Blood 2009 - 1 Blood 2010 Blood is a specialized CT...

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1 Blood 2010 Blood is a specialized CT composed of cells, fragments of cells (platelets), and the extracellular substance (plasma). Unlike other CT, fibers are only present when blood is clotting. Blood accounts for approximately 7% of our body weight. The average adult has 5-6 liters of blood Erythrocytes, leukocytes and platelets constitute the formed elements of blood which are suspended in plasma. Plasma is the aqueous solution containing proteins, vitamins, hormones, lipids, salts, and ions. Albumin is the most abundant protein, and is responsible for osmotic pressure in capillaries and venules. Blood has multiple functions. It maintains a constant internal environment; it transports oxygen, nutrients, and hormones to tissues; it carries waste products to the kidneys and carbon dioxide to the lungs; it conveys white cells to places where they are needed in defense and protection of the body. Erythrocytes are by far the most numerous cells in blood (5-6 million per microliter). They are in the shape of biconcave disks which provides a large surface-to-volume ratio, thus facilitating gas exchange. The cytoskeleton is responsible for this shape. Mature erythrocytes lack nuclei. Platelets also lack nuclei. The diameter of red cells is fairly constant at 7.5 micrometer. This figure is useful to know so that the size of other cells can be estimated by comparing them to a red cell. Mature red cells also lack organelles but the cytoplasm is packed with hemoglobin, the protein which binds reversibly O 2 and CO 2 . The membrane of red cells is very flexible to allow passage through tiny capillaries. Carbohydrates that bind to protein and lipids in the membrane determine the blood group antigens (A, B, O, Rh, etc). The lifespan of erythrocytes is 100-120 days. They are removed from the circulation by macrophages present in the spleen, liver, and bone narrow. Every second, 2.5 million new red cells enter the circulation and 2.5 million are removed! The function of erythrocytes is to carry oxygen to
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2 the tissues and to carry some of the carbon dioxide to the lungs for removal. Platelets are involved in the cascade of events that leads to formation of a blood clot. Like red cells, platelets do not have a nucleus but they do have organelles: mitochondria, lysosomes, ribosomes, and a complex array of microtubules. Unlike red cells, platelets never possessed a nucleus. They are really cytoplasmic fragments of giant polyploid cells found in bone marrow and know as megakaryocytes. Platelets are about 3 micrometer (so less than half the size of a red cell). Platelets contain several types of granules. These granules hold proteins and calcium ions as well as serotonin , all of which play important roles in hemostasis. Platelets survive just 5-10 days in circulating blood. Normally there are around 300,000 platelets per microliter of blood. Note that platelets, unlike red cells, never possessed a nucleus. Hemostasis is a very complex process which involves many proteins and
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Blood 2009 - 1 Blood 2010 Blood is a specialized CT...

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