What is the most important electrolyte for the maintenance of osmotic pressure and volume in the extracellular fluid compartment? The most abundant positive ion or cation in the extracellular fluid  is sodium (Na+). Chloride (Cl-) is the most abundant negative ion  or anion.
Medical Definition of MACROPHAGE. : a phagocytic tissue cell of the immune system that may be fixed or freely motile, is derived from a monocyte, functions in the destruction of foreign antigens (as bacteria and viruses), and serves as an antigen-presenting cellâsee histiocyte, mononuclear phagocyte system. a phagocytic tissue cell of the immune system that may be fixed or freely motile, is derived from a monocyte, functions in the destruction of foreign antigens (as bacteria and viruses), and serves as an antigen-presenting cell â compare histiocyte. â macÂ·roÂ·phagÂ·ic \Ëma-krÉ-Ëfa-jik\ adjective.
Before the lung macrophages get to the lungs, they have to be formed. The first step in a lung macrophage's life is to be formed in bone marrow as a monocyte, another type of white blood cell. Monocytes circulate in the blood, and only when they move into other tissues do they mature into macrophages.nside the lungs, macrophages can either populate the surface of the lungs as lung macrophages or move into the alveoli of the lungs, where they are known as alveolar macrophages.