Composition of Blood
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2.1 Blood is composed of roughly 55 percent plasma, which carries the red and white blood cells and the platelets (Blood, 2004). Blood normally contains from 4.5-6.0 million red cells per uL, and they have a lifespan of roughly 120 days in the circulation (Normal, 2004). Red cells, known as erythrocytes, carry oxygen from the lungs to the tissues, and pick up carbon dioxide which they take back for expulsion from the lungs (Blood, 2004). These gases are carried bound to hemoglobin, a protein that contains the element iron, and is red in color. The second major category of cells in blood are the white cells, which number between 5,000 and 10,000 per uL (Normal, 2004). These are divided into lymphocytes, polymorphonuclear leukocytes (PMN), basophils, eosinophils, and monocytes (Blood, 2004). White blood cells tend to have a shorter life-cycle than red cells, usually from a few days to a few weeks. Platelets are the third formed element in blood, and are involved in blood clotting, and usually number from 200,000 to 500,000 per uL. All the various blood cells are produced in the bone marrow, and arise from a single pluripotent stem cell (Blood, 2004). These cells are fairly rare, and are attached to osteoblasts lining the inner surfaces of bone cavities. By mitosis, they produce two kinds of progeny - more stem cells, and cells that differentiate along the paths that lead to the different kinds of blood cells. Cytokines and hormones determine which types of blood
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arasitic invaders. Basophils leave the bloodstream and accumulate at the site of an infection, where they release granules containing histamine, serotonin, prostaglandins, and leukotrienes which increase blood flow to the area and add to the inflammatory response. These mediators also play a role in response to allergens in hay fever and the anaphylactic response to insect stings.
Monocytes leave the bloodstream to become macrophages (Blood, 2004). They are large phagocytic cells which can engulf bacteria, or dead and dying cells of the body, and other foreign material (antigens) that enters the body. They generally migrate to sites of infections or tissue damage. Platelets are fragments of megakaryocytes. Their numbers are usually maintained by a negative feedback homeostatic mechanism. If their numbers drop below 50,000 per uL, there is a danger of uncontrolled bleeding because of the major role they play in blood clotting.
The white blood cells protect the body from invasion by foreign organisms such as bacteria, viruses and parasites, protection it against invasion by toxic substances, and help clear up the debris left over by infections and tissue damage from traumatic events. They are a surveillance mechani
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Approximate Word count = 1482
Approximate Pages = 6 (250 words per page)
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