The RBCs play a major role in the transport of both oxygen and carbon dioxide.
In the lungs, oxygen diffuses from the air in alveoli into the blood of surrounding capillaries. Most of the oxygen enters RBCs and combines with the heme portions of hemoglobin (Hb) to form oxyhemoglobin (HbO2). About 98.5% of the oxygen is transported as oxyhemoglobin. The remaining 1.5% is dissolved in the plasma.
In body tissues, oxyhemoglobin releases oxygen and it diffuses from capillary blood into the interstitial fluid before entering the tissue cells. Actually, only about 25% of the oxygen is released in healthy individuals at rest, so oxyhemoglobin is present even in deoxy- genated blood.
The reason that hemoglobin is such an effective carrier of oxygen is that the chemical bond between oxygen and hemoglobin is relatively unstable. When the surrounding oxygen concentration is high, as in the lungs, hemoglobin combines readily with oxygen; but when the surrounding oxygen concentration is low, as in body tissues, hemoglobin releases oxygen.
The transport of carbon dioxide is more complex. Carbon dioxide diffuses from body cells into the interstitial fluid before entering the capillary blood. After carbon dioxide enters the blood, it is transported in one of the three ways:
- About 7% is dissolved in the plasma.
- About 23% enters RBCs and combines with hemoglobin to form carbaminohemoglobin (HbCO2). Carbon dioxide combines with the globin (protein) portion of hemoglobin, so carbon dioxide and oxygen have different binding sites on hemoglobin. Therefore, hemoglobin can transport oxygen and carbon dioxide at the same time.
The remaining 70% of the carbon dioxide also enters RBCs, but it quickly combines with water to form carbonic acid (H2CO3). This reaction is catalyzed by the enzyme carbonic anhydrase. Carbonic acid rapidly breaks down (dissociates) into hydrogen ions (H+) and bicarbonate ions (HCO3-). Carbon dioxide is now part of the bicarbonate ions, which then diffuse out of the RBCs and are transported to the lungs in plasma.
When the blood returns to the lungs, all of these reactions run in reverse, releasing carbon dioxide, which diffuses into the alveoli.