Compensatory reactions of the body
When ascending to an altitude of 2.5 to 5.0 km, the body encounters oxygen deprivation. The primary trigger of adaptation is the stimulation of specialized structures—carotid chemoreceptors. They are sensitive to a decrease in the partial pressure of oxygen ($pO_2$) in arterial blood.
Having received a signal about oxygen starvation, the nervous system triggers a cascade of defense reactions. Their global goal is to maximize the supply of oxygen to the starving tissues.
The main physiological effects at this stage include:
- Increased pulmonary ventilation — compensatory hyperventilation develops, designed to capture more oxygen from the rarefied air.
- Increased blood pressure (BP) — the vascular bed adapts for accelerated blood delivery to vital organs.
- Increased heart rate (HR) — the heart begins to beat faster, increasing cardiac output and compensating for the lack of oxygen in each portion of blood.
Pathogenesis of hypocapnia: why is respiration depressed?
One of the most important and paradoxical mechanisms of altitude hypoxia is the development of hypocapnia, a condition in which carbon dioxide levels in the blood critically drop.
This process proceeds in the form of a strict pathogenetic chain:
- A compensatory increase in pulmonary ventilation occurs due to a lack of oxygen.
- Enhanced breathing leads to excessive "washout" of carbon dioxide ($CO_2$) from the body.
- As a result, the partial pressure of carbon dioxide ($pCO_2$) directly in the alveolar air decreases, leading to hypocapnia.
- Since carbon dioxide is the main natural stimulant of the respiratory center (via action on central chemoreceptors), its deficiency leads to a decrease in stimulating impulses.
- The logical outcome is restriction or even depression of respiration, which can exacerbate the initial oxygen shortage.
Altitude (mountain) sickness
If a person ascends to an altitude of 4–5 km, the body's compensatory capabilities may be exhausted, and altitude (mountain) sickness develops.
Initial symptoms of this condition include pronounced weakness, the appearance of cyanosis (bluish discoloration of the skin and mucous membranes due to lack of oxygen in the blood), and intense headache.
With further stay in adverse conditions, the stage of decompensation sets in. At this point, the body's defense systems give up, which manifests in effects directly opposite to the initial adaptation:
- A pathological decrease in heart rate (HR) occurs.
- Blood pressure (BP) rapidly drops.
- The depth of respiratory movements decreases, making gas exchange extremely inefficient.
Critical disorders at extreme altitudes
Ascending to an altitude above 7 km without special oxygen equipment is considered a zone of critical disorders. Under such conditions, acute hypoxia develops.
Main risks of extreme altitude:
- Occurrence of life-threatening disorders of the respiratory system functions.
- Catastrophic failures in the circulatory system.
- With the rapid development of hypoxic phenomena, sudden loss of consciousness is possible, making staying at such an altitude deadly dangerous.
Briefly: answers to topical questions
What mechanisms provide long-term tissue adaptation to chronic altitude hypoxia?
Long-term tissue adaptation to chronic altitude hypoxia is provided by vascular, tissue, and biochemical changes. Mechanisms include:
- Increasing the density of the capillary network in tissues (increasing the length and tortuosity of capillaries).
- Vasodilation of cerebral vessels, leading to increased cerebral blood flow.
- Increasing cell resistance to hypoxia, especially nerve cells.
- Accelerating oxyhemoglobin dissociation in tissue capillaries due to a rightward shift of the oxyhemoglobin dissociation curve caused by the accumulation of 2,3-glycerophosphate in erythrocytes.
How does the oxygen capacity of blood change during a prolonged stay at a significant altitude?
During a prolonged stay at a significant altitude, the oxygen capacity of the blood increases. This occurs due to adaptive changes in the blood system, such as enhanced erythropoiesis (increase in the number of red blood cells) and an increase in total hemoglobin content.
In which direction and under the influence of what factors does the shift of the oxyhemoglobin dissociation curve occur during hypoxia?
During hypoxia, the oxyhemoglobin dissociation curve shifts to the right, which facilitates the release of oxygen to tissues. This shift occurs under the influence of the following factors:
- Accumulation of 2,3-bisphosphoglycerate (2,3-BPG) or 2,3-diphosphoglycerate (2,3-DPG) in erythrocytes.
- Increase in carbon dioxide content ($pCO_2$).
- Accumulation of acids and acidification of the medium (increase in hydrogen ion concentration $H^+$, acidosis).
- Increase in temperature.
The hypocapnia chain: "Low oxygen → Frequent breathing → Losing CO2 → Receptors are silent → Breathing is inhibited".
Class and exam questions
What serves as the primary trigger for initiating compensation at high altitudes?
Decrease in the partial pressure of oxygen (pO2), which is detected by carotid chemoreceptors.
What is the paradox of hypocapnia during ascent to high altitudes?
Due to hyperventilation, the body actively loses carbon dioxide. The drop in its level (hypocapnia) reduces the stimulation of central chemoreceptors, leading to respiratory depression.
How do cardiovascular parameters change at the decompensation stage of altitude sickness?
Unlike the initial stage, where pulse and pressure rise, during decompensation (at an altitude of 4-5 km), a dangerous decrease in heart rate and a drop in blood pressure occur.
What are the risks of rapid development of acute hypoxia at altitudes above 7 km?
The rapid increase in oxygen starvation at such altitudes leads to severe respiratory and circulatory failures, as well as sudden loss of consciousness.
What else to study on this topic
- Mechanisms of carotid chemoreceptor function
- Differences in the functions of central and peripheral chemoreceptors
- Pathophysiology of cyanosis in hypoxic states
- Hemodynamic shifts at the stage of decompensation
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