Blood, one of the body's major fluid tissues, continuously circulates through the heart and blood vessels, carrying vital elements to every part of the body.
Reviewing blood basics
Blood performs several physiologically vital functions through its special components: the liquid portion (plasma) and the formed constituents (erythrocytes, leukocytes, and platelets) that are suspended in it. Erythrocytes, or red blood cells (RBCs), carry oxygen to the tissues and remove carbon dioxide from them. Leukocytes, or white blood cells (WBCs), participate in inflammatory and immune responses. Plasma carries antibodies and nutrients to tissues and carries waste away; coagulation factors in plasma, with platelets (thrombocytes), control clotting.
The average person has 5 to 6 L of circulating blood, Which constitutes 5% to 7% of body weight (as much as 10% in premature neonates). Blood is three to five times more viscous than water, with an alkaline pH of 7.35 to 7.45, and is either bright red (arterial blood) or dark red (venous blood), depending on the degree If oxygen saturation and the hemoglobin level.
Formation and characteristics
Hematopoiesis occurs primarily in the red bone marrow of the long bones and axial skeleton where primitive cells (stem cells) produce the precursors of erythrocytes (normoblasts), leukocytes, and thrombocytes (megakaryocytes). During embryonic development, blood cells are derived from mesenchyma and form in the yolk sac. As the fetus matures, blood cells are produced in the liver, spleen, and thymus; by the 5th month of gestation, blood cells also begin to form in the bone marrow. After birth, blood cells are usually Produced only in the marrow.
The most important function of blood is to transport oxygen (bound to RBCs inside hemoglobin) from the lungs to the body tissues and to return carbon dioxide from these tissues to the lungs. Blood also performs the following vital defensive and protective functions:
Other functions of blood include control of hemostasis by platelets, plasma, and coagulation factors that repair tissue injuries and prevent or halt bleeding; regulation of acid-base and fluid balance; regulation of body temperature by carrying off excess heat generated by the internal organs for dissipation through the skin; and transportation of nutrients and regulatory hormones to body tissues and transportation of metabolic wastes to the organs of excretion (kidneys, lungs, and skin).
Because of the rapid reproduction of bone marrow cells and the short life span and minimal storage in the bone marrow of circulating cells, bone marrow cells and their precursors are particularly vulnerable to physiologic changes that can affect cell production. Resulting blood disorders may be primary or secondary, quantitative or qualitative, or both; they may involve some or all blood components.
Quantitative blood disorders result from increased or decreased cell production or cell destruction; qualitative blood disorders stem from intrinsic cell abnormalities or plasma component dysfunction. Specific causes of blood disorders include trauma, chronic disease, surgery, malnutrition, drugs, exposure to toxins and radiation, and genetic and congenital defects that disrupt production and function. For example, depressed bone marrow production or mechanical destruction of mature blood cells can reduce the number of RBCs, platelets, and granulocytes, resulting in pancytopenia (anemia, thrombocytopenia, granulocytopenia). Increased production of multiple bone marrow components can follow myeloproliferative disorders.
Diagnosing hematologic disorders
Laboratory studies that help determine blood composition, production, and function are vital in diagnosing hematologic disorders. Other common studies include tests to evaluate the coagulation and agglutination properties of the blood and biopsies to evaluate the blood's formed elements.
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