Gas Exchange

External exchange is the movement of gases between the alveoli and the capillary blood in the lungs (see Fig. 14- 1). The barrier that separates alveolar air from the blood is composed of the alveolar wall and the capillary wall, both of which are extremely thin. This respiratory membrane is not only very thin, it is also moist. The moisture is important because the oxygen and carbon dioxide must go into solution before they can diffuse across the membrane. Recall that diffusion refers to the movement of molecules from an area in which they are in higher concentration to an area in which they are in lower concentration. Therefore, the relative concentrations of a gas on the two sides of a membrane determine the direction of diffusion. Normally, inspired air contains about 21% oxygen and 0.04% carbon dioxide; expired air has only 16% oxygen and 3.5% carbon dioxide. These values illustrate that a two-way diffusion takes place through the walls of the alveoli and capillaries (Fig. 14-10).
Internal exchange takes place between the blood and the tissues. In metabolism, the cells constantly consume oxygen and produce carbon dioxide. Based on relative concentrations of these gases, oxygen diffuses out of the blood and carbon dioxide enters. At this point, blood returning from the tissues and entering the lung capillaries through the pulmonary circuit is relatively low in oxygen and high in carbon dioxide. Again, the blood will pick up oxygen and give up carbon dioxide. After a return to the left side of the heart, it starts once more on its route through the systemic circuit.

Figure 14-1 Overview of respiration. In ventilation, gases are moved into and out of the lungs. In external exchange, gases move between the air sacs (alveoli) of the lungs and the blood. In internal exchange, gases move between the blood and body cells. The circulation transports gases in the blood.

Transport of Oxygen

A very small percentage (1.5%) of the oxygen in the blood is carried in solution in the plasma. (Oxygen does dissolve in water, as shown by the fact that aquatic animals get their oxygen from water.) However, almost all (98.5%) of the oxygen that diffuses into the capillary blood in the lungs binds to hemoglobin in the red blood cells. If not for hemoglobin and its ability to hold oxygen in the blood, the heart would have to work much harder to supply enough oxygen to the tissues. The hemoglobin molecule is a large protein with four small iron-containing “heme” regions. Each heme portion can bind one molecule of oxygen. Oxygenated blood (in systemic arteries and pulmonary veins) is 97% saturated with oxygen. That is, the total hemoglobin in the red cells is holding 97% of the maximum amount that it can hold. Deoxygenated blood (in systemic veins and pulmonary arteries) is usually about 70% saturated with oxygen. This 27% difference represents the oxygen that has been taken up by the cells. Note, however, that even blood that is described as deoxygenated still has a reserve of oxygen. Even under conditions of high oxygen consumption, as in vigorous exercise, for example, the blood is never totally depleted of oxygen.

In clinical practice, gas concentrations are expressed as pressure in millimeters of mercury (mmHg), as is blood pressure. Because air is a mixture of gases, each gas exerts only a portion of the total pressure, or a partial pressure (P). The partial pressures of oxygen and carbon dioxide are symbolized as Po2 and Pco2 respectively. To enter the cells, oxygen must separate from hemoglobin. Normally, the bond between oxygen and hemoglobin is easily broken, and oxygen is released as blood travels into areas where the oxygen concentration is relatively low. Cells are constantly using oxygen in metabolism and obtaining fresh supplies by diffusion from the blood.

Figure 14-10 Gas exchange. (A) External exchange between the alveoli and the blood. Oxygen diffuses into the blood and carbon dioxide diffuses out, based on concentrations of the two gases in the alveoli and in the blood. (B) Internal exchange between the blood and the cells. Oxygen diffuses out of the blood and into tissues, while carbon dioxide diffuses from the cells into the blood.

The poisonous gas, carbon monoxide (CO) at low partial pressure binds with hemoglobin at the same sites as does oxygen. However, it binds more tightly and displaces oxygen. Even a small amount of carbon monoxide causes a serious reduction in the blood’s ability to carry oxygen. For an interesting variation on normal gas transport.

Transport of Carbon Dioxide

Carbon dioxide is produced continuously in the tissues as a byproduct of metabolism. It diffuses from the cells into the blood and is transported to the lungs in three ways:
* About 10% is dissolved in the plasma and in the fluid within red blood cells. (Carbonated beverages are examples of water in which CO2 is dissolved.).
* About 15% is combined with the protein portion of hemoglobin and plasma proteins.
* About 75% is transported as an ion, known as a bicarbonate ion, which is formed when carbon dioxide undergoes a chemical change after it dissolves in blood fluids.
The bicarbonate ion is formed slowly in the plasma but much more rapidly inside the red blood cells, where an enzyme called carbonic anhydrase increases the speed of the reaction. The bicarbonate formed in the red blood cells moves to the plasma and then is carried to the lungs. In the lungs, the process is reversed as bicarbonate reenters the red blood cells and releases carbon dioxide for diffusion into the alveoli and exhalation. For those with a background in chemistry, the equation for these reactions follows. The arrows going in both directions signify that the reactions are reversible. The upper arrows describe what happens as CO2 enters the blood; the lower arrows indicate what happens as CO2 is released from the blood to be exhaled from the lungs.

Carbon dioxide is important in regulating the blood’s pH (acid-base balance). As a bicarbonate ion is formed from carbon dioxide in the plasma, a hydrogen ion (H) is also produced. Therefore, the blood becomes more acidic as the amount of carbon dioxide in the blood increases to yield more hydrogen and bicarbonate ions. The exhalation of carbon dioxide shifts the blood’s pH more toward the alkaline (basic) range. The bicarbonate ion is also an important buffer in the blood, acting chemically to help keep the pH of body fluids within a steady range of 7.35 to 7.45.