![]() ![]() The diver starts with a low carbon dioxide content, a high pH, and a normal oxygen tension. Hyperventilation before diving enables breath hold divers to stay down longer but is very dangerous. The hyperventilation reduces the body’s carbon dioxide content but does not affect oxygen content much, but the Fi o 2 of 100 kPa considerably increases the total oxygen content. It may be possible to hold a breath for over 5 minutes by hyperventilation on 100% oxygen. Hyperventilation does not alter the rates of oxygen consumption and carbon dioxide production, but the lower initial carbon dioxide content means that the hypoxic stimulus triggers respiration before the pH of the cerebrospinal fluid falls enough to do so. Hyperventilation has little effect on the oxygen content of the body but blows off carbon dioxide so that you start with a higher cerebrospinal fluid pH. The breath hold can be extended further by hyperventilation immediately beforehand. With practice you can resist the stimulus to breathe for longer but it remains carbon dioxide accumulation that causes release of the breath hold. During the breath hold the oxygen content of tissues decreases, but the breath hold is broken as a result of carbon dioxide production and resulting acidosis, which stimulates the respiratory centre. The haemoglobin in arterial blood is virtually saturated at an inspired partial pressure of oxygen (Fi o 2) of 21 kPa, and increasing the partial pressure of oxygen has little effect on the amount of oxygen bound to haemoglobin.Īn average healthy person with no special training can hold his (or her) breath for about half a minute. Doubling our inspired partial pressure of oxygen doubles the amount of oxygen in solution but does not double the amount of oxygen in the body since a large part of our oxygen content is bound to oxygen carrying pigments. The effect of the increased partial pressures of oxygen is more complex. If the partial pressure of nitrogen is doubled (by breathing air at 10 m depth) for long enough for equilibration to take place we will contain twice as many dissolved nitrogen molecules as at sea level. Nitrogen is fat soluble and at sea level we have several litres dissolved in our bodies. Gases dissolve in the liquid with which they are in contact. According to Dalton’s law the partial pressure of oxygen at any depth will be 21% of the total pressure exerted by the air and the partial pressure of nitrogen will be 78% of total pressure. The pressure on a diver increases by 100 kPa for every 10 m he or she descendsĭry air is composed of roughly 21% oxygen, 78% nitrogen, and 1% other gases.
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