Pulmonary Gas Exchange

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Pulmonary gas exchange is driven by passive diffusion and thus does not require energy for exchange. Substances move down a concentration gradient. Oxygen moves from the alveoli (high oxygen concentration) to the blood (lower oxygen concentration, due to the continuous consumption of oxygen in the body). Conversely, carbon dioxide is produced by metabolism and has a higher concentration in the blood than in the air.

Oxygen in the lungs first diffuses through the alveolar wall and dissolves in the fluid phase of blood. The amount of oxygen dissolved in the fluid phase is governed by Henry's Law. Oxygen dissolved in the blood may diffuse into red blood cells and bind to hemoglobin. Binding of oxygen to hemoglobin allows a greater amount of oxygen to be transported in the blood. Although carbon dioxide and oxygen are the most important molecules exchanged, other gases are also transported between the alveoli and blood. The amount of a gas that is exchanged depends on the water solubility of the gas the affinity of the gas for hemoglobin. Water vapor is also excreted through the lungs, due to humidification of inspired air by the lung tissues.

Red blood cells transit the alveolar capillaries in about 3/4 of a second.

Most gases (including carbon dioxide and nitrous oxide) reach equilibrium with the blood before the red blood cells leave the alveolar capillaries. Gases that reach equilibrium before the blood leaves the alveolar capillaries are perfusion limited, since the amount of the gas exchanged depends solely on the volumetric flow rate of blood past the alveoli. However, carbon monoxide is stored in such high concentrations in the blood, due to its strong binding to hemoglobin, that equilibrium is not reached before the blood leaves the alveolar capillary. Thus, the concentration of carbon monoxide in the arterial system can be used to assess the resistance of the alveolar walls to gas diffusion. Transport of carbon monoxide is thus termed diffusion...
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