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Exercise at altitude- ch 12.docx

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Human Biology
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Exercise at altitude- chapter 12  Low partial pressure of oxygen at altitude is what limits exercise performance.  The reduced barometric pressure at altitude is referred to as hypobaric environment or simply hypobaria (low atmospheric pressure).  Low atmospheric pressure also means a lower partial pressure of oxygen which limits pulmonary diffusion of oxygen from the lungs and oxygen transport to the tissues.  When oxygen delivery to the body tissues is compromised, the result is cellular hypoxia (oxygen deficiency)  Few negative physiological effects on performance are seen below 1500m  At an elevation, the air always contains 20.93% oxygen, 0.03 carbon dioxide, 79.04% nitrogen.  The pressure that oxygen molecules in the air exert at various altitudes drops proportionally with decreases in the barometric pressure.  The very low water vapour pressure at high altitudes promotes evaporation of moisture from the skin surface, because of the high radiant between skin and air, and can lead quickly to dehydration.  Because the atmosphere in thinner and drier at altitude, solar radiation is more intense at higher elevations. This effect is magnified when the ground is snow covered.  Altitude causes hypobaric hypoxia, resulting in decreased partial pressures of oxygen throughout the body.  With acute exposure to altitude a series of adaptations occur in an attempt to minimize the drop in oxygen delivery to the tissues. Pulmonary ventilation increases, and pulmonary diffusion is reasonably well maintained; but oxygen transport is slightly impaired because hemoglobin saturation at altitude is reduced.  The diffusion gradient that allows oxygen exchange between the blood and active tissue is substantially reduced at moderate and high altitudes; thus, oxygen uptake by muscle is impaired. A decrease in plasma volume initially increases red blood cell concentration, allowing more oxygen to be transported per unit of blood, partially compensating for this impaired oxygen uptake.  Upon initial ascent to altitude, the body increases its cardiac output during submaximal work to compensate for the decreased oxygen content per liter of blood. It does so by increasing heart rate, because stroke volume falls with the fall in plasma volume.  During maximal work at altitude, stroke volume and heart rate are both lower, which reduces cardiac output. The reduced cardiac output, combined with the decreased pressure gradient, severely impairs oxygen delivery and uptake.  Ascent to altitude increases metabolic rate by increasing sympathetic nervous system activity. There is an increased reliance on carbohydrate for fuel,
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