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Red blood cells, also called erythrocytes, are the predominant cell type in the blood. They are responsible for transport of oxygen from the lungs to body’s tissues, and removal of carbon dioxide in the reverse direction. Erythrocytes lack most of typical cell structures, they have no nucleus, and almost no organelles. This means they cannot regenerate, cannot synthesize new proteins, and cannot use the oxygen they are carrying. Erythrocytes are shaped almost like a donut, with biconcave surfaces. This unique shape increases the cell’s surface area for efficient gas exchange, while also being flexible to change when needed. Red cells contain structural proteins actin and spectrin, which make them resilient but also elastic, like pieces of memory foam. This elasticity, together with the donut shape, enables the cells to bend and fold on themselves, to squeeze through narrow capillaries, then spring back to their original shape in larger vessels.
The major component of red blood cells is a protein named hemoglobin. Hemoglobin is composed of four polypeptide chains, each of which is bound to a red pigment molecule called heme. Heme binds oxygen to a ferrous iron in its center. Thus, a molecule of hemoglobin can bind up to four molecules of oxygen. Binding of oxygen is a cooperative process: binding at one site changes the protein conformation in a way that facilitates further binding at other sites. Formation of the hemoglobin-oxygen complex, known as oxyhemoglobin, is reversible, depending on oxygen partial pressure. Oxygen binds in the lungs where its pressure is high, and disassociates in tissues, where its pressure is low.
While hemoglobin is responsible for transport of most of the oxygen, it only carries a small portion of carbon dioxide. Carbon dioxide binds to the polypeptide part of hemoglobin, and not the heme, but its binding changes the conformation of the molecule and decreases its affinity for oxygen. In other words, the two gases compete for binding on hemoglobin; oxygen binding is favored in the lungs, while carbon dioxide binding is more favorable in tissues.
The majority of carbon dioxide is transported in the blood in the form of bicarbonate ions. Conversion of carbon dioxide to carbonic acid, which dissociates into bicarbonate and hydrogen ions, is catalyzed by an enzyme present in red blood cells, called carbonic anhydrase. Bicarbonate ions then diffuse out to the plasma to be exchanged for chloride ions, while hydrogen ions bind to hemoglobin that has released oxygen.
When red blood cells reach the lungs, the reverse happens: high oxygen pressure favors its binding to hemoglobin, which releases hydrogen ions and carbon dioxide; the same carbonic anhydrase then converts bicarbonate and hydrogen ions back to carbon dioxide to be breathed out.
