Fig. 24.6 ). Thus small amounts of CO can greatly influence the binding of O2 to Hgb. In the presence of CO, the affinity of Hgb for O2 is enhanced. This causes the dissociation curve to shift to the left, which further prevents the unloading and delivery of O2 to tissues. As the PCO of blood approaches 1.0 mm Hg, all of the Hgb binding sites are occupied by CO, and Hgb is unable to bind to O2. This situation is not compatible with life and is the cause of death in cases of CO poisoning. In healthy individuals, HgbCO occupies about 1% to 2% of the Hgb binding sites; however, in cigarette smokers and in individuals who reside in high-density urban traffic areas, occupation of Hgb binding sites can be increased to 10%. Levels above 5% to 7% are considered hazardous. Treatment of individuals with high levels of CO, such as those who have inhaled car exhaust or smoke from a burning building, consists of administering high concentrations of O2 to displace CO from Hgb. Increasing the ambient pressure above atmospheric pressure, through the use of a barometric chamber, substantially increases the O2 tension, which promotes the dissociation of CO from Hgb. Another gas, nitric oxide, has great affinity (200,000 times greater than O2) for Hgb, and it binds irreversibly to Hgb at the same site that O2 does. Endothelial cells synthesize nitric oxide, which has vasodilation properties. Thus nitric oxide is used therapeutically as an inhalant in patients with pulmonary hypertension to reduce pressure. Although nitric oxide poisoning is not common, the clinician should be cautious when administering nitric oxide therapy for long periods. Hgb-bound CO and nitric oxide is referred to as methemoglobin. Under normal conditions, about 1% to 2% of Hgb is bound to CO and nitric oxide.
Oxygen Saturation, Content, and Delivery
Each Hgb molecule can bind up to four O2 atoms, and each gram of Hgb can bind up to 1.34 mL of O2. The term O2 saturation (SO2) refers to the amount of O2 bound to Hgb in relation to the maximal amount of O2 (100% O2 capacity) that can bind Hgb. At 100% O2 capacity, the heme groups of the Hgb molecules are fully saturated with O2, and at 75% O2 capacity, three of the four heme groups are occupied. Binding of O2 to each heme group increases the affinity of the Hgb molecule to bind additional O2. The O2 content in blood is the sum of the O2 bound to Hgb and the dissolved O2. Oxygen content is decreased in the presence of increased CO2 and CO and in individuals with anemia (
Fig. 24.7
Oxygen delivery from the lungs to tissues is dependent on several factors, including cardiac output, the Hgb content of blood, and the ability of the lung to oxygenate the blood. Not all of the O2 carried in blood is unloaded at the tissue level. The actual O2 extracted from blood by the tissue is the difference between the arterial O2 content and the venous O2 content, multiplied by cardiac output. Under normal conditions, Hgb leaves the tissue 75% saturated with O2, and only about 25% is actually used by tissues. Hypothermia, relaxation of skeletal muscles, and an increase in cardiac output reduce O2 extraction. Conversely, a decrease in cardiac output, anemia, hyperthermia, and exercise increase O2 extraction.