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THE BLOOD

1. Blood transports dissolved gases, nutrients, waste products,enzymes, and hormones; regulates the pH and electrolyte concentration of body fluids; restricts fluid losses; defends against pathogens and toxins; and regulates body temperature.

THE COMPOSITION OF THE BLOOD

1. Blood is a connective tissue because it contains specialized blood cells, a ground substance (plasma), and protein fibers.

2. Whole blood can be fractionated for analytical or clinical purposes.

3. There are 5-6 liters of whole blood in the circulatory system of an adult man, and 4-5 liters in that of an adult woman.

The Plasma:

1. Plasma accounts for 55 percent of the volume of whole blood. Roughly 92 percent of plasma is water, and the rest consists of electrolytes and dissolved organic compounds of various kinds.

2. The exchange of dissolved materials occurs by diffusion across

the walls of capillaries.

3. The dissolved concentrations of gases, nutrients, and waste products differs depending on the region sampled and the metabolic state of the tissue and individual.

4. Plasma resembles interstitial fluid except for its relatively high concentration of dissolved proteins, including albumins, globulins, and fibrinogen.

The Plasma Proteins:

1. Albumin, the most abundant plasma protein, contributes to the osmotic pressure of the blood and provides a transport mechanism for specific insoluble or valuable materials in the blood.

2. Globular proteins are important in binding and transporting hormones, lipids (lipoproteins), and metal ions (metalloproteins). The immunoglobulins (antibodies) are proteins that attack foreign proteins and pathogens.

3. Under the proper stimulation, fibrinogen molecules aggregate to form large insoluble strands of fibrin that establish the basis for a blood clot. The removal of fibrinogen from plasma leaves a fluid called serum.

Cellular Components:

1. The blood contains red blood cells, white blood cells (granular andagranular), and platelets.

Red Blood Cells:

1. Red blood cells account for slightly less than half the blood volume. The hematocrit averages 46 in adult men and 42 in adult women.

2. There are roughly 5 million RBCs in each microliter of blood; they transport oxygen and carbon dioxide, and have large surface-to volume ratios.

3. Erythrocytes lack mitochondria, ribosomes, and nuclei.They are unable to perform normal maintenance operations and usually degenerate after about 120 days in the circulation.

4. Each red blood cell contains molecules of hemoglobin, a globular protein formed from four subunits. Each subunit contains a single molecule of the porphyrin heme, which can reversibly bind oxygen.

2. Heme molecules bind to oxygen when plasma concentrations are high; the oxygen is released when plasma concentrations decline.

Carbon dioxide molecules can be bound to the globin portion of the hemoglobin molecule.

3. Damaged or expired red blood cells are recycled by phagocytes. The proteins are disassembled into amino acids, and the iron gets bound to transferrins for transport to the bone marrow and liver. The heme units are not recycled, but removed from the circulation by the liver and excreted in the bile.

4. Agglutinogens A, B, and D (Rh) on the exposed surfaces of the red blood cells determine an individual's blood type. Agglutinins within the plasma will react with red blood cells bearing different agglutinogens.

5. Anti-Rh agglutinins are only synthesized after an Rh-negative individual becomes sensitized to the Rh agglutinogen.

6. Testing for compatibility involves the determination of blood type and a cross-match test.

7.Standard blood typing detects the A, B, and D (Rh) agglutinogens.The most common blood type used for transfusion is O-negative, sometimes called the universal donor.

The following list, from The American National Red Cross, shows the percentage of people in the United States with a particular blood type. These percentages may vary in certain sections of the country depending upon the cultural make up of the population.
O Positive - 38.4%
A Positive - 32.3%
B Positive - 9.4%
O Negative - 7.7%
A Negative - 6.5%
AB Positive - 3.2%
B Negative - 1.7%
AB Negative - 0.7%

8. Cross-matching involves exposing donor RBCs to the recipient's plasma to detect other possible cross reactions.

White blood cells:

1. White blood cells are components of the immune system that defends the body against pathogens, toxins, wastes, and abnormal or damaged cells and tissues.

2. There are 6,000-9,000 white blood cells in each microliter of whole blood.

3. Leukocytes show chemotaxis (attraction to chemicals) and diapedesis (ability to move through vessel walls).

4. Granular leukocytes include neutrophils, eosinophils , and basophils.

5. Neutrophils are abundant, highly mobile phagocytes. Eosinophils (less common) are attracted to foreign compounds coated with antibodies. Basophils (rare) migrate into damaged tissues and release histamine, aiding in the inflammation response.

6. Monocytes migrating into peripheral tissues become free macrophages.

7. Lymphocytes, cells of the lymphatic system, include T cells and B cells. T cells migrate to peripheral tissues and attack foreign or abnormal cells; B cells produce antibodies.

Platelets:

1. Megakaryocytes in the bone marrow release packets of cytoplasm, called platelets, into the circulating blood. There are 150,000-500,000 platelets in each microliter of whole blood.

THE CLOTTING SYSTEM

1. Hemostasis prevents the loss of blood at damaged vessels. It can be divided into the vascular, platelet, and coagulation phases.

2. The coagulation phase occurs as factors released by

endothelial cells ( extrinsic pathway ) and platelets (intrinsic pathway) interact with clotting factors to yield thromboplastin.

3. Thromboplastin triggers the conversion of prothrombin to thrombin, and thrombin catalyzes the formation of fibrin from circulating plasma fibrinogen.This results in the formation of a blood clot.

4. The blood clot gradually retracts (syneresis), pulling the damaged surfaces together. As repairs proceed the clot dissolves (fibrinolysis) through the action of plasmin, the activated form of circulating plasminogen.

5. The coagulation process requires calcium ions, and Vitamin K must be available for the synthesis of five of the clotting factors.

6. Excessive or inappropriate clotting can lead to the formation of a thrombus or an embolus . Anticoagulant drugs include heparin and coumadin; in some cases clots may be dissolved by activating plasminogen with streptokinase, urokinase, or tissue plasminogen activator.

HEMOPOIESIS

1. Circulating stem cells giving rise to embryonic blood cells migrate into the liver, spleen, thymus, and bone marrow.

2. The bone marrow is the major site of blood cell production in the normal adult.

3. Hemocytoblastsdivide to give rise to several populations of stem cells.The divisions of these cells produce the various blood cells throughout life.

Erythropoiesis

1. Erythropoiesis primarily occurs within red marrow of the sternum, vertebrae, skull, scapulae, pelvis, and proximal limb bones.

2. Red blood cell formation increases undererythropoiesis- stimulating factor (erythropoietin) stimulation. This hormone is released from the kidneys (or other tissues) when they are not receiving adequate supplies of oxygen.

3. Stages in red blood cell development include erythroblasts and reticulocytes. Reticulocytes usually account for 0.8 percent of circulating red blood cells.

White Blood Cell Formation

1. Granulocytes and monocytes are produced by stem cells in the bone marrow. Lymphocytes are produced in bone marrow, thymus, spleen, and other lymphoid tissues.

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