Medical brain notes 27

  CHARACTERISTICS OF BLOOD   Blood has distinctive physical characteristics:   Amount —a person has 4 to 6 liters of blood, depend-ing on his or her size. Of the total blood volume in the human body, 38% to 48% is composed of the various blood cells, also called  formed elements . The remaining 52% to 62% of the blood volume is plasma, the liquid portion of blood (Fig. 11–1).   Figure 11–1.  Components of blood and the relationship of blood to other body tissues.   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 oxy-gen 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. pH —the n...

  PLASMA   Plasma  is the liquid part of blood and is approxi-mately 91% water. The solvent ability of water enables the plasma to transport many types of sub-stances. 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 urine. Hormones produced by endocrine glands are carried in the plasma to their tar-get 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 (HCO 3 – ). When the blood reaches the lungs, the CO 2  is re-formed, diffuses into the alveoli, and is exhaled. Also in the plasma are the  plasma proteins . The clotting factors  prothrombin ,  fibrinogen , and others are synthesized by the liver and circulate until acti-vated to form a clot in a ruptured or damaged ...

  BLOOD CELLS   There are three kinds of blood cells: red blood cells, white blood cells, and platelets. Blood cells are pro-duced 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 lympho-cytes.     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 that red blood cells are the only human cells with-out nuclei. Their nuclei disintegrate as the red blood cells mature and are not needed for normal func-tioning.   A normal RBC count ranges from 4.5 to 6.0 million cells per microliter ( m L) of blood...

  WHITE BLOOD CELLS   White blood cells (WBCs) are also called  leukocytes . There are five kinds of WBCs; all are larger than RBCs and have nuclei when mature. The nucleus may be in one piece or appear as several lobes or segments. Special staining for microscopic examination gives each kind of WBC a distinctive appearance (see Figs. 11–2 and 11–3). A normal WBC count (part of a CBC) is 5,000 to 10,000 per  m L. Notice that this number is quite small compared to a normal RBC count. Many of our WBCs are not circulating within blood vessels but are carrying out their functions in tissue fluid or in lym-phatic tissue.     Classification   The five kinds of white blood cells, all produced in the red bone marrow (and some lymphocytes in lymphatic tissue), may be classified in two groups: granular and agranular. The granular leukocytes are the  neu-trophils ,  eosinophils , and  basophils , which usually   have nuclei in two or more lobe...

  PLATELETS   The more formal name for platelets is  thrombocytes , which are not whole cells but rather fragments or pieces of cells. Some of the stem cells in the red bone marrow differentiate into large cells called  megakary-ocytes  (see Figs. 11–2 and 11–3), which break up into   small pieces that enter circulation. These small, oval, circulating pieces are platelets, which may last for 5 to 9 days, if not utilized before that.  Thrombopoietin  is a hormone produced by the liver that increases the rate of platelet production.   A normal platelet count (part of a CBC) is 150,000 to 300,000/ m L (the high end of the range may be extended to 500,000). Thrombocytopenia  is the term for a low platelet count.   Function   Platelets are necessary for  hemostasis , which means prevention of blood loss. There are three mechanisms,  and platelets are involved in each. Two of these mech-anisms are shown in Fig. 11–6.  ...