Porth's Essentials of Pathophysiology, 4e

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Control of Respiratory Function

C h a p t e r 2 1

Tissue capillaries

3

Oxygen Dissociation in the Tissues. The dissociation or release of O 2 from hemoglobin occurs in the tissue capillaries where the PO 2 is less than that of the arte- rial blood. As oxygen dissociates from hemoglobin, it dissolves in the plasma and then moves into the tis- sues where the PO 2 is less than that in the capillaries. The affinity of hemo- globin for O 2 is influenced by the carbon dioxide (PCO 2 ) content of the blood and its pH, temperature, and 2,3-diphosphoglycerate (2,3- DPG), a by-product of glycolysis in red blood cells. Under conditions of high metabolic demand, in which the PCO 2 is increased and the pH is decreased, the binding affinity of hemoglobin is decreased, and during decreased metabolic demand, when the PCO 2 is decreased and the pH is increased, the affinity is increased.

HbO 2

PO 2

O 2

O 2

Body cells

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with hemoglobin at the same site as oxygen, has a bind- ing tenacity that is 250 times that of oxygen. Therefore, small concentrations of carbon monoxide in the air (less than 1 part per thousand of air) can be lethal. Even though the oxygen content of the blood is greatly reduced in carbon monoxide poisoning, the PO 2 may be normal, making detection difficult because the blood is bright red and there are no obvious signs of hypoxemia, such as a bluish discoloration of the lips or fingertips. The use of a hyperbaric chamber, in which 100% oxy- gen can be administered at high atmospheric pressures (e.g., 3 ATM), increases the PO 2 , or the amount of oxy- gen carried in the dissolved form, to life-saving levels. Oxygen–Hemoglobin Dissociation Curve The relationship between the oxygen carried in com- bination with hemoglobin and the PO 2 of the blood is described by the oxygen–hemoglobin dissociation curve , depicted in Figure 21-17. The x axis of the graph depicts the PO 2 or dissolved oxygen. It reflects the partial pres- sure of oxygen in the lungs (i.e., the PO 2 ranges from 95 to 100 mm Hg when breathing room air, but can rise to 200 mm Hg or higher when oxygen-enriched air is breathed). The y axis on the left depicts hemoglobin saturation or the amount of oxygen that is carried by hemoglobin. The right y axis depicts oxygen content or

total amount of oxygen (i.e., mL O 2

/dL) that is being

carried in the blood. The S-shaped oxygen dissociation curve reflects the effect that oxygen saturation has on the conformation of the hemoglobin molecule and its affinity for oxygen. Its flat upper-right portion represents the binding of oxygen to hemoglobin in the lungs (see Fig. 21-17A). Notice that this plateau occurs at approximately 100 mm Hg PO 2 , at which point the hemoglobin is approximately 98% saturated. Increasing the alveolar PO 2 above this level does not increase hemoglobin saturation. Even at high altitudes, when the partial pressure of oxygen is considerably decreased, the hemoglobin remains rela- tively well saturated. At 60 mm Hg PO 2 , for example, the hemoglobin is still approximately 89% saturated. The steeper lower-left portion of the dissociation curve—between 60 and 40 mm Hg—represents the removal of oxygen from hemoglobin as it moves through the tissue capillaries. This portion of the curve reflects the fact that there is considerable transfer of oxygen from hemoglobin to the tissues with only a small drop in PO 2 , thereby ensuring a gradient for oxygen to move into body cells. The tissues normally remove approximately 5 mL of oxygen per dL of blood, and the hemoglobin of mixed venous blood is approximately 75% saturated as it returns to the right side of the heart. In this portion of the dissociation curve (saturation <75%), the rate