Oxygen is relatively insoluble in aqueous solutions such as blood. Dissolved oxygen alone is insufficient to meet the demands of tissue metabolism. Therefore, an alternate means of transporting oxygen is essential. Oxygen binds reversibly to hemoglobin, enabling the transport of large amounts of oxygen—approximately 20 mL/100 mL of blood at a hemoglobin concentration of 15 g/100 mL.
Oxygen Dissociation Curve
The oxygen dissociation curve represents the relationship between the oxygen content of blood and the partial pressure of oxygen to which it is exposed (Fig. 15-1).1 Oxygen content is expressed as the volume of oxygen contained in 100 mL of blood but may also be expressed as either volumes % or mL/dL. The standard oxygen dissociation curve (Fig. 15-1) demonstrates the effects of oxygen–hemoglobin interaction at standard pH (7.40), temperature (37°C), and atmospheric pressure (760 mm Hg). The blue line at the bottom of the graph in Fig. 15-1 shows the amount of oxygen dissolved in blood, and the red line shows the total amount of oxygen in blood at any given oxygen tension. Almost the entire quantity of oxygen transported in blood is bound to hemoglobin. However, the role of dissolved oxygen cannot be ignored. Oxygen diffuses across the alveolar-capillary membrane, enters the plasma, traverses the red cell membrane, and enters the erythrocyte interior—all while dissolved in aqueous solutions. It then combines with hemoglobin enabling the transport of large amounts of oxygen to the metabolizing tissues. Dissolved oxygen, although present in very low concentration in blood, is a critical component of the process of O2 exchange because hemoglobin cannot traverse endothelial and erythrocytic membranes.
Oxygen dissociation curve. Relationship between oxygen content and pressure in normal human blood. The total oxygen content of blood as a function of the partial pressure of oxygen is indicated by the red line. The blue line indicates the content of dissolved oxygen resulting from changes in PO2. The partial pressure of oxygen (P50) necessary to saturate one-half of hemoglobin in blood is indicated in green.
Changes in the quaternary structure of hemoglobin that accompany oxygen binding result in a sigmoid, rather than hyperbolic, oxygen dissociation curve. The S-shaped dissociation curve is the result of changes in oxygen affinity of unbound heme groups following the binding of oxygen to another heme group in the same hemoglobin molecule. As illustrated in Fig. 15-1, once the partial pressure of oxygen reaches 90 to 100 mm Hg, hemoglobin is almost completely saturated with bound oxygen. There is little additional oxygen binding even at higher oxygen tensions. The relatively flat nature of the curve in this range of arterial oxygen tension is an advantage because reductions in arterial PO2 (as might be caused by lung disease) will ...