CHARACTERISTICS OF BLOOD
Blood has distinctive physical characteristics: Amount—a person has 4 to 6 liters of blood, depending on his or her size. Of the total blood volume in the human body, 38% to 48% is composed of various blood cells, also called formed elements. The remaining 52% to 62% of the blood volume is plasma, the liquid portion of blood. Color—you’re probably saying to yourself, “Of course, it’s red!” Mention is made of this obvious fact, however, because the color does vary. Arterial blood is bright red because it contains high levels of oxygen. Venous blood has given up much of its oxygen in tissues and has a darker, dull red color. This may be important in the assessment of the source of bleeding. If blood is bright red, it is probably from a severed artery, and dark red blood is probably venous blood.
Plasma is the liquid part of blood and is approximately 91% water. The solvent ability of water enables the plasma to transport many types of substances. Nutrients absorbed in the digestive tract, such as glucose, amino acids, and minerals, are circulated to all body tissues. Waste products of the tissues, such as urea and creatinine, circulate through the kidneys and are excreted in the urine. Hormones produced by endocrine glands are carried in the plasma to their target organs, and antibodies are also transported in plasma. Most of the carbon dioxide produced by cells is carried in the plasma in the form of bicarbonate ions (HCO3 –). When the blood reaches the lungs, the CO2 is re-formed, diffuses into the alveoli, and is exhaled.
There are three kinds of blood cells: red blood cells, white blood cells, and platelets. Blood cells are produced from stem cells in hemopoietic tissue. After birth, this is primarily the red bone marrow, found in flat and irregular bones such as the sternum, hip bone, and vertebrae. Lymphocytes mature and divide in lymphatic tissue, found in the spleen, lymph nodes, and thymus gland. The thymus contains stem cells that produce T lymphocytes, and the stem cells in other lymphatic tissue also produce lymphocytes.
RED BLOOD CELLS
Also called erythrocytes, red blood cells (RBCs) are biconcave discs, which means their centers are thinner than their edges. You may recall from Chapter 3 that red blood cells are the only human cells without nuclei. Their nuclei disintegrate as the red blood cells mature and are not needed for normal functioning.
A normal RBC count ranges from 4.5 to 6.0 million cells per microliter (L) of blood (1 microliter 1 mm3 one-millionth of a liter, a very small volume). RBC counts for men are often toward the high end of this range; those for women are often toward the low end. Another way to measure the number of RBCs is the hematocrit. This test involves drawing blood into a thin glass tube called a capillary tube and centrifuging the tube to force all the cells to one end.
Red blood cells contain the protein hemoglobin (Hb), which gives them the ability to carry oxygen. Each red blood cell contains approximately 300 million hemoglobin molecules, each of which can bond to four oxygen molecules. In the pulmonary capillaries, RBCs pick up oxygen and oxyhemoglobin is formed.
Production and Maturation
Red blood cells are formed in red bone marrow (RBM) in flat and irregular bones. Within red bone marrow are precursor cells called stem cells. Recall that stem cells are unspecialized cells that may develop, or differentiate, in several ways. The stem cells of the red bone marrow may also be called hemocytoblasts (hemo “blood,” cyto “cell,” blast “producing”), and they constantly undergo mitosis to produce all the kinds of blood cells, many of which are RBCs. The rate of production is very rapid (estimated at several million new RBCs per second), and a major regulating factor is an oxygen. If the body is in a state of hypoxia, or lack of oxygen, the kidneys produce a hormone called erythropoietin, which stimulates the red bone marrow to increase the rate of RBC production (that is, the rate of stem cell mitosis). This will occur following hemorrhage or if a person stays for a time at a higher altitude. As a result of the action of erythropoietin, more RBCs will be available to carry oxygen and correct the hypoxic state.
Red blood cells live for approximately 120 days. As they reach this age they become fragile and are removed from circulation by cells of the tissue macrophage system (formerly called the reticuloendothelial or RE system). The organs that contain macrophages (literally, “big eaters”) are the liver, spleen, and red bone marrow. The old RBCs are phagocytized and digested by macrophages, and the iron they contained is put into the blood to be returned to the red bone marrow to be used for the synthesis of new hemoglobin.
All of the functions of blood described in this chapter—transport, regulation, and protection—contribute to the homeostasis of the body as a whole. However, these functions could not be carried out if the blood did not circulate properly. The circulation of blood throughout the blood vessels depends upon the proper functioning of the heart, the pump of the circulatory system, which is the subject of our next chapter.