The exchange of gases between the blood and tissues at the cellular level.
The part of respiration concerned with the passage of oxygen from the blood to the tissues, and the passage of carbon dioxide from the tissues to the blood.
The exchange of gases in body tissues; oxygen diffuses from the blood to the cells and carbon dioxide diffuses from the cells to the blood. Oxygen is carried in combination with hemoglobin. Oxyhemoglobin gives arterial blood its red color; reduced hemoglobin gives venous blood its dark red color. Most carbon dioxide is carried in the blood as bicarbonate ions; a small amount is bonded to hemoglobin. Normally the partial pressure of oxygen in the blood is 75 to 100 mm Hg, depending on age; for carbon dioxide it is 35 to 45 mm Hg.