Mechanism of Development and Types of Hypoxia
Entering an environment with low atmospheric pressure causes the body to immediately respond to changes in gas composition. The main challenge is not pressure as a physical magnitude itself, but the proportional drop in the partial pressure of oxygen ($pO_2$). Hypoxia develops due to insufficient oxygen in the inspired air.
In physiology, hypoxic states are generally divided into two main categories:
- Exogenous hypoxia: A direct result of changes in the gas composition of the external environment (e.g., ascending to high altitude).
- Endogenous hypoxia: Arises from internal functional disorders or pathological states within the body when oxygen delivery or utilization is impaired.
Emergency Respiration Normalization
If respiration is critically impaired at high altitudes, a specialized gas mixture called Carbogen is used for rapid recovery.
This mixture consists of pure oxygen with the addition of 5% carbon dioxide ($CO_2$). The presence of carbon dioxide is critically important here: hypocapnia (low blood $CO_2$ due to hyperventilation) frequently develops at high altitudes, causing the respiratory center to lose its primary physiological stimulus. The addition of 5% $CO_2$ effectively stimulates the respiratory center, eliminating the negative effects of hypocapnia and forcing the body to breathe deeper and more efficiently.
Acclimatization: Systemic Changes
During prolonged stay in mountainous regions, the body initiates adaptation processes to chronic oxygen deprivation. These physiological shifts affect several vital life support systems simultaneously.
Blood System Changes The body strives to compensate for oxygen shortage by increasing transport capacity. Erythropoiesis is enhanced, leading to an increase in the total erythrocyte count. Concurrently, hemoglobin concentration rises, thereby increasing the total oxygen-carrying capacity of the blood.
Respiratory Function Changes To capture more rarefied air, pulmonary ventilation naturally increases.
Biochemical and Tissue Shifts
Adaptation occurs not only at the organ level, but also at microscopic and molecular levels.
Hemoglobin-Oxygen Affinity To ensure tissues receive adequate nutrition, oxyhemoglobin dissociation in tissue capillaries is accelerated. A specific substance—2,3-bisphosphoglycerate (2,3-BPG)—accumulates in erythrocytes. Its accumulation shifts the oxyhemoglobin dissociation curve to the right, which physiologically means a decrease in hemoglobin's affinity for $O_2$ and facilitates its release to cells.
Vascular Remodeling
- Capillary network density increases: capillaries become longer and more tortuous.
- Marked vasodilation of cerebral vessels occurs. This reaction aims to increase cerebral blood flow to protect nerve cells.
- As a result, overall cell resistance (especially neurons) to prolonged hypoxia increases.