Body Fluids

A body fluid is simply water with dissolved substances in it, and every animal cell lives bathed in, or filled with, one of these fluids. Physiologists split body fluid into two broad compartments: the intracellular fluid (ICF), held inside the cell membrane, and the extracellular fluid (ECF), everything outside it. The ECF is further divided into three related fluids - interstitial (tissue) fluid, which directly surrounds each cell; plasma, the fluid part of blood inside vessels; and lymph, the fluid draining through the lymphatic vessels. These three are chemically similar but not identical, and fluid constantly moves between them at the capillary bed. Blood entering a capillary from an arteriole arrives under high hydrostatic (pumping) pressure, which pushes water and small solutes out through the thin, permeable capillary wall into the surrounding tissue, forming tissue fluid. At the arterial end this outward hydrostatic push exceeds the inward pull of osmotic (water) potential, so fluid nets outward; by the time blood reaches the venous end of the same capillary bed, hydrostatic pressure has dropped and the balance reverses, so fluid (with the wastes it has picked up from the cells) nets back into the capillary. Large plasma proteins are mostly left behind in the blood because they cannot cross the capillary wall, which is why tissue fluid has a much lower protein concentration than plasma. Blood itself is a specialised connective tissue made of two components: plasma, the liquid matrix, which makes up about 55% of total blood …