Externe en interne ademhaling

External respiration involves the exchange of oxygen and carbon dioxide between alveoli and pulmonary capillaries in the lungs.

This gas exchange always occurs from an area of higher partial pressure to lower or from P_high to P_low .

Oxygen diffuses from the alveolar air, where PO₂  is 105 mmHg, into the blood within pulmonary capillaries, where PO₂ is 40 mmHg.

Similarly, carbon dioxide is driven from blood at a PCO₂ of 45 mmHg to alveolar air with a PCO₂ of 40 mmHg.

In internal respiration, oxygen is transferred from systemic capillaries to tissue cells throughout the body, while CO₂ moves in the opposite direction.

Systemic capillaries have a higher PO₂ than tissue cells because oxygen is spent for ATP production in cells, prompting oxygen to diffuse from the capillaries into the cells.

Conversely, CO₂ production by active metabolism results in higher PCO₂ in cells than in systemic capillaries, causing CO₂ to diffuse from cells into these capillaries.