A the tissues the partial pressures of oxygen and carbon dioxide are opposite to those in the lungs. The partial pressure of oxygen in plasma is high compared with that in the tissues, so oxygen enters tissues. The partial pressure of carbon dioxide is lower in the plasma, so carbon dioxide leaves the tissues and enters the plasma.
Nearly all of the oxygen is carried bound to hemoglobin. Oxygen must first be released from hemoglobin to exit the red blood cell and enter the plasma where it is available for exchange with the interstitial fluids bathing the tissue. Oxygen release from hemoglobin is more efficient when oxygen levels are low and in actively metabolizing tissues. This is a direct result of the shape of the oxygen-hemoglobin saturation curve, which shifts to release more oxygen at lower pH, higher temperature and increased PCO2, as is found in metabolizing tissues. This causes more oxygen to be released at sites where it is most needed.
Carbon dioxide dissolves more readily in plasma than does oxygen, but most of it is carried as bicarbonate dissolved in the plasma. Carbon dioxide released from the tissues enters red blood cells which use the enzyme carbonic anhydrase to convert it to bicarbonate that enters plasma. This significantly increases the ability of the blood to carry carbon dioxide.
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