Blood is composed of two major ingredients: the liquid blood plasma and several types of cells suspended within the plasma. The cells constitute approximately 40% of the total blood volume and are grouped into three major categories: erythrocytes (red blood cells), leukocytes (white blood cells), and thrombocytes (platelets). The red blood cells remain within the blood vessels, and their function is to transport oxygen and carbon dioxide. The white blood cells fight infection, and thus are able to migrate out of the blood vessels and into the tissues. Platelets are not complete cells, but are small detached fragments of much larger cells called megakaryocytes. Their principal function is to aid in blood clotting.Leukocytes themselves are subdivided into a number of categories: granulocytes (approximately 65%), lymphocytes(30%),monocytes(5%), and natural killer cells. Gran-ulocytes are further subdivided into neutrophils (95%), eosinophils, (4%) and basophils (1%). Neutrophils phagocytose (i.e., ingest) and destroy small foreign bodies such as bacteria, basophils help mediate inflammatory reactions by secreting histamine, while eosinophils help to destroy parasites and modulate allergic responses.
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