Exchange of oxygen and carbon dioxide

Dalton's law and Henry's law

The exchange of oxygen and carbon dioxide between alveolar air and pulmonary blood is a passive process, which is explained by two gas laws. Dalton's law explains how gases move by diffusion based on pres­sure differences; Henry's law describes the diffusion of gas based on its solubility.

Dalton's law

Dalton's law states that each gas within a mixture exerts its own pressure independent of the other gases present. The pressure of an individual gas is called its partial pressure, designated P_x. The total pressure of a gas mixture is calculated by summing all its partial pressures. Atmospheric air is a combination of nitro­gen (N₂), oxygen (O₂), carbon dioxide (CO₂), and water vapor.

At sea level, atmospheric pressure is 760 mmHg. Atmospheric air is 78.6% nitrogen, 20.9% oxygen, 0.04% carbon dioxide, 0.06% other gases, and varying water vapor, depending on the humidity. Therefore, atmospheric air can be computed as follows:

External and internal respiration

External respiration, also called pulmonary gas exchange, is the diffusion of O₂ and CO₂ from the alveoli to pulmonary blood. Pulmonary blood is deox­ygenated blood arriving from the right ventricle. Blood circulating through the body picks up CO₂ and delivers O₂. As this blood travels through the pulmo­nary capillaries, CO₂ diffuses into the alveoli while O₂ diffuses from the alveoli to pulmonary blood. The exchange of these gases occurs independently and passively.

Pulmonary gas exchange is facilitated by a very thin respiratory membrane. In addition, there is a close association between the amount of gas reaching the alveoli, that is, ventilation, and the blood flow through the pulmonary capillaries, that is, perfusion. When ventilation becomes inadequate within the alveoli, the Po, will decrease. This causes an autoregulatory response in which pulmonary arterioles constrict. Conversely, if P_o, increases, the pulmonary arterioles dilate, allowing more blood to flow to those areas that can maximize gas exchange. Note that this is the oppo­site of what happens in systemic circulation, where a decrease in P_o, results in vasodilation.

Internal respiration, or systemic gas exchange, occurs at the tissue level, where there is an exchange of O₂ and CO₂ between systemic capillaries and tissue. O₂ diffuses from the capillaries into the cells; CO₂ diffuses from the cells into the systemic capillaries.