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.
PULMONARY IMMUNE CELLS: PULMONARY MACROPHAGES the pulmonary artery in order to remove peripheral bloodmonocytes and instillation of collagenase into the pul-monary artery and the trachea.Finally the lung was cutand further digested in collagenase.ocalization and origin of lung macrophages Based on their localization, the macrophages in thelung include four different types: 1) the alveolar macro-phage; 2) the interstitial macrophage; 3) the intravas-cular macrophage; and 4) the dendritic cell.