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Gas molecules move from areas of higher partial pressure to lower partial pressure. When lungs expand, pressure decreases, drawing oxygen-rich air inward. Oxygen diffuses from alveoli into capillaries where partial pressure is lower, while carbon dioxide moves in the opposite direction down its pressure gradient.

Hemoglobin is a respiratory pigment in red blood cells that binds oxygen in the lungs where partial pressure is high. As blood travels to tissues where oxygen partial pressure is lower, hemoglobin releases oxygen for cellular use. This reversible binding enables efficient oxygen delivery throughout the body.

Carbon dioxide is a byproduct of cellular respiration and accumulates in tissues. If CO2 builds up, it changes cellular pH and damages cells. The circulatory system continuously transports CO2 from tissues back to the lungs for exhalation, maintaining proper cellular conditions and enabling continued energy production.

Alveoli and capillaries are intertwined and physically touch, with both typically one cell thick, enabling easy gas diffusion. The lungs contain hundreds of millions of alveoli with approximately 100 square meters of surface area, allowing massive oxygen and carbon dioxide exchange despite the lungs' relatively small size.

When oxygen enters the lungs, it mixes with residual air already present, decreasing alveolar oxygen partial pressure and increasing oxygen uptake by the lungs. Oxygen then diffuses through the alveoli into neighboring capillaries, moving toward the lower partial pressure medium in the blood.

Oxygen-depleted blood from tissues has lower oxygen partial pressure than oxygen-rich air in alveoli, creating a pressure gradient that drives oxygen diffusion into blood. Simultaneously, carbon dioxide diffuses from blood into alveoli where partial pressure is lower, then exits through exhalation, completing the continuous cycle.

Each gas moves down its own pressure gradient independently. Oxygen and carbon dioxide have different partial pressures in the lungs and tissues, so they move in opposite directions simultaneously. This independent movement allows efficient simultaneous uptake of oxygen and removal of carbon dioxide without direct interaction between the two gases.