Oxygen molecules (O2) take advantage of two important properties that facilitate its bodily transport. First, O2 is lipid soluble and crosses cell membranes without the aide of membrane transporters. Second, the free movement across cell membranes relies on a partial pressure gradient for diffusion according to Fick’s law. When O2 reaches the alveolar capillary blood, it diffuses into erythrocytes and bonds to hemoglobin where the interaction is governed by the oxyhemoglobin dissociation curve.
Oxygen exerts a partial pressure of 160 mm Hg in the atmosphere at sea level. In the alveolus, water vapor and carbon dioxide dilute atmospheric gas, slightly decreasing the partial pressure of O2 to 150 mm Hg. Pulmonary arterial blood in alveolus capillaries has O2 partial pressure of 20–40 mm Hg. According to this decreasing pressure gradient from atmosphere to alveolar capillaries, O2 easily diffuses into erythrocytes. Increasing FiO2 to 100% O2 increases alveolar partial pressure of O2 and creates a larger gradient, aiding in O2 diffusion. Several variables affect oxygen uptake (Table 145-1).
TABLE 145-1Factors Affecting Oxygen Uptake ||Download (.pdf) TABLE 145-1 Factors Affecting Oxygen Uptake
|Increases Oxygen Uptake ||Decreases Oxygen Uptake |
|Left shift in oxyhemoglobin dissociation curve ||Anemia |
|Blood transfusion ||Blood dyscrasias |
|Increased alveolar ventilation ||Dead space |
|Increased FiO2 ||V/Q mismatch |
| ||Chronic obstructive lung disease |
| ||Diffusion limitation (ie, pulmonary edema or interstitial lung disease) |
In erythrocytes, O2 is readily taken up by hemoglobin. Hemoglobin is a tetrameric metalloprotein that acts as an O2 carrier, increasing O2 carrying capacity of blood by seven times when compared to dissolved O2 alone. A normal hemoglobin protein has the capability of carrying 1.34 mL of oxygen per gram of hemoglobin. This protein consists of four subunits, two alpha subunits and two beta subunits, each with an iron-containing heme moiety. The iron ion functions as a site of reversible binding for oxygen molecules and exists as ferrous iron (Fe2+) or ferric (Fe3+) when oxidized. As one O2 binds to a heme group, molecular conformational changes occur causing other heme groups to increase their O2 affinity. This is known as cooperativity. Several variables affect the carrying capacity of hemoglobin for oxygen (Table 145-2). The degree of O2–hemoglobin binding (O2 saturation) is represented by the oxygen–hemoglobin dissociation curve. At an O2 partial pressure of 80 mm Hg, 95.8% of hemoglobin is saturated with O2. After O2 diffusion has occurred from alveoli, pulmonary arterial blood partial pressure of O2 is 100 mm Hg, an almost 100% saturation of hemoglobin.
TABLE 145-2Factors Affecting Oxygen Carrying Capacity