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Blood

Composition and Functions of Blood

1. Blood is composed of a nonliving fluid matrix (plasma) and formed elements (Figure 10.1). It is scarlet to dull red, depending on the amount of oxygen carried. Normal adult blood volume is 5 to 6 liters.

2. Dissolved in plasma (primarily water) are nutrients, gases, hormones, wastes, proteins, salts, and so on. Plasma composition changes as body cells remove or add substances to it, but homeostatic mechanisms act to keep it relatively constant. Plasma makes up 55 percent of whole blood.

3. Formed elements, the living blood cells that make up about 45 percent of whole blood, include:

   1. Erythrocytes, or RBCs - disk-shaped cells, lacking a nucleus, that transport oxygen bound to their hemoglobin molecules. Their lifespan is 100 to 120 days.

   2. Leukocytes, or WBCs - ameboid cells involved in protection of the body.

   3. Platelets - cell fragments that act in blood clotting.

4. A decrease in oxygen-carrying ability of blood is anemia. Possible causes are decrease in number of functional RBCs or decrease in amount of hemoglobin they contain. Polycythemia is an excessive number of RBCs that may result from bone marrow cancer or a move to a location where less oxygen is available in the air (at high altitude, for example).

5. Leukocytes are nucleated cells, classed into two groups:

   1. Granulocytes include neutrophils, eosinophils, and basophils.

   2. Agranulocytes include monocytes and lymphocytes.

6. When bacteria, viruses, or other foreign substances invade the body, WBCs increase in number (leukocytosis) and fight them in various ways.

7. An abnormal decrease in number of WBCs is leukopenia. An abnormal increase in WBCs is seen in infectious mononucleosis and leukemia (cancer of leukocytes).

8. All formed elements arise in red bone marrow from a common stem cell, the hemocytoblast. However, their developmental pathways differ. The stimulus for hematopoiesis is hormonal (erythropoietin in the case of RBCs).

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Hemostasis

1. Stoppage of blood loss from an injured blood vessel, or hemostasis, involves three steps: vascular spasms, platelet plug formation, blood clot formation (Figure 10.2).

2. Hemostasis is started by a tear or interruption in the blood vessel lining. Platelets adhere to the damaged site and release serotonin, which causes vasoconstriction. Platelet PF3 and tissue cell thromboplastin initiate the clotting cascade, leading to formation of fibrin threads. Fibrin traps RBCs as they flow past, forming the clot.

3. Normally, clots are digested when a vessel has been permanently repaired. An attached clot mat forms or persists in an unbroken blood vessel is a thrombus: a clot traveling in the bloodstream is an embolus.

4. Abnormal bleeding may reflect a deficit of platelets (thrombocytopenia), genetic factors (hemophilia), or inability of the liver to make clotting factors.

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Blood Groups and Transfusions

1. Blood groups are classified on the basis of proteins (antigens) on RBC membranes. Complementary antibodies may (or may not) be present in blood. Antibodies act to agglutinate (clump) and lyse foreign RBCs.

2. The blood group most commonly typed for is ABO (Figure 10.3). Type 0 is most common; least common is AB. ABO antigens are accompanied by preformed antibodies in plasma, which act against RBCs with "foreign" antigens.

3. Rh factor is found in most Americans. Rh- people do not have preformed antibodies to Rh+ RBCs but form them once exposed to Rh+ blood.