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Five Ways In Which Human Body Adapts To Low Oxygen Pressure At High Altitudes
Acclimatization of Human Body To High Altitudes
At sea level, the barometric pressure is 760 mm of mercury (Hg); at 10,000 feet it remains around 523 mm Hg; and at 50,000 feet above the sea level it is only 87 mm Hg.
This decrease in barometric pressure is accompanied by a proportionate reduction in partial pressure of Oxygen in the air, that remains at all times slightly less than 21% of the total barometric pressure.
A person remaining at high altitudes for days, weeks or years becomes more and more acclimatized to the low partial pressure of Oxygen, so that it causes fewer deleterious effects on the body, and also that it becomes possible for the person to work harder or ascend still higher altitudes without suffering from ill effects due to low Oxygen saturation.
The principal means by which acclimatization comes about are as follows:
Increased Ventilation - An Adaptation To Survive At High Altitudes
1. Increased Ventillation
Lungs are like windows to our body that ventilate causing outflow and inflow of air to and from the atmosphere.
At high altitudes, the Oxygen pressure in the atmosphere is much lower than that at the sea level. As a person begins to ascend high altitudes, the resulting hypoxia or decrease in oxygen content is sensed by the chemoreceptors in the brain that result in increased pulmonary ventilation to a maximum of about 65%.
This is an immediate compensation for the high altitude, and this alone allows the person to rise several thousand feet higher than would otherwise be possible.
Then if the person remains at a very high altitude for several days, the ventilation gradually increases to an average of about five times the normal.
The immediate increase in ventilation on rising to a high altitude blows off large quantities of carbon dioxide from the body, increasing the pH of body fluids. This inhibits the respiratory center in the brain. However, during the next 2 to 5 days, with a reduction in blood carbon dioxide levels, the concentration of bicarbonate ions also reduces in the brain. This in turn reduces the pH in the fluids surrounding the respiratory center, thus increasing its activity and improving lung function.
2. Increase in Red Blood Cell Count and Hemoglobin Level at High Altitudes
Lack of Oxygen is an important stimulus for causing an increase in red cell production. As the quantity of Oxygen being transported to various organs reduces drastically, the bone marrow immediately begins to produce large quantities of red blood cells.
In acclimatization to high altitudes, the hematocrit (or the percent of blood that is cells. Thus if a person has a hematocrit of 40, then it means 40% of his blood volume is cells and the remainder is plasma) rises from a normal value of 40-45 to an average of 60-65.
The hemoglobin concentration also increases from a normal of 15 gms/dl to about 22 gm/dl. This adaptation helps to improve the oxygen-carrying capacity of blood.
In addition, the blood volume also increases by as much as 20-30%, resulting in a total increase in circulating hemoglobin of as much as 50-90%.
This improvement in hemoglobin levels is a very slow process and takes many months to show.
Lungs - the seat of Oxygen Uptake from the Atmosphere
3. Increased Diffusing Capacity of Lung Membranes During Acclimatization
Breathing apparatus of our lungs consists of the nose and nasal passages through which the air is inhaled, the trachea or windpipe and its branching bronchi and bronchioles, which are the passages through which air passes into the lungs, and alveoli or air sacs, that are the terminal units in which exchange of oxygen and carbon dioxide occurs with the blood present in capillaries.
With every breath, the air from the atmosphere flows into the lungs. The oxygen from this air diffuses through lung membranes to reach the blood.
The diffusing capacity of lung membranes for oxygen increases at high altitudes. Part of this increase is due to increased blood volume that expands blood capillaries in the lungs and increases the surface area through which oxygen can diffuse into the blood.
Another part results from an increase in the lung volume which expands the surface area of alveolar membranes.
A final part results from an increase in arterial pressure in the lungs, forcing blood into a greater number of lung capillaries than normally, especially in the upper parts of the lungs that are poorly perfused under usual conditions.
4. Increased Capillarity as an Adaptation for High Altitudes
As a person ascends to a high altitude, the heart begins to pump more blood and the cardiac output increases by as much as 30%. But as the red cell count increases, the cardiac output decreases back towards normal and that the amount of oxygen carried to the body organs remains constant.
Another circulatory adaptation is an increase in the number of capillaries in various body organs.
This feature is more marked in individuals who are born and brought up at high altitudes, and less so in those who become exposed to high altitudes later in life.
The capillary density is increased especially in structures that are active and exposed to chronic low oxygen concentration, like the right ventricular muscles that are subject to chronic hypoxia and excess workload due to pulmonary hypertension at high altitudes (the increase in blood pressure in arteries of lungs due to constriction of lung capillaries as a result of low oxygen concentration).
5. Cellular Acclimatization to High Altitudes
In individuals native to altitudes of 13,000 to 17,000 feet, mitochondria and some other cellular oxidative enzyme systems are more plentiful than in sea level inhabitants.
Such individuals can utilize oxygen more effectively than their counterparts on the plains.
Tibetans - A Sherpa Family Living at High Altitudes
Natural Acclimatization of Natives Living at High Altitudes
Many natives in the Andes and in the Himalayas are born and live at altitudes above 13,000 feet all their lives.
In all aspects of acclimatization, these natives are superior to the people who were born at lower altitudes and have lived at high altitudes for 10 or more years
The process of adaptation in such individuals begins in infancy.
Their chests are broader and well developed, the chest size is greatly increased while the body size is somewhat decreased, giving a high ratio of ventilation capacity to body mass.
Their hearts, particularly the right sides of their hearts which provide a high pressure to pump the blood through a greatly expanded lung capillary system are considerably larger than the hearts of lowlanders.
Their hemoglobin levels are high which greatly increases the oxygen-carrying capacity of their blood and its efficient delivery to vital organs.
Naturally acclimatized natives can achieve a daily work output even at high altitudes almost equal to that of a normal person at sea level.