Chapter 139

### 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 139-1).

TABLE 139-1Factors Affecting Oxygen Uptake

### 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 139-2). 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 139-2Factors Affecting Oxygen Carrying Capacity

Sign in to your MyAccess profile while you are actively authenticated on this site via your institution (you will be able to verify this by looking at the top right corner of the screen - if you see your institution's name, you are authenticated). Once logged in to your MyAccess profile, you will be able to access your institution's subscription for 90 days from any location. You must be logged in while authenticated at least once every 90 days to maintain this remote access.

Ok

## Subscription Options

### AccessAnesthesiology Full Site: One-Year Subscription

Connect to the full suite of AccessAnesthesiology content and resources including procedural videos, interactive self-assessment, real-life cases, 20+ textbooks, and more