Disorders Caused by High Partial Pressures of Gases

The total atmospheric pressure increases by one atmosphere (760 mmHg) for every 10 m (33 ft) below sea level. If a diver descends 10 meters below sea level, therefore, the partial pressures and amounts of dissolved gases in the plasma will be twice those values at sea level. At 20 meters, they are three times, and at 30 meters they are four times the values at sea level. The increased amounts of nitrogen and oxygen dissolved in the blood plasma under these conditions can have serious effects on the body.

Oxygen Toxicity

Although breathing 100% oxygen at one or two atmospheres pressure can be safely tolerated for a few hours, higher partial

Ventilation Perfusion Ratio Apex

■ Figure 16.24 Lung ventilation/perfusion ratios. The ventilation, blood flow, and ventilation/perfusion ratios are indicated for the apex and base of the lungs. The ratios indicate that the apex is relatively overventilated and the base underventilated in relation to their blood flows. As a result of such uneven matching of ventilation to perfusion, the blood leaving the lungs has a PO2 that is slightly lower (by about 5 mmHg) than the PO2 of alveolar air.

oxygen pressures can be very dangerous. Oxygen toxicity may develop rapidly when the Po2 rises above about 2.5 atmospheres. This is apparently caused by the oxidation of enzymes and other destructive changes that can damage the nervous system and lead to coma and death. For these reasons, deep-sea divers commonly use gas mixtures in which oxygen is diluted with inert gases such as nitrogen (as in ordinary air) or helium.

Hyperbaric oxygen therapy, in which a patient is given 100% oxygen gas at 2 to 3 atmospheres pressure to breathe for varying lengths of time, is used to treat carbon monoxide poisoning, decompression sickness, severe traumatic injury (such as crush injury), infections that could lead to gas gangrene, and other conditions. While normal plasma oxygen concentration is 0.3 ml O2/|00 ml blood (as previously described), breathing |00% oxygen at a pressure of 3 atmospheres raises the plasma concentration to about 6 ml O2/|00 ml blood. This helps to kill anaerobic bacteria, such as those that cause gangrene; promote wound healing; reduce the size of gas bubbles (in the case of decompression sickness); and to quickly eliminate carbon monoxide from the body. Although hyperbaric oxygen was formerly used to treat premature infants for respiratory distress, the practice was discontinued because it caused a fibrotic deterioration of the retina that frequently resulted in blindness.

Respiratory Physiology

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