18.5 Transport of Gases in Human Bodily Fluids
Mary Ann Clark; Jung Choi; and Matthew Douglas
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe how oxygen is bound to hemoglobin and transported to body tissues
- Explain how carbon dioxide is transported from body tissues to the lungs
Once the oxygen diffuses across the alveoli, it enters the bloodstream and is transported to the tissues where it is unloaded, and carbon dioxide diffuses out of the blood and into the alveoli to be expelled from the body. Although gas exchange is a continuous process, the oxygen and carbon dioxide are transported by different mechanisms.
Transport of Oxygen in the Blood
Although oxygen dissolves in blood, only a small amount of oxygen is transported this way. Only 1.5 percent of oxygen in the blood is dissolved directly into the blood itself. Most oxygen—98.5 percent—is bound to a protein called hemoglobin and carried to the tissues.
Hemoglobin
Hemoglobin, or Hb, is a protein molecule found in red blood cells (erythrocytes) made of four subunits: two alpha subunits and two beta subunits (see figure below) .Each subunit surrounds a central heme group that contains iron and binds one oxygen molecule, allowing each hemoglobin molecule to bind four oxygen molecules. Molecules with more oxygen bound to the heme groups are brighter red. As a result, oxygenated arterial blood where the Hb is carrying four oxygen molecules is bright red, while venous blood that is deoxygenated is darker red.
It is easier to bind a second and third oxygen molecule to Hb than the first molecule. This is because the hemoglobin molecule changes its shape, or conformation, as oxygen binds. The fourth oxygen is then more difficult to bind. The binding of oxygen to hemoglobin can be plotted as a function of the partial pressure of oxygen in the blood (x-axis) versus the relative Hb-oxygen saturation (y-axis). The resulting graph—an oxygen dissociation curve—is sigmoidal, or S-shaped (see figure below). As the partial pressure of oxygen increases, the hemoglobin becomes increasingly saturated with oxygen.
Visual Connection
Factors That Affect Oxygen Binding
The oxygen-carrying capacity of hemoglobin determines how much oxygen is carried in the blood. In addition to [latex]{\text{P}}_{{\text{O}}_{\text{2}}}[/latex], other environmental factors and diseases can affect oxygen carrying capacity and delivery.
Carbon dioxide levels, blood pH, and body temperature affect oxygen-carrying capacity (see figure below). When carbon dioxide is in the blood, it reacts with water to form bicarbonate [latex]{\text{(HCO}}_{\text{3}}^{-}\text{)}[/latex] and hydrogen ions (H+). As the level of carbon dioxide in the blood increases, more H+ is produced and the pH decreases. This increase in carbon dioxide and subsequent decrease in pH reduce the affinity of hemoglobin for oxygen. The oxygen dissociates from the Hb molecule, shifting the oxygen dissociation curve to the ri