Anatomy & Physiology

Anatomy & Physiology Study Guide proper or lymphoid organs at any given moment. Only a minute fraction of the total WBC population is found in circulation. WBC Circulation and Movement Unlike RBCs, WBCs circulate for only a short portion of their lifespan. White blood cells migrate through the loose and dense connective tissues of the body, using the bloodstream primarily to travel from one organ to another and for rapid transportation to areas of infection or injury. WBCs detect the chemical signs of damage to surrounding tissues and exit the bloodstream and go to the damaged area. Circulating WBCs have four characteristics: • All can migrate out of the bloodstream: When white blood cells in the bloodstream become activated, they contact and adhere to the vessel walls in a process called margination. After further interaction with endothelial cells, the activated WBCs squeeze between adjacent endothelial cells and enter the surrounding tissue. This process is called emigration, or diapedesis. • All are capable of amoeboid movement: The gliding motion of amoeboid movement is accomplished by the flow of cytoplasm into slender cellular processes extended in front of the cell. (The movement is similar to that of an amoeba, a type of protozoan). The mechanism involves the continuous rearrangement of bonds in the cytoskeleton between actin filaments, and it requires ATP and calcium ions. However, it is not fully understood. This mobility allows WBCs to move through the endothelial lining and into peripheral tissues. • All are attracted to specific chemical stimuli: This characteristic, called positive chemotaxis, guides WBCs to damaged tissues, invading pathogens, and other active WBCs. • Neutrophils, eosinophils, and monocytes are capable of phagocytosis: These cells may engulf pathogens, cell debris, or other materials. Neutrophils and eosinophils are sometimes called microphages; this is to distinguish them from the larger macrophages in connective tissues. Macrophages are monocytes that have traveled from of the bloodstream and have become actively phagocytic. Types of WBCs Neutrophils, monocytes, basophils, and eosinophils contribute to the nonspecific defenses of the body. A variety of stimuli activate these defenses, but they do not discriminate between one type of threat and another. Fifty to seventy percent of the flowing WBCs are neutrophils. This name reflects the fact that the granules of these WBCs are chemically neutral and thus are difficult to stain with either acidic or basic dyes. Neutrophils are highly mobile, and consequently are the first of the WBCs to arrive at the site of an injury. These very active cells specialize in attacking and digesting bacteria that have been “marked” with antibodies or with complement proteins—plasma proteins involved in tissue defenses. Upon encountering a bacterium, a neutrophil quickly engulfs it, and the metabolic rate of the neutrophil increases dramatically. Most neutrophils have a short lifespan. They can survive in the bloodstream for only about 10 hours. However, they last less than 30 minutes when actively engulfing debris or pathogens. A neutrophil will die after engulfing one to two dozen bacteria, but its breakdown releases chemicals that attract

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