Osmotic gradients are rather transient by nature Osmosis from one fluid compartment to another is determined by the relative concentration of solutes in each compartment. Electrolytes are the more abundant solute molecules and thereby play a principal role in governing the body's water distribution and content. Water Gain and Loss A person is in a state of fluid balance when daily gains and losses are equal. Water gained is from two sources Metabolic water is produced as a byproduct of aerobic respiration. Preformed water is ingested in food and drink Routes of water loss include water in urine, feces, expired breath, sweat at rest, and cutaneous transpiration.
Water losses vary greatly with physical activity and environmental conditions.
Regulation of fluid balance Regulation of Intake Fluid intake is governed mainly by thirst. Dehydration reduces blood volume and pressure and raises its osmolarity. This results in a smaller volume and greater viscosity of saliva. Also during dehydration, salivation is inhibited by sympathetic output from the thirst center. Reduced salivation is not the primary motivation to drink Long-term satiation of thirst depends on absorbing water from the small intestine and lowering the osmolarity of the blood.