The kidneys cannot restore fluid volume or osmolarity, but in dehydration they can support existing fluid levels and slow down the rate of loss until water and electrolytes are ingested Changes in urine volume are usually linked to adjustments in sodium reabsorption. In true dehydration, blood volume declines and sodium concentration rises. ADH targets the collecting tubules of the nephrons, and the kidneys resabsorb more water and produce less urine If blood volume and pressure are too high or blood osmolarity too low, ADH release is inhibited. The renal tubules absorb less water, urine output increases, and total body water declines Disorders of Water Balance Fluid Deficiency Volume depletion occurs when proportionate amounts of water and sodium are lost without replacement. Total fluid volume declines but osmolarity remains normal.
Volume depletion occurs during hemorrhage, severe burns, and chronic vomiting or diarrhea.
The simplest cause of dehydration is a lack of drinking water. Dehydration from diarrhea is a major cause of infant mortality, and can be a serious problem for elderly or bedridden people who depend on others to provide them with water. Dehydration affects all fluid compartments. As the blood loses water its osmolarity rises and water from the tissue fluid enters the bloodstream to balance the loss. This raises the osmolarity of the tissue fluid, and water moves out of the cells to balance that.