Concept
Body fluid is extracellular fluid which leaves the heart and returns again to the heart
after circulating through various tissues. The flow of this fluid is due only to a difference
in fluid pressure which is mainly caused by the contractile action of the heart，vascular
walls and the surrounding tissues and to osmosis.
Mechanism
The body fluid leaves the heart，circulates through the whole body and returns again to
the heart and its changes in velocity，pressure，distribution，volume and back flow is under
the control of many regulatory mechanisms of the circulation system. The main force of
circulation is due to a difference in fluid pressure produced by contraction and dilatation of the atriums and ventricles of the heart. The flow of body fluid receives further control
of pressure，velocity and distribution in the arterial system. In the capillary area，the
contractility of the walls regulates the fluid flow，and the body fluid flows to the tissue
spaces by the difference in hydrostatic pressure. In the tissues，the body fluid flows in two ways; one is from the tissue spaces to the blood capillaries by the difference in effective
osmosis，and the other way is from the tissue spaces to the lymph capillaries by the difference in hydrostatic pressure. Then，the volume of this circulating fluid is regulated by
the spleen，liver，large veins and lymphatic net work.The returning flow from the blood
and lymph capillaries to the heart is regulated during the passage through the venous and
lymphatic systems.
The heart is the center of circulation of the body fluid，and the tissue spaces are the
most peripheral parts. All vessels; arteries，veins and lymphatics are not simply tubes.
These vessels themselves have an important ro¥e in the control of the pressure，velocity，
distribution，outward flow，volume and returning flow of fluid to the heart.