Blood accounts for 8% of human body weight.Platelets are critically important for wound healing, which can only occur once the clot forms and bleeding ceases completely.
Platelets secrete factors that increase local platelet aggregation (e.g., thromboxane A), enhance vasoconstriction (e.g., serotonin), and promote blood coagulation (e.g., thromboplastin, fibrinogen). The sticky surface of platelets allows them to accumulate at the site of broken blood vessels to form a clot, due in part to the release of clotting factors that occurs during endothelial injury to blood vessels.
Platelets contain mitochondrial DNA, but not nuclear DNA. Platelets are produced at a rate of 200 billion per day, a process regulated by the hormone thrombopoietin. They result from fragmentation of large cells called megakaryocytes, which are derived from stem cells in the bone marrow. These membrane-bound cell fragments lack nuclei and are responsible for blood clotting (coagulation). Thrombocytes measure between one to two micrometers in diameter. There are several different types of white blood cells: basophils, eosinophils, neutrophils, monocytes, natural killer cells, B- and T-cell lymphocytes, macrophages, and dendritic cells, all of which perform distinct functions. They destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. WBCs are the main functional component of the body’s immune system. They lack hemoglobin but contain organelles, a nucleus, and nuclear DNA. Leukocytes are usually larger in size (10–14 micrometers in diameter) than red blood cells. Leukocytes (WBCs)Ĭomponents of Blood: From left to right, a diagram of an erythrocyte, thrombocyte, and leukocyte. The combined surface area of all red blood cells of the human body would be roughly 2,000 times greater than the body’s exterior surface. The ratio of RBCs to blood plasma is referred to as the hematocrit, and is normally about 45%. RBCs, endothelial vessel cells, and other blood cells are also marked by glycoproteins that define the different blood types. Mature RBCs lack a nucleus and organelles and have no nuclear DNA. RBCs contain hemoglobin molecules which bind to oxygen so it can be transported to tissues. Erythrocytes (RBCs)Įrythrocytes are discs measuring about seven to eight micrometers in diameter. Blood cells are essential for normal metabolic and immune system function. Oxygen, carbon dioxide, and glucose are among the most vital molecules transported in blood. Although it consists of cells suspended in fluid, blood is still considered a tissue as it is technically a type of extracellular matrix.īlood enables transport of cells and molecules between parts of the body.
By volume, the RBCs constitute about 45% of whole blood, the plasma about 54.3%, and white blood cells about 0.7%. The cellular components of blood are erythrocytes (red blood cells, or RBCs), leukocytes (white blood cells, or WBCs), and thrombocytes (platelets). Left tube: after standing, the RBCs have settled at the bottom of the tube.īlood is a circulating tissue composed of fluid, plasma, and cells. Composition of Blood: Two tubes of EDTA-anticoagulated blood.
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