The body requires a particular volume of fluid within it in order to function normally. Most of the fluids which are taken into the body by drinking or eating are excreted by the kidneys to make sure the body does not have too much fluid (fluid over-load) or too little fluid (dehydration).
A relatively small amount of fluid is excreted from the body by sweating, opening bowels and breathing, but most of the fluid excretion occurs by urine production in the kidneys.
Healthy kidneys are extremely good at regulating the amount of fluid in the body. If a person drinks an extra 1.5 litres of fluid in a day, the kidneys will excrete an extra 1.5 litres of urine.
When kidney function deteriorates, there is usually an inability to excrete sufficient fluid resulting in swelling of the feet, hands and face. Patients can also retain excessive fluid in the abdomen causing ascites. Renal failure can also result in excessive fluid in the lungs causing shortness of breath.
Patients whose kidneys do not make sufficient fluids may be treated with diuretics which are medicines that promote fluid production by the kidneys.