Red Blood Cells
Objective 3
Describe the structure and function of the red blood cells (RBCs). Identify the components of the hemoglobin molecule and describe its function. Identify the ABO and Rh antigens that may be present on red blood cells and describe their inheritance and corresponding antibody development.
The primary function of red blood cells is to transport oxygen. Their unique characteristics enable them to do this with remarkable efficiency. Among these characteristics is their biconcave shape, their reversible deformity, their lack of a nucleus, and the fact that they do not consume the oxygen they carry.
Their biconcave shape gives them a high surface-to-volume ratio, thus allowing them to carry a great deal of oxygen in relation to their size. Think of this shape as a donut, but the hole in the middle doesn’t go all the way through.
RBCs are required to squeeze through really small spaces, specifically blood capillaries. As they pass through these small spaces, they become almost torpedo-like, but when they come out the other side, they revert back to their biconcave shape. The ability to change shape like this is called reversible deformity.
Red blood cells lack mitochondria, so they don’t produce ATP by oxidative metabolism. Instead, they utilize glycolysis to stay alive. By so doing, they use glucose for energy, but they transport all of the oxygen they pick up.
Because a RBC lacks a nucleus, it allows all of the cytoplasmic space to carry oxygen. Before the nucleus is lost, it provides the instructions to produce hemoglobin, the oxygen-carrying molecule of the red cell.
Red blood cells live for approximately 120 days. This isn’t too bad due to the fact that they are constantly damaged from squeezing through small spaces, and they don’t have a nucleus or any organelles to repair the damage that takes place. Once the damage has rendered the cells dysfunctional, they are removed from the circulation by phagocytic white blood cells, the spleen, and the liver.
As previously mentioned, hemoglobin is the oxygen-carrying molecule of the red blood cell. There are approximately 280 million hemoglobin molecules in each red cell. One hemoglobin molecule consists of two main components, heme and globin. Each polypeptide chain has a heme molecule bound to it. Heme is a ringed molecule with one iron (Fe2+) atom at the center. The iron atom is the binding site for oxygen; each Fe2+ can pick up one oxygen molecule (O2) at the lungs. So, if there are 280 million hemoglobin molecules in a red cell and each hemoglobin molecule has four hemes, and each heme can pick up one oxygen molecule, one red cell can carry 1.1 billion oxygen molecules. The concentration of hemoglobin in the blood ranges from 14-16 g/dl.
At the lungs, hemoglobin has a high affinity, or attraction to oxygen; it wants to pick it up and hold on to it. At the tissue level, hemoglobin has a lower affinity for oxygen. Hemoglobin wants to let go of oxygen, thereby allowing it to