All moments of fluids within the body occurs as a result of following forces;
Osmotic pressure; is contributed by dissolved substances in a solution. All organic and inorganic constituents dissolved in body fluids contribute to osmotic pressure. This pressure tends to retain water in the compartment. Osmotic pressure in various fluid compartments is equal and normally maintained by shift of water from relatively dilute solution to concentrated one.
Hydrostatic pressure; is exerted by the force of fluid present in the compartment. It tends to push fluid out of the compartment. For example in a hose pipe, the water pressure at the tap end is greater than the water pressure at the nozzle end and this pressure gradient is necessary to maintain the flow. This is hydrostatic pressures - which "try" to force the water out of the pipe. In a blood vessel, the hydrostatic pressure at the arterial end is usually higher than at the venous end. In vascular compartment, the blood pressure is equal to the hydrostatic pressure and vice versa. Balance of above forces (resultant force) determines the magnitude and direction of flow of fluid between various compartments.
The human body is composed of trillions of cells all working together for the maintenance of the entire organism. While cells may perform very different functions, all the cells are quite similar in their metabolic requirements. Maintaining a constant internal environment with all that the cells need to survive (oxygen, glucose, mineral ions, waste removal, and so forth) is necessary for the well-being of individual cells and the well-being of the entire body. The varied processes by which the body regulates its internal environment are collectively referred to as homeostasis
Homeostasis in a general sense refers to stability, balance or equilibrium. It is the body's attempt to maintain a constant internal environment. Maintaining a stable internal environment requires constant monitoring and adjustments. This adjusting of physiological systems within the body is called homeostatic regulation.
The body fluids play an important role in homeostasis by providing an immediate environment to the living cell. In order to maintain the life processes the cells must get appropriate concentrations of oxygen, carbon dioxide, nutrients, electrolytes, and other substances within the tissue fluid. For this, digestive system helps to maintain a steady concentration of nutrients, respiratory system helps to maintain steady concentrations of oxygen and the removal of carbon dioxide and urinary system eliminates surplus and waste products.