Life Span of the White Blood Cells
The life of the granulocytes after being released from the bone marrow is normally 4 to 8 hours circulating in the blood and another 4 to 5 days in tissues where they are needed. In times of serious tissue infection, this total life span is often shortened to only a few hours because the granulocytes proceed even more rapidly to the infected area, perform their functions, and, in the process, are themselves destroyed.
The monocytes also have a short transit time, 10 to 20 hours in the blood, before wandering through the capillary membranes into the tissues. Once in the tissues, they swell to much larger sizes to become tissuemacrophages, and, in this form, can live for monthsunless destroyed while performing phagocytic func-tions. These tissue macrophages are the basis of the tissue macrophage system, discussed in greater detaillater, which provides continuing defense against infection.
Lymphocytes enter the circulatory system continu-ally, along with drainage of lymph from the lymph nodes and other lymphoid tissue. After a few hours, they pass out of the blood back into the tissues by dia-pedesis. Then, still later, they re-enter the lymph and return to the blood again and again; thus, there is con-tinual circulation of lymphocytes through the body. The lymphocytes have life spans of weeks or months; this life span depends on the body’s need for these cells.
The platelets in the blood are replaced about once every 10 days; in other words, about 30,000 platelets are formed each day for each microliter of blood.