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In order for our body cells to function properly, they must be surrounded in extracellular fluid that is relatively constant with regard to osmolality. The kidneys, in concert with neural and endocrine input, regulate the volume and osmolality of the extracellular fluid by altering the amount of sodium and water excreted. This is accomplished primarily through alterations in sodium and water reabsorption, the mechanisms of which differ within each nephron segment.
Goal:
Discuss the mechanism by which the kidney integrates signals from other body systems to control extracellular fluid volume and osmolality.
Objective:
1. Describe the mechanisms by which sodium and water input and output are controlled.
2. Outline the neurological and endocrine effects on sodium and water homeostasis.
3. Compare and contrast the mechanisms for sodium, chloride and water transport in the proximal tubule, the thick ascending limb and the collecting duct.
Proper functioning of the cells of the body requires that their environment, the extracellular fluid (ECF), contain the fluid and electrolyte balance within narrow limits. All body systems, principally the cardiovascular, nervous and endocrine systems, contribute to maintaining fluid and electrolyte homeostasis. This article describes the mechanisms by which the kidneys integrate signals from other body systems to control the ECF volume and osmolality.
Paramount in the control of the ECF volume and osmolality is the regulation of sodium and water. The osmolality is set by the ratio of solute to water so that controlling either the amount of solute or water can set the extracellular osmolality to the optimal level. The osmolality and ionic composition of ECF are controlled within very narrow limits, as illustrated by the observation that a rise in body osmolality of as little as 1% to 2% results in a conscious desire to drink and near maximal renal water conservation.
Body Requirements for Sodium and Water
Table 1 shows the typical routes of water gain and loss in adults in steady state. The majority of water gain comes from fluid and food intake. In addition, oxidation of food produces a small amount of water, primarily as a result of carbohydrate metabolism. While water loss is primarily in the form of urine, a sizable, insensible component of fluid loss occurs through evaporation from the skin and respiratory...