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.
Macrophages. A type of white blood cell that surrounds and kills microorganisms, removes dead cells, and stimulates the action of other immune system cells. Monocytes. A type of immune cell that is made in the bone marrow and travels through the blood to tissues in the body where it becomes a macrophage.acrophages. A type of white blood cell that surrounds and kills microorganisms, removes dead cells, and stimulates the action of other immune system cells. Monocytes. A type of immune cell that is made in the bone marrow and travels through the blood to tissues in the body where it becomes a macrophage.