Chapter 11 The Blood 327 Copyright Goodheart-Willcox Co., Inc. Hemoglobin is actually composed of two molecules: a large protein called globin and an iron-containing molecule called heme. Oxygen binds to the heme molecule, and carbon dioxide binds to globin. Each hemoglobin molecule has four heme binding sites for oxygen (Figure 11.5). In the capillaries of the lung, when oxygen binds to the heme molecule on hemoglobin, an oxyhemo- globin molecule is formed. These oxygen-rich RBCs travel to systemic tissue capillaries. Within these capillaries, the hemoglobin in the RBCs unloads its oxygen, and the oxygen diffuses from the blood into the oxygen-deprived tissues. As carbon dioxide diffuses from the tissues into the RBCs, it binds to the globin protein of a hemoglobin molecule, and a carbaminohemoglobin molecule is formed. The carbaminohemoglobin molecule trans- ports some of the carbon dioxide from the tissues back to the lungs for elimination. Erythropoiesis The kidneys regulate red blood cell production through a process called erythropoiesis. When blood oxygen content decreases, a condition known as hypoxia, the kidneys secrete a hormone called erythropoietin (EPO). Erythropoietin stimulates stem cell production of RBCs in the red bone marrow (Figure 11.6). When additional RBCs are produced and blood oxygen levels rise, erythropoietin levels diminish, slowing RBC production. Any condition or factor, such as emphysema or high-altitude exposure, that results in hypoxia causes the kidneys to release erythropoietin nutrients needed due to the increase in metabolism. Plasma plays an important role in each of these responses. It would be impossible to maintain the body’s homeostasis without it. Erythrocytes Red blood cells (RBCs), also called erythrocytes, are part of one of the most important functions in the body—gas exchange. They carry oxygen to every living cell in the body and carry carbon dioxide away. RBCs are the most abundant cells in the blood, numbering between 4 and 6 million per cubic millimeter. A cubic millimeter is so small that it is almost invisible to the naked eye. Imagine how small RBCs must be to fit 4 to 6 million in a tiny, nearly invisible speck! Red blood cells measure only 7 or 8 micrometers in diameter. To put this measurement into perspective, 1 millimeter is equal to 1,000 micrometers. Shape and Size Mature red blood cells are biconcave disc-shaped cells that look like a disc with a flattened middle because mature red blood cells have no nucleus. As the RBC develops, its nucleus is forced out, causing the center of the cell to collapse. This mechanism of development serves three important functions: • It increases the surface area of the cell, providing a larger area for oxygen and carbon dioxide diffusion. • It increases the flexibility of the cell, allowing it to change shape so it can fit into capillary openings that are half its size. • It limits the cell’s life span to 120 days. Without a nucleus, the cell is unable to replicate. Hemoglobin The hemoglobin molecule is the functional part of the red blood cell because it carries out the important job of gas exchange. Approximately 98% of oxygen and 20%–30% of carbon dioxide in the body attach or bind to the hemoglobin molecules of the RBCs. As a result, hemoglobin is known as the binding site of a red blood cell. Hemoglobin levels differ between men and women. In men, they range from 13.5 to 17.5 grams per deciliter (g/dL). In women, they range from 12 to 15 g/dL. One RBC contains approximately 280 million hemoglobin molecules. This means that one RBC can carry over 1 billion oxygen molecules. Red blood cell Heme binding site for oxygen Heme binding site for oxygen Heme binding site for oxygen Heme binding site for oxygen © Body Scientifi c International Figure 11.5 During gas exchange, oxygen binds to hemoglobin at each of the four heme binding sites.