endocrine and metabolic disorders
Water and Sodium Balance
Water intake and excretion
The average daily fluid intake is about 2.5 L. The amount needed to replace losses from the urine and other sources is about 1 to 1.5 L/day in healthy adults. However, on a short-term basis, an average young adult with normal kidney function may ingest as little as 200 mL of water each day to excrete the nitrogenous and other wastes generated by cellular metabolism. More is needed in people with any loss of renal concentrating capacity. Renal concentrating capacity is lost in Older people People with ArginineVasopressin Deficiency (Central Diabetes Insipidus) , certain kidney disorders, People who ingestethanol,phenytoin,lithium,demeclocycline, oramphotericin B People with osmotic diuresis (eg, due to high-protein diets or hyperglycemia) Other obligatory water losses are mostly insensible losses from the lungs and skin, averaging about 0.4 to 0.5 mL/kg/hour or about 650 to 850 mL/day in a 70-kg adult. With fever, another 50 to 75 mL/day may be lost for each degree Celsius of temperature elevation above normal. Gastrointestinal losses are usually negligible, except when marked vomiting, diarrhea, or both occur. Sweat losses can be significant during environmental heat exposure or excessive exercise. Water intake is regulated by thirst. Thirst is triggered by receptors in the anterolateral hypothalamus that respond to increased plasma osmolality (as little as 2%) or decreased body fluid volume. Rarely, hypothalamic dysfunction decreases the capacity for thirst. Water excretion by the kidneys is regulated primarily byvasopressin(antidiuretic hormone [ADH]).Vasopressinis released by the posterior pituitary and results in increased water reabsorption in the distal nephron.Vasopressinrelease is stimulated by any of the following: Increased plasma osmolality Decreased blood volume Decreased blood pressure Stress Vasopressinrelease may be impaired by certain substances (eg,ethanol,phenytoin), by tumors or infiltrative disorders affecting the posterior pituitary, and by trauma to the brain. In many cases a specific cause cannot be identified. Water intake decreases plasma osmolality. Low plasma osmolality inhibitsvasopressinsecretion, allowing the kidneys to produce dilute urine. The diluting capacity of healthy kidneys in young adults is such that maximum daily fluid intake can be as much as 25 L; greater amounts quickly lower plasma osmolality. 