The function of the circulatory system is to transport and distribute substances either used or produced by cells or both. Excluded are those materials that are discharged directly from sweat glands, digestive glands, and renal tubule cells. Included, however, are nutritive and metabolic substances, hormones, waste products, water, and heat.
Disturbances in the normal pattern of circulation can either result in or from disease conditions. An example is edema, which is an abnormal accumulation of fluid in the cells, tissue spaces, or cavities of the body. There are three main factors in the formation of generalized edema and a fourth which plays a role in the formation of local edema. They are the permeability of the capillary wall, the colloid osmotic pressure of the plasma proteins, and the hydrostatic pressure in the capillaries. The fourth factor, which is of importance in local edema formation, is lymphatic obstruction.
Increased permeability of the capillary walls plays an important role in the formation of inflammatory edema, the edema of severe infections, metabolic intoxications, asphyxia, anaphylactic reactions, secondary shock, and acute nephritis. See also Edema.
A deficiency of circulating blood volume, both cellular elements and fluid, is called oligemia. This may be the result of an acute blood loss or it may be of a chronic nature, such as an anemia combined with dehydration. Anemia, or oligochromemia, is a deficiency of circulating red cell volume or, more specifically, hemoglobin content. It can result from a variety of causes. See also Anemia; Blood.
Pancytopenia, or oligocythemia, is a deficiency of all circulating blood cellular elements. This is usually the result of a deficiency of the blood-forming tissue, the bone marrow.
The decrease of blood flow to an organ or tissue is known as ischemia. This can be sudden as when a vessel is ligated, when a thrombus or blood clot forms, or when an embolus comes to lodge in the vessel. A gradual occlusion can follow arteriosclerotic changes in the vessel wall. The effect of a sudden occlusion depends to a great extent on the collateral circulation to the organ or tissue involved. If an adequate collateral circulation comes to be established, the tissue survives; if not, it dies and an infarct results. See also Infarction.
An excess of blood is referred to as plethora. This increase may be the result of an increase in the size or the number of red blood cells. The increase in red cell volume may be a polycythemia vera, or true polycythemia, which is a primary increase in the number of red blood cells with no regard to the needs of the organism. Polycythemia, or erythrocytosis, is usually a secondary increase in red cells following conditions of chronic hypoxia. Serous plethora is an increase in the watery part of the blood.
Hyperemia, or congestion, refers to an excess of blood within an organ or tissue. This condition may be localized or generalized. Active hyperemia, caused by an active dilatation of arterioles and capillaries, occurs under certain physiological conditions, such as in the muscle when there is an increased need for blood during exercise. It also occurs in pathological states such as inflammation. Passive hyperemia, a condition which results in an accumulation of blood in the venous system, may be generalized or localized and can result from any obstruction or hindrance to the outflow of blood from the venous circuit. Diseases of the lungs such as emphysema, fibrosis, or pulmonary hypertension of any origin can result in right ventricular failure and generalized congestion. Localized venous congestion results when a main vein from a region is occluded either by a thrombus or some extrinsic pressure such as a tumor or enlarged lymph nodes.
The escape of blood from within the vascular system is hemorrhage. This process can be the result of trauma to, or disease of, the vessel wall. The causes of hemorrhage other than trauma can be divided into three main groups. In the first group are those conditions in which there is a disease process affecting the vessel wall, such as arteriosclerosis or aneurysm formation. In the second group are conditions in which there is an acute process affecting the vessel wall such as in septicemia, poisoning by heavy metals, or even anoxia. The third group consists of those conditions in which there is a defect in the blood itself, which results in hemorrhage. Apoplexy, or stroke, is an acute vascular lesion of the brain. This can be the result of hemorrhage of, thrombosis in, or embolism to a cerebral vessel. See also Hemorrhage.
Thrombosis is the formation of a thrombus, a solid body formed during life and composed of the elements of the blood: platelets, fibrin, red cells, and leukocytes. Thrombosis may occur on a vessel wall anywhere that the endothelium is damaged. However, because the platelets release thromboplastinogen, which activates the clotting mechanism, thrombosis and blood coagulation may occur together.
The sudden blocking of an artery or vein by a clot or other substance which has been brought to its place by the blood current is an embolism. The material carried in the circulation in this process is an embolus. Emboli may be composed of thrombi, fat, air, tumor cells, masses of bacteria or parasites, bone marrow, amniotic fluid, or atheromatous material from the vessel wall. See also Embolism.
