6.1 Derived copy of gas exchange across respiratory surfaces  (Page 3/24)

and for carbon dioxide:

At high altitudes, P _atm decreases but concentration does not change; the partial pressure decrease is due to the reduction in P _atm .

When the air mixture reaches the lung, it has been humidified. The pressure of the water vapor in the lung does not change the pressure of the air, but it must be included in the partial pressure equation. For this calculation, the water pressure (47 mm Hg) is subtracted from the atmospheric pressure:

and the partial pressure of oxygen is:

These pressures determine the gas exchange, or the flow of gas, in the system. Oxygen and carbon dioxide will flow according to their pressure gradient from high to low. Therefore, understanding the partial pressure of each gas will aid in understanding how gases move in the respiratory system.

Gas exchange across the alveoli

Oxygen and carbon dioxide move independently of each other; they diffuse down their own pressure gradients. As blood leaves the lungs through the pulmonary veins, the venous {\text{P}}_{{\text{O}}_{\text{2}}}    = 100 mm Hg, whereas the venous {\text{P}}_{{\text{CO}}_{\text{2}}}    = 40 mm Hg. As blood enters the systemic capillaries, the blood will lose oxygen and gain carbon dioxide because of the pressure difference of the tissues and blood. In systemic capillaries, {\text{P}}_{{\text{O}}_{\text{2}}} = 100 mm Hg, but in the tissue cells, {\text{P}}_{{\text{O}}_{\text{2}}} = 40 mm Hg. This pressure gradient drives the diffusion of oxygen out of the capillaries and into the tissue cells. At the same time, blood {\text{P}}_{{\text{CO}}_{\text{2}}} = 40 mm Hg and systemic tissue {\text{P}}_{{\text{CO}}_{\text{2}}} = 45 mm Hg. The pressure gradient drives CO ₂ out of tissue cells and into the capillaries. The blood returning to the lungs through the pulmonary arteries has a venous {\text{P}}_{{\text{O}}_{\text{2}}} = 40 mm Hg and a {\text{P}}_{{\text{CO}}_{\text{2}}} = 45 mm Hg. The blood enters the lung capillaries where the process of exchanging gases between the capillaries and alveoli begins again ( [link] ).

Art connection

Which of the following statements is false?

1. In the tissues, {\text{P}}_{{\text{O}}_{\text{2}}} drops as blood passes from the arteries to the veins, while {\text{P}}_{{\text{CO}}_{\text{2}}} increases.
2. Blood travels from the lungs to the heart to body tissues, then back to the heart, then the lungs.
3. Blood travels from the lungs to the heart to body tissues, then back to the lungs, then the heart.
4. {\text{P}}_{{\text{O}}_{\text{2}}} is higher in air than in the lungs.

In short, the change in partial pressure from the alveoli to the capillaries drives the oxygen into the tissues and the carbon dioxide into the blood from the tissues. The blood is then transported to the lungs where differences in pressure in the alveoli result in the movement of carbon dioxide out of the blood into the lungs, and oxygen into the blood.

Section summary

The lungs can hold a large volume of air, but they are not usually filled to maximal capacity. Lung volume measurements include tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume. The sum of these equals the total lung capacity. Gas movement into or out of the lungs is dependent on the pressure of the gas. Air is a mixture of gases; therefore, the partial pressure of each gas can be calculated to determine how the gas will flow in the lung. The difference between the partial pressure of the gas in the air drives oxygen into the tissues and carbon dioxide out of the body.

Art connections