All the cells in the body, apart from those of the outer skin, are surrounded by a fluid called the extracellular fluid (ECF).
80% of ECF is interstitial fluid (which includes lymph, synovial fluid, cerebrospinal fluid, GI tract fluids, and fluids in the eyes and ears), and 20% of ECF is blood plasma.
For the cells of the body to work properly, the extracellular fluid needs to have a stable composition of salts - such as potassium and sodium - and acidity (often referred to as pH).
The kidneys are the most important organs for regulating the characteristics of body fluids. This regulation is apparent in the control of
*Blood volume,
*Extracellular fluid volume and
*Osmolality of body fluids.
Osmotic pressure (created by the dissolved electrolytes in body fluids) and hydrostatic pressure (created by the water in body fluids) are the main forces behind any molecular movement between body compartments.
99 % of water and Na is re-absorbed by tubules to maintain osmotic balance. If more Na is excreted, this leads to water excretion and dehydration.
The kidneys are central to maintaining these correct balances and the effective functioning of all the cells of the body.
Thirst also plays a vital role in controlling some characteristics of body fluids. The salt and water balance is maintained by a series of hormones acting on the kidney. The kidneys recognize and act upon a series of messages that vary according to how much fluid is drunk.
If an excess of fluid is drunk, the body fluids become more diluted, and the kidneys excrete a more dilute urine, getting rid of the excess that has been taken in.
Function of kidney: Balance the body's fluids
