Acclimatization is the process by which an organism undergoes temporary, non-genetic changes in its physiology or behavior to adjust to a change in its environment. This adjustment allows the organism to maintain normal function and survive in conditions that would otherwise cause strain or injury. It is a highly flexible, short-term response that occurs within the lifetime of a single individual. The changes made are reversible, meaning the body will revert to its previous state once the environmental stressor is removed.
Defining Acclimatization vs. Adaptation
The concept of acclimatization is frequently confused with biological adaptation, but a fundamental distinction exists between the two processes. Acclimatization is a phenotypic change, meaning it is an observable adjustment in an individual’s traits or characteristics without altering the underlying genetic code. It is rapid, often occurring over a matter of hours, days, or weeks, and the effects disappear if the individual returns to the original environment.
In contrast, adaptation is a long-term, permanent change that occurs at the population level across many generations. This process involves genetic modifications, which are heritable traits passed down through natural selection, making the entire species better suited to a specific environment. For example, the high oxygen efficiency seen in certain long-established high-altitude human populations is considered an adaptation, whereas the temporary increase in red blood cells in a sea-level resident visiting the mountains is acclimatization.
Common Environmental Triggers
The body’s acclimatization mechanisms are typically triggered by three primary environmental stressors that challenge physiological stability. The most recognized trigger is a change in altitude, which introduces the stress of hypobaric hypoxia, or a reduction in the partial pressure of available oxygen. At higher elevations, the lower air pressure makes it more difficult for the lungs to transfer oxygen into the bloodstream.
Another major trigger is temperature variation, encompassing both extreme heat and cold stress. Exposure to a hot environment forces the body to shed excess heat to prevent core temperature from rising. Conversely, prolonged exposure to cold necessitates physiological adjustments to maintain core body temperature. Changes in humidity often accompany temperature stress and are also significant, particularly in hot environments where high moisture content impairs the body’s primary cooling mechanism: the evaporation of sweat.
The Physiological Process of Adjustment
When an individual rapidly ascends to high altitude, the body immediately registers the reduced oxygen availability and initiates a series of physiological responses. One of the quickest changes is a marked increase in ventilation, which involves breathing more rapidly and deeply to increase the amount of oxygen taken into the lungs. This increased respiratory drive, known as the hypoxic ventilatory response, helps to raise the oxygen saturation in the blood, though it also causes a temporary shift in blood chemistry.
Over the course of several days to weeks, the body begins to increase its oxygen-carrying capacity. The kidneys release the hormone erythropoietin (EPO), which stimulates the bone marrow to produce more red blood cells. The concentration of hemoglobin increases over time, a process that takes a few weeks for a measurable effect. Additionally, the acute reduction in plasma volume temporarily concentrates the existing red blood cells to enhance oxygen delivery until the new cells are produced.
Acclimatization to a hot environment involves a different set of coordinated responses focused on thermoregulation and fluid balance. Following repeated heat exposure, the cardiovascular system stabilizes, resulting in a lower heart rate and reduced strain during work in the heat. This is partly due to an expansion of plasma volume, which helps to maintain blood pressure and allows more blood to be diverted to the skin for cooling.
The sweating mechanism also becomes significantly more efficient, with full heat acclimatization typically taking between seven and fourteen days. The onset of sweating begins earlier in response to a rise in core temperature, and the total sweat rate increases, allowing for a greater evaporative cooling effect. Furthermore, the sweat glands become better at conserving electrolytes, leading to sweat that is less salty and reducing the risk of electrolyte imbalance.