much as threefold during exercise. A similar increase in diffusing capacity occurs at high altitude.
Part of the increase results from increased pul-monary capillary blood volume, which expands the capillaries and increases the surface area through which oxygen can diffuse into the blood. Another part results from an increase in lung air volume, which expands the surface area of the alveolar-capillary interface still more. A final part results from an increase in pulmonary arterial blood pressure; this forces blood into greater numbers of alveolar capil-laries than normally—especially in the upper parts of the lungs, which are poorly perfused under usual conditions.
Peripheral Circulatory System Changes During Acclimatiza-tion—Increased Tissue Capillarity. The cardiac outputoften increases as much as 30 per cent immediately after a person ascends to high altitude but then decreases back toward normal over a period of weeksas the blood hematocrit increases, so that the amountof oxygen transported to the peripheral body tissues remains about normal.
Another circulatory adaptation is growth ofincreased numbers of systemic circulatory capillaries in the nonpulmonary tissues, which is called in-creased tissue capillarity (or angiogenesis). This occurs especially in animals born and bred at high altitudes but less so in animals that later in life become exposed to high altitude.
In active tissues exposed to chronic hypoxia, the increase in capillarity is especially marked. For instance, capillary density in right ventricular muscle increases markedly because of the combined effects of hypoxia and excess workload on the right ventricle caused by pulmonary hypertension at high altitude.
Cellular Acclimatization. In animals native to altitudes of13,000 to 17,000 feet, cell mitochondria and cellular oxidative enzyme systems are slightly more plentiful than in sea-level inhabitants. Therefore, it is presumed that the tissue cells of high altitude–acclimatized human beings also can use oxygen more effectively than can their sea-level counterparts.
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