Body Fluids – Circulation
Many sorts of evidences have led biologists to the conclusion that life had its origin in the ancient seas. It is not surprising, therefore, that the protoplasm of early organisms had many properties in common, with seawater that bathed them, and that ' life processes evolved a close dependence on the stable conditions existing in seawater.
Figure: Fluid compartments of the body. All exchanges with the environment occur across plasma.
The evolution of multicellular animals involved the development of body fluids - tissue fluids, blood and lymph that could provide all body cells with a stable .and relatively non-fluctuating environment, and fill all spaces between cells and cell layers. The body fluids are divided into two main phases intracellular and extracellular. The extracellular fluid is further subdivided into blood plasma and interstitial fluid. The blood plasma is contained in blood vessels and interstitial fluid fills the spaces around the cells. Nutrients and gases passing between blood vessels and cells have to cross this fluid. The interstitial fluid is formed constantly by filtration from the blood plasma through the capillary walls.