![]() ![]() Venous PO2 is much lower and Pco2 much higher after exercise, for example, than at rest, whereas arterial values are not significantly affected by moderate physical activity. Blood gas measurements of venous blood are not as useful because these values are far more variable. The values in arterial blood are relatively constant and clinically significant because they reflect lung function. The PO2 and PCO2 values of blood are a result of gas exchange in the lung alveoli and gas exchange between systemic capillaries and body cells. After gas exchange in the alveoli of the lungs, blood in the pulmonary veins and systemic arteries has a PO2 of about 100 mmHg and a Diminished pulses may reflect poor peripheral circulation or low blood pressure, while patient movement is frequently caused by the pain associated with arterial puncture. Blood in the systemic veins, which is delivered to the lungs by the pulmonary arteries, usually has a PO2 of 40 mmHg and a PCO>2 of 46 mmHg. However, the necessary sample of arterial blood can be difficult to obtain due to diminished pulses or patient movement. For this reason, breathing from a tank of 100% oxygen (with a PO2 of 760 mmHg) would significantly increase oxygen delivery to the tissues, although it would have little effect on the total oxygen content of blood.Īn electrode that produces a current in response to dissolved carbon dioxide is also used, so that the PCO>2 of blood can be measured together with its PO2. Since the oxygen carried by red blood cells must first dissolve in plasma before it can diffuse to the tissue cells, however, a doubling of the blood PO2 means that the rate of oxygen diffusion to the tissues would double under these conditions. This is because the plasma contains relatively little oxygen compared to the red blood cells. If the PO2 doubles, the amount of oxygen dissolved in the plasma also doubles, but the total oxygen content of whole blood increases only slightly. It can, however, significantly increase the amount of oxygen dissolved in the plasma (because the amount dissolved is directly determined by the PO2). Thus an increase in blood PO2-produced, for example, by breathing 100% oxygen from a gas tank-cannot significantly increase the amount of oxygen contained in the red blood cells. At a normal PO2 of about 100 mmHg, hemoglobin is almost completely loaded with oxygen. Conclusions: Our results suggest that the likelihood of discrepant sO2 is 27 among patients with pO2 less than 50 mm Hg. When the lungs are functioning properly, the PO2 of systemic arterial blood is only 5 mmHg less than the PO2 of alveolar air. Measurements of arterial PO2 thus provide valuable information in treating people with pulmonary diseases, in performing surgery (when breathing may be depressed by anesthesia), and in caring for premature babies with respiratory distress syndrome. If the inspired air had a normal PO2 but the arterial PO2 was below normal, for example, you could conclude that gas exchange in the lungs was impaired. It does, however, provide a good index of lung function. Since blood PO2 measurements are not directly affected by the oxygen in red blood cells, the PO2 does not provide a measurement of the total oxygen content of whole blood. ![]()
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