Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

The Doctrine of Viscera (Splanchnology)
Respiratory System
Respiration - Gas Exchange in the Lungs

Gas exchange takes place in the Lungs between the air entering the alveoli and the Blood flowing through the capillaries (Fig. 60). This intensive gas exchange is facilitated by the exceptionally thin air-blood barrier. Formed by the alveolar and capillary walls, this barrier separates the air from the blood and is only about 2.5 µm thick. The alveolar walls consist of a single layer of squamous epithelium (alveolocytes) lined on the inner surface facing the alveolar lumen with a thin phospholipid film known as surfactant. Surfactant prevents the alveoli from collapsing during expiration and reduces surface tension. The alveoli are enveloped by a dense network of blood capillaries, which significantly increases the surface area available for gas exchange between the air and the blood.

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Fig. 60. Gas exchange between blood and alveolar air:

1 — alveolar lumen; 2 — alveolar wall; 3 — Capillary Wall; 4 — capillary lumen; 5 — erythrocyte within the capillary lumen. The arrows indicate The pathway of oxygen (O₂) and carbon dioxide (CO₂) across the air-blood barrier (between blood and air)

The concentration (partial pressure) of oxygen in inhaled air within the alveoli is much higher (100 mm Hg) than in the venous blood (40 mm Hg) flowing through the pulmonary capillaries. Consequently, oxygen readily diffuses from the alveoli into the blood, where it rapidly binds to Hemoglobin in the erythrocytes. Simultaneously, carbon dioxide, whose concentration in the capillary venous blood is high (47 mm Hg), diffuses into the alveoli, where the capillary pressure of CO₂ is considerably lower (40 mm Hg). Carbon dioxide is then eliminated from the lungs with exhaled air.

Thus, the pressure gradient (tension) of oxygen and carbon dioxide between alveolar air, arterial blood, and venous blood enables oxygen to diffuse from the alveoli into the blood and carbon dioxide to diffuse from the blood into the alveoli.