Final answer:
White blood cells migrate from the bloodstream into surrounding tissues through a process known as diapedesis or emigration, primarily occurring in venules. Leukocytes adhere to the vessel's endothelial lining and squeeze through to reach the site of infection or injury, performing functions critical to the immune response.
Step-by-step explanation:
The process by which white blood cells, or leukocytes, migrate out of the bloodstream and into surrounding tissues is called diapedesis or emigration. This phenomenon occurs primarily in venules, the very small veins which are the site where multiple capillaries converge after exchanging materials in the capillary beds. Venules and capillaries are the main sites where leukocytes adhere to the endothelial lining, a process facilitated by the thin walls consisting of endothelium, and then move through the vessel wall by squeezing between adjacent endothelial cells.
During an immune response, leukocytes are drawn to the site of infection or injury. They exit the bloodstream through diapedesis—leaving capillaries and venules—before moving through the connective tissue. This migration allows them to perform their essential functions, such as releasing chemicals to destroy pathogens and carrying out phagocytosis, the engulfing and digesting of pathogens. This is crucial for the immune response as different types of leukocytes, including granular ones like eosinophils and neutrophils, as well as agranular ones like monocytes which transform into macrophages, can effectively target and eliminate invaders in affected tissues.