What explains adequate oxygen delivery to tissues at rest in an acclimatized person at high altitudes?

From the Guidelines

The best explanation for adequate oxygen delivery to tissues at rest in an acclimatized person at high altitudes is that hemoglobin concentration increases. At high altitudes, where atmospheric oxygen is reduced, the body compensates through several physiological adaptations, as described by 1. The most significant of these is an increase in hemoglobin concentration, which occurs through increased erythropoietin production by the kidneys in response to hypoxia. This leads to greater red blood cell production in the bone marrow. With more hemoglobin molecules available in the bloodstream, the oxygen-carrying capacity of the blood increases substantially, allowing adequate oxygen delivery to tissues despite lower arterial PO2 levels.

Some key points to consider in this context include:

• The reduction in barometric pressure at higher altitudes decreases the partial pressure of inspired oxygen, leading to a condition known as ‘hypobaric hypoxia’ 1.
• A series of physiological responses help to maintain adequate tissue O2 delivery and supply at high altitudes, through a process called ‘acclimatization’, which includes increase in ventilation, cardiac output, red cell mass and blood O2 carrying capacity, and other metabolic modifications at the microvascular and cellular levels 1.
• The efficacy of acclimatization depends on the duration of individual’s exposure to altitude, age, sea level partial pressure of oxygen in arterial blood (PaO2) and minute ventilation 1.
• While other adaptations occur during acclimatization, including increased ventilation, cardiac output adjustments, and tissue-level changes in oxygen utilization, the increased hemoglobin concentration provides the most direct and significant compensation for the reduced oxygen availability at altitude.

It's also worth noting that, as described by 1, the SaO2 at a given altitude varies with age, sex, ethnic group and degree of acclimatisation to altitude, but native populations living at high altitudes tend to have higher SaO2 levels than non-acclimatized individuals. However, the increase in hemoglobin concentration remains the primary mechanism by which the body adapts to high-altitude hypoxia and ensures adequate oxygen delivery to tissues.

From the Research

Oxygen Delivery at High Altitudes

The following factors contribute to adequate oxygen delivery to tissues at rest in an acclimatized person at high altitudes:

• Increased hemoglobin concentration, which improves arterial oxygen content 2
• Ventilatory acclimatization, which increases alveolar P(O2) and reduces equivalent altitude 3
• Changes in blood hemoglobin content and/or hemoglobin-oxygen affinity, which can mitigate the effects of arterial hypoxemia 4

Physiological Adaptations

Physiological adaptations to high altitude involve:

• Cardiovascular adaptations, including changes in cardiac output, systolic blood pressure, and heart rate at rest 2
• Respiratory adaptations, including hypoxic ventilatory response and alveolar hyperventilation 2
• Changes in energy metabolism, including alterations in cardiac and skeletal muscle energy metabolism 5

Acclimatization Process

The acclimatization process involves:

• Immediate adaptation responses, such as hypoxic ventilatory response and alveolar hyperventilation 2
• Longer-lasting compensation processes, such as increases in blood hemoglobin concentration and changes in hemoglobin-oxygen affinity 2, 4
• Individual variability in the acclimatization process, with some people adapting more quickly or completely than others 3