Water UK

Water and the kidneys

The main function of the kidneys is to excrete a number of water-soluble compounds such as the nitrogen-containing breakdown products of protein metabolism (principally urea) and sodium chloride, plus other substances such as sulfates, phosphates and other electrolytes, which if retained by the body would be toxic and lethal. The kidneys filter the blood and excrete the waste products in the urine, leaving the nutrients in the bloodstream. The kidneys also remove excess water from the body and help to maintain normal blood pressure.(i)

The kidneys play a vital role in maintaining water homeostasis by adjusting urine output according to water availability. Dehydration causes increased osmotic pressure in the blood and in severe instances in cells, resulting in thirst and hypersecretion of antidiuretic hormone (ADH, or vasopression) from the brain. The increase in plasma ADH acts first on the kidneys, which respond by excreting less water. However, in order to fulfil their main role, to excrete waste products, the kidneys require a minimum amount of water. This limits the extent to which they can concentrate urine, in order to save water and prevent further dehydration.(ii) The trigger of thirst is then most important in stimulating the individual to drink.(iii)

Dehydration has been implicated in a number of conditions affecting the kidneys and urinary tract, including kidney stones, cancer of the kidney and urinary tract infection. (See Urinary Tract)

Last updated: November 2004

(i) National Kidney Research Fund. Patient Information: Keeping your kidneys healthy. NKRF, 2001
(ii) Principles of Human Nutrition. Ed M Eastwood. Chapter 8: Water, electrolytes, minerals and trace elements. London: Chapman & Hall 1997
(iii) Sansevero AC. Dehydration in the elderly: strategies for prevention and management. Nurse Pract. 1997;22:41-42,51-57,63-72