Outline Gas Exchange Steps at Alveoli and Tissue Capillaries

Gas exchange in humans takes place mainly at two levels — in the alveoli of lungs and at the tissue capillaries. It occurs through diffusion based on the differences in partial pressures of oxygen (pO₂) and carbon dioxide (pCO₂).

A. At the alveoli (lungs):

1. Alveolar air has higher pO₂ (~104 mm Hg) and lower pCO₂ (~40 mm Hg) than deoxygenated blood.
2. Oxygen diffuses from alveolar air into the pulmonary capillary blood.
3. CO₂ diffuses from blood (pCO₂ ~45 mm Hg) into alveoli to be exhaled.
4. Oxygen binds with hemoglobin forming oxyhemoglobin.

B. At the tissue level (capillaries):

1. Tissues have low pO₂ (~40 mm Hg) and high pCO₂ (~45 mm Hg) due to metabolism.
2. O₂ diffuses from blood to tissues; CO₂ diffuses from tissues into the blood.
3. Oxyhemoglobin releases oxygen; CO₂ forms bicarbonate or binds to hemoglobin.

Thus, gas exchange ensures oxygen uptake in lungs and carbon dioxide removal in tissues, maintaining the balance necessary for aerobic respiration.