Acclimatization

Definition: Acclimatization is the physiological process in which an individual adjusts to changes in the environment to survive. Acclimatization is also known as acclimation or acclimation. This occurs in a brief period –hours to weeks and may persist lifelong.

Acclimatization to new environments occurs in many species.

Adaptation is an evolution that involves many generations.

Acclimatization to high altitude:

Barometric pressure decreases when altitude increases linearly, but the percentage of air composition remains the same up to the mesosphere layer. For example, the percentage of oxygen in the atmosphere remains constant at 21%, but the atmospheric partial pressure reduces proportionately to the barometric pressure. Above the mesosphere layer, the composition of the air is variable.

What is high altitude?

High altitude usually refers to an attitude over 2500 meters to 3000 meters.

The partial pressures:

The partial pressure of vapor depends on body temperature and is independent of high altitude. According to the ‘Environmental Lapse Rate,’ air temperature changes at high altitudes. But our body temperature remains within a normal range.

The alveolar partial pressure of carbon dioxide depends on the body’s metabolism and respiration. When metabolism increases, carbon dioxide production increases. Increased carbon dioxide stimulates ventilation.

Increased ventilation removes carbon dioxide from the body, reducing partial pressure falls.

Metabolic production of carbon dioxide does not alter with increasing altitude, so the partial pressure of carbon dioxide( pco2 )will not change until there are some underlying renal or pulmonary diseases.

There is a progressive decrease in partial pressure of Oxygen and Nitrogen with increasing altitude. Decrease partial pressure of Nitrogen produces no ill effect.

But decreased partial pressure of oxygen at high altitude produces ill effects. The effects produced are similar to the effects of hypoxic hypoxia.

The harmful effects of partial pressure of oxygen depend on

1 . Rapidity of development of hypoxia: that is in sudden or acute ascent and

2. Gradually, hypoxia develops in slow ascent, and the duration of exposure is prolonged. It is present in permanent residents at high altitudes.

Rapid ascent to 10,000 feet can produce cerebral and pulmonary edema in persons engaged in heavy physical work during the first three days after arrival. After that, the conditions respond to rest and oxygen therapy.

The cerebral and pulmonary edemas do not develop in individuals who gradually ascend to high altitude and avoid physical exertion at rest for a few days.

The exact cause is not known but, probable causes of hypoxic pulmonary edema are:

Hypoxia increases sympathetic activity, which causes vasoconstriction leading to an increase in the pulmonary capillary hydrostatic pressure. As a result, plasma and fluid will come out from the pulmonary capillary and cause pulmonary edema.

In the brain, the exact mechanism causes cerebral edema.

Mountain sickness At a higher altitude, a person may suffer from mountain sickness when the effects of hypoxia are not very serious. During gradual ascent, the body gets time to adjust to the new environment.

Mountain sickness starts at a high altitude after 8 to 12 hours of arrival and remains for 28 days. The cause is not well known, but it is associated with cerebral edema and alkalosis.

Mountain sickness is characterized by :

Nausea, vomiting, headache, irritability, and dyspnea may occur.

Pathophysiology of development of cerebral edema:

At higher altitudes, when the autoregulatory mechanism of the brain fails, capillary pressure increases, causing transduction of fluid in the brain, tissue. Accumulation of fluid in the brain will increase intracranial pressure, disturb its normal functions, and cause herniation of the brain tissue.

Respiratory alkalosis is due to hyperventilation:

Respiratory alkalosis is a severe condition and must be treated promptly. Otherwise, it may cause disorientation, coma, and death.

Prevention of mountain sickness:

Mountain sickness can be prevented by:

1. Very slow ascent,

2. Administration of Oxygen,Glucocorticoids, Calcium channel blockers, and Carbonic anhydrase.

Acclimatization

Acclimatization to high altitudes is a process by which an individual response to the reduced atmospheric partial pressure of oxygen. Reduction in partial pressure of oxygen initiates many different changes to increase the oxygen level at the tissue level.

This acclimatization reduces the effects of hypoxia by changes so that persons living at high altitudes and permanent residents can enjoy ordinary lives.

The main changes are:

  1. Increase in pulmonary ventilation

  2. Affinity of the hemoglobin for oxygen increases due to the rise in 2-3 DPG in red blood cells, secondary to increased pH. This 2-3 DPG shifts the oxygen hemoglobin dissociation curve to the right, releasing more oxygen from hemoglobin at the tissue level.

3. An increase in hemoglobin concentration occurs.

Hypoxia is a powerful stimulation for erythropoietin secretion that stimulates erythropoeisis at all stages and causes the release of red blood cells in circulation.

In permanent residents, the total count of red blood cells is about 8 million per cubic ml of blood. In addition, hemoglobin concentration increases, so the oxygen-carrying capacity of the blood increases. However, It is prolonged, and hemoglobin concentration rises after 2-3 weeks. Therefore, hemoglobin concentration is 20 grams per deciliter of blood in a permanent resident of a higher altitude. The average hemoglobin concentration is about 14.5 grams per deciliter of blood.

1. Hypoxia causes increased vascularity of the tissues by opening many existing tissue capillaries and forming new blood vessels. Vascular neogenesis is the formation of new blood vessels.

. Hypoxia causes vasodilatation, so that blood flow increases. In addition, hypoxia induces vascular endothelial growth factor, which stimulates vascular neogenesis. Other growth factors also help in vascular neogenesis.

1. Hypoxia increases Pulmonary artery pressure that opens many pulmonary capillaries and increases diffusion at the tissue level.
2. Hypoxia increases myoglobin synthesis, number of Mitochondria.

Acclimatized subjects may suffer from mountain sickness due to renal failure and if the cardiovascular regulatory mechanism fails or there is a decrease in sensitivity of peripheral chemoreceptors.

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