The Binding of Oxygen and Hemoglobin Is Determined by Oxygen Tension
Figure 48-2 shows that the oxygen content of blood—that is, the amount of oxygen combined with hemoglobin—is determined by Po2. At a Po2 of more than approximately 70 mm Hg, the oxyhemoglobin dissociation curve is virtually flat, which indicates that further increases in Po2 add little oxygen to hemoglobin.
At this point, the hemoglobin is saturated with oxygen because each iron atom is associated with an oxygen molecule. The fact that hemoglobin becomes virtually saturated with oxygen at a Po2 of more than 70 mm Hg has important clinical consequences. Many animals live at altitudes considerably above sea level, where the lower barometric pressure results in a low PIo2 (inspired oxygen tension). Although these animals have a lower Pao2 (arterial oxygen tension) than their sea-level-dwelling counterparts, they are still able to transport sufficient oxygen to their tissues because their hemoglobin is well saturated with oxygen. Clearly, at extreme altitudes, hemoglobin begins to desaturate.One gram of saturated hemoglobin can hold 1.36 to 1.39 mL of oxygen; therefore, average mammalian blood with 10 to 15 g ofhemoglobin per deciliter has an oxygen capacity of 13.6 to 21 mL of oxygen per deciliter (volume percentage ∣vol%]) when hemoglobin is saturated with oxygen. The oxygen capacity of the blood is the maximal amount of oxygen that can be carried in the blood at any given time. Anemiai a reduction in the number of circulating erythrocytes (red blood cells) with a consequent reduction in the amount of hemoglobin in the blood, decreases oxygen capacity. When the hemoglobin content of blood increases, oxygen capacity increases as well. The latter occurs during exercise; contraction of the spleen forces more erythrocytes into the circulation. More erythrocytes than normal in the blood is known as polycythemia, and these red blood cells increase the oxygen capacity of the blood.
When the Po2 is less than 60 mm Hg, the oxyhemoglobin dissociation curve has a steep slope. This is in the range of tissue Po2 at which oxygen is unloaded from the blood. Tissue Po2 varies in accordance with the blood IlowZmetabolisin ratio, but average tissue Po2 is 40 mm Hg. Blood exposed to such a Po2 loses 250∕o of its oxygen to the tissues. In rapidly metabolizing tissues in which tissue Po2 is lower, more oxygen is unloaded from the blood. The oxygen remaining in combination with hemoglobin forms a reserve that can be tapped in emergencies.
Oxygen content is a term that describes the amount of oxygen in the blood, most bound to hemoglobin. When hemoglobin is saturated with oxygen, oxygen content and oxygen capacity are equal. When oxygen leaves the blood in the tissues, oxygen content decreases, but the oxygen capacity remains the same.