Mechanism of development of mountain sickness

Dry atmospheric air contains: nitrogen 78.08%, oxygen—20.94%, carbon dioxide—0.03%, argon—0.94% and other gases—0.01%. When rising to a height, this percentage does not change, but the density of the air changes, and, consequently, the values ​​of the partial pressures of these gases.

According to the law of diffusion, gases move from a medium with a higher partial pressure to a medium with a lower pressure. Gas exchange both in the lungs and in the human blood is carried out thanks to the existing gas. the difference between these pressures.

At normal atmospheric pressure 760 mm Hg. the partial pressure of oxygen is: 760X0.2094=159 mmHg. Art., where 0.2094 is the percentage of oxygen in the atmosphere, equal to 20.94%.

Under these conditions, the partial pressure of oxygen in the alveolar air (inhaled with air and entering the alveoli of the lungs) is about 100 mm Hg. Art. Oxygen is poorly soluble in the blood, but it is bound by the hemoglobin protein found in red blood cells—erythrocytes. Under normal conditions, due to the high partial pressure of oxygen in the lungs, hemoglobin in arterial blood is saturated with oxygen up to 95%.

When passing through tissue capillaries, blood hemoglobin loses about 25% of oxygen. Therefore, venous blood carries up to 70% oxygen, the partial pressure of which, as can be easily seen from the graph (Fig. 2), is only 40 mm Hg at the moment of venous blood flow to the lungs at the end of the blood circulation cycle. Art. Thus, between venous and arterial blood there is a significant pressure difference equal to 100-40 = 60 mm Hg. Art.

Between carbon dioxide inhaled with air (partial pressure 40 mm Hg) and carbon dioxide flowing with venous blood to the lungs at the end of the circulatory cycle (partial pressure 47-50 mm Hg), the pressure difference is 7-10 mm Hg. Art.

As a result of the existing pressure difference, oxygen passes from the pulmonary alveoli into the blood, and directly in the tissues of the body, this oxygen from the blood diffuses into the cells (into an environment with an even lower partial pressure). Carbon dioxide, on the contrary, first passes from the tissues into the blood, and then, when venous blood approaches the lungs, from the blood into the alveoli of the lung, from where it is exhaled into the surrounding air (Fig. 3).

With increasing altitude, the partial pressures of gases decrease. So, at an altitude of 5550 m (which corresponds to an atmospheric pressure of 380 mm Hg) for oxygen it is equal to

380X0.2094=80 mm Hg. Art., that is, it is reduced by half. At the same time, naturally, the partial pressure of oxygen in arterial blood also decreases, as a result of which not only the saturation of hemoglobin in the blood with oxygen decreases, but also due to the sharp reduction in the pressure difference between arterial and venous blood, the transfer of oxygen from the blood to the tissues significantly worsens. This is how oxygen deficiency occurs—hypoxia, which can lead to mountain sickness in a person.

Naturally, a number of protective compensatory and adaptive reactions occur in the human body. So, first of all, the lack of oxygen leads to the excitation of chemoreceptors—nerve cells that are very sensitive to a decrease in the partial pressure of oxygen. Their excitement serves as a signal for deepening and then increased breathing. The expansion of the lungs that occurs in this case increases their alveolar surface and thereby contributes to a more rapid saturation of hemoglobin with oxygen. Thanks to this, as well as a number of other reactions, a large amount of oxygen enters the body.

However, with increased breathing, ventilation of the lungs increases, during which increased removal (“washing out”) of carbon dioxide from the body occurs. This phenomenon is especially intensified with intensification of work in high altitude conditions. Tax, if on the plain at rest approximately 0.2 liters of CO₂ are removed from the body within one minute, and during strenuous work - 1.5-1.7 liters, then in high altitude conditions on average per minute the body loses about 0.3-0.35 liters of CO₂ at rest and up to 2.5 liters with tense muscle work. As a result, a lack of CO₂ occurs in the body—the so-called hypocapnia, characterized by a decrease in the partial pressure of carbon dioxide in the arterial blood. But carbon dioxide plays an important role in regulating the processes of respiration, blood circulation and oxidation. A serious deficiency of CO₂ can lead to paralysis of the respiratory center, a sharp drop in blood pressure, deterioration of heart function, and disruption of nervous activity. Thus, a decrease in blood pressure CO₂ by 45 to 26 mmHg. Art. reduces blood circulation to the brain by almost half. That is why cylinders intended for breathing at high altitudes are filled not with pure oxygen, but with its mixture with 3-4% carbon dioxide.

A decrease in the CO₂ content in the body disrupts the acid-base balance towards an excess of alkalis. Trying to restore this balance, the kidneys spend several days intensively removing this excess of alkalis from the body along with urine. This achieves acid-base balance at a new, lower level, which is one of the main signs of the end of the adaptation period (partial acclimatization). But at the same time, the amount of the body’s alkaline reserve is disrupted (decreased). When suffering from mountain sickness, a decrease in this reserve contributes to its further development. This is explained by the fact that a fairly sharp decrease in the amount of alkalis reduces the blood’s ability to bind acids (including lactic acid) formed during hard work. This in a short time changes the acid-base ratio towards an excess of acids, which disrupts the functioning of a number of enzymes, leads to disorganization of the metabolic process and, most importantly, inhibition of the respiratory center occurs in a seriously ill patient. As a result, breathing becomes shallow, carbon dioxide is not completely removed from the lungs, accumulates in them and prevents oxygen from reaching hemoglobin. In this case, suffocation quickly sets in.