Hemostasis is the process that prevents blood loss from the circulation when a blood vessel is ruptured by an injury. Events in hemostasis include the following:
1. Contraction of the smooth muscles in the blood vessel wall. This reduces the flow of blood and loss from the defect in the vessel wall. The term for this reduction in the diameter of a vessel is vasoconstriction.
2. Formation of a platelet plug. Activated platelets be come sticky and adhere to the defect to form a temporary plug.
3. Formation of a blood clot.
The many substances necessary for blood clotting, or coagulation, are normally inactive in the bloodstream. A balance is maintained between compounds that promote clotting, known as procoagulants, and those that prevent clotting, known as anticoagulants. In addition, there are chemicals in the circulation that act to dissolve any unnecessary and potentially harmful clots that may form. Under normal conditions, the substances that prevent clotting prevail. When an injury occurs, however, the procoagulants are activated, and a clot is formed. The clotting process is a well-controlled series of separate events involving 12 different factors, each designated by a Roman numeral. The final step in these reactions is the conversion of a plasma protein called fibrinogen into solid threads of fibrin, which form the clot. A few of the final steps involved in blood clot formation are described below and diagrammed in Figure 9-8:
* Substances released from damaged tissues result in the formation of prothrombinase, a substance that triggers the final clotting mechanism.
* Prothrombinase converts prothrombin in the blood to thrombin. Calcium is needed for this step.
* Thrombin, in turn, converts soluble fibrinogen into insoluble fibrin. Fibrin forms a network of threads that entraps plasma and blood cells to form a clot.
Blood clotting occurs in response to injury. Blood also clots when it comes into contact with some surface other than the lining of a blood vessel, for example, a glass or plastic tube used for a blood specimen. In this case, the preliminary steps of clotting are somewhat different and require more time, but the final steps are the same as those described above. The fluid that remains after clotting has occurred is called serum (plural, sera). Serum contains all the components of blood plasma except the clotting factors, as expressed in the formula: Plasma = serum + clotting factors
Figure 9-8 Final steps in blood clot formation.
Blood cells are constantly formed through a process called hemopoiesis (fig. 16.6). The term erythropoiesis refers to the formation of erythrocytes; leukopoiesis refers to the formation of leukocytes. These processes occur in two classes of tissues. Myeloid tissue is the red bone marrow of the humeri, femora, ribs, sternum, pelvis, and portions of the skull that produces erythrocytes, granular leukocytes, and platelets.
Lymphoid tissue-including the lymph nodes, tonsils, spleen, and thymus-produces the agranular leukocytes (monocytes and lymphocytes). During embryonic and fetal development, the hemopoietic centers are located in the yolk sac, liver, and spleen. After birth, the liver and spleen become the sites of blood cell destruction. Erythropoiesis is an extremely active process. It is estimated that about 2.5 million erythrocytes are produced every second in order to replace the number that are continuously destroyed by the liver and spleen. (Recall that the life span of an erythrocyte is approximately 120 days.) During the destruction of erythrocytes, iron is salvaged and returned to the red bone marrow where it is used again in the formation of erythrocytes. Agranular leukocytes remain functional for 100 to 300 days under normal body conditions. Granular leukocytes, by contrast, have an extremely short life span of 12 hours to 3 days. Hemopoiesis begins the same way in both myeloid and lymphoid tissues (fig. 16.6).
Undifferentiated mesenchymal-like cells develop into stem cells called hemocytoblasts. These stem cells are able to divide rapidly. Some of the daughter cells become new stem cells (hence, the stem cell population is never depleted), whereas other daughter cells become specialized along different paths of blood cell formation. Hemocytoblasts, for example, may develop into proerythroblasts which form erythrocytes; myeloblasts, which form granular leukocytes (neutrophils, eosinophils, and basophils); lymphoblasts, which form lymphocytes; monoblasts, which form monocytes; or megakaryoblasts, which form platelets. Stem cells are currently being harvested from fetal organs (the placenta and umbilical cord) with the intent of promoting their differentiation into an array of adult cells, making up specific organs such as the brain and pancreas. It is hoped that differentiated stem cells will replace diseased cells in vital organs. Other research is focusing on utilizing stem cells from one’s own red bone marrow and thus being more ethically acceptable and less likely to be rejected as foreign cells. The major purpose of a bone marrow transplant is to provide competent hemopoietic hemocytoblasts to the recipient. If the bone marrow is returned to the same person, the procedure is called an autotransplant. If the donor and recipient are different people, it is termed an allogenic transplant.
FIGURE 16.6 The processes of hemopoiesis. Formed elements begin as hemocytoblasts (stem cells) and differentiate into the various kinds of blood cells, depending on the needs of the body.