Thus, the oxygen saturation is the ratio of the quantity of oxygen actually bound to the quantity that can be potentially bound. For example, if oxygen content is 16 mL O2/dL

blood and oxygen capacity is 20 mL O2/dL blood, then the blood is 80% saturated. Arterial blood saturation (Sao2) is normally about 98%.

Blood Po2, O2 saturation, and oxygen content are three closely related indices of oxygen transport. The relationship between Po2, oxygen saturation, and oxygen content is illustrated by the oxyhemoglobin equilibrium curve, an S-shaped curve over a range of arterial oxygen tensions from 0 to 100 mm Hg (Fig. 21.6). The shape of the curve results because the hemoglobin affinity for oxygen increases progressively as blood Po2 increases.

The shape of the oxyhemoglobin equilibrium curve reflects several important physiological advantages. The plateau region of the curve is the loading phase, in which oxygen is loaded onto hemoglobin to form oxyhemoglobin in the pulmonary capillaries. The plateau region illustrates how oxygen saturation and content remain fairly constant despite wide fluctuations in alveolar Po2. For example, if Pao2 were to rise from 100 to 120 mm Hg, hemoglobin would become only slightly more saturated (97 to 98%). For this reason, oxygen content cannot be raised appreciably by hyperventilation. The steep unloading phase of the curve allows large quantities of oxygen to be released or unloaded from hemoglobin in the tissue capillaries where a lower capillary Po2 prevails. The S-shaped oxyhemoglobin equilibrium curve enables oxygen to saturate hemoglobin under high partial pressures in the lungs and to give up large amounts of oxygen with small changes in Po2 at the tissue level.

A change in the binding affinity of hemoglobin for O2 shifts the oxyhemoglobin-equilibrium curve to the right or left of normal (Fig. 21.7). The P50—the Po2 at which 50% of the hemoglobin is saturated—provides a functional way to assess the binding affinity of hemoglobin for oxygen. The normal P50 for arterial blood is 26 to 28 mm Hg. A high P 50 signifies a decrease in hemoglobin's affinity for oxygen and results in a rightward shift in the oxyhemoglo-bin equilibrium curve, whereas a low P50 signifies the opposite and shifts the curve to the left. A shift in the P50 in either direction has the greatest effect on the steep phase and only a small effect on the loading of oxygen in the normal lung, because loading occurs at the plateau.

Oxyhemoglobin Curve
Po2 (mm Hg)

An oxyhemoglobin equilibrium curve.

The oxygen saturation (left vertical axis) or oxygen content (right vertical axis) is plotted against partial pressure of oxygen (horizontal axis) to generate an oxy-hemoglobin equilibrium curve. The curve is S-shaped and can be divided into a plateau region and a steep region. The dashed line indicates amount of oxygen dissolved in the plasma. a = arterial; v = venous,- So2 = oxygen saturation; and P50 = partial pressure of O2 required to saturate 50% of the hemoglobin with oxygen.

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