Chapter 145: Oxygen Transport

INTRODUCTION

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 UPTAKE

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).

HEMOGLOBIN

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.