Red blood cells, or erythrocytes (e-rith'ro-sitz), are tiny, approximately 7.5 |im in diameter. They are biconcave disks which means that they are thin near their centers and thicker around their rims (fig. 14.4). This special shape is an adaptation for the red blood cell's function of transporting gases; it increases the surface area through which gases can diffuse. The shape also places the cell membrane closer to oxygen-carrying hemoglobin molecules within the cell. Because of its shape, a red blood cell can readily squeeze through the narrow passages of capillaries.
Each red blood cell is about one-third hemoglobin by volume. This protein is responsible for the color of the blood. The rest of the cell mainly consists of membrane, water, electrolytes, and enzymes. When the hemoglobin combines with oxygen, the resulting oxy-hemoglobin is bright red, and when the oxygen is released, the resulting deoxyhemoglobin is darker. Blood rich in deoxyhemoglobin may appear bluish when it is viewed through blood vessel walls.
A person experiencing prolonged oxygen deficiency (hypoxia) may become cyanotic. The skin and mucous membranes appear bluish due to an abnormally high blood concentration of deoxyhemoglobin. Exposure to low temperature may also result in cyanosis. Such exposure constricts superficial blood vessels, which slows blood flow and removes more oxygen than usual from blood flowing through the vessels.
Red blood cells have nuclei during their early stages of development but extrude them as the cells mature, which provides more space for hemoglobin. Since they lack nuclei, red blood cells cannot synthesize messenger RNA or divide. Because they also lack mitochondria, red blood cells produce ATP through glycolysis only, and use none of the oxygen they carry. As long as cytoplasmic enzymes function, these can carry on vital energy-releasing processes. With time, however, red blood cells become less and less active. Typically, this leads to more rigid red blood cells that are more likely to be damaged or worn, and eventually removed by the spleen and liver.
In sickle cell disease, a single DNA base change causes an incorrect amino acid to be incorporated into hemoglobin, causing hemoglobin to crystallize in a low oxygen environment. This bends the red blood cells containing the hemoglobin into a sickle shape, which blocks circulation in small vessels, causing excruciating joint pain and damaging many organs. As the spleen works harder to recycle the abnormally short-lived red blood cells, infection becomes likely.
Most children with sickle cell disease are diagnosed at birth and receive antibiotics daily for years to prevent
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.