Body fluid volume and electrolyte concentration are normally maintained within very narrow limits despite wide variations in dietary intake, metabolic activity, and environmental stresses. Homeostasis of body fluids is preserved primarily by the kidneys.
Water and Na balance are closely interdependent. Total body water (TBW) is about 60% of body weight (ranging from about 50% in obese people to 70% in lean people). Almost two thirds of TBW is in the intracellular compartment (intracellular fluid, or ICF); the other one third is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF is in the intravascular compartment; the other 75% is interstitial fluid (see see Fluid compartments in an average 70-kg man.).
The major intracellular cation is K. The major extracellular cation is Na. Concentrations of intracellular and extracellular cations are as follows:
Intracellular K concentration averages 140 mEq/L.
Extracellular K concentration is 3.5 to 5 mEq/L.
Intracellular Na concentration is 12 mEq/L.
Extracellular Na concentration averages 140 mEq/L.
The concentration of combined solutes in water is osmolarity (amount of solute per L of solution), which, in body fluids, is similar to osmolality (amount of solute per kg of solution). Plasma osmolality can be measured in the laboratory or estimated according to the formula
Plasma osmolality (mOsm/kg) =
where serum Na is expressed in mEq/L and glucose and BUN are expressed in mg/dL. Osmolality of body fluids is normally between 275 and 290 mOsm/kg. Na is the major determinant of plasma osmolality. Apparent changes in osmolality may result from errors in the measurement of Na with electrodes that are not ion selective (see Diagnosis). An osmolar gap is present when measured osmolality exceeds estimated osmolality by ≥ 10 mOsm/kg. It is caused by unmeasured osmotically active substances present in the plasma. The most common are alcohols (ethanol, methanol, isopropanol, ethylene glycol), mannitol
, and glycine.
Water crosses cell membranes freely from areas of low solute concentration to areas of high solute concentration. Thus, osmolality tends to equalize across the various body fluid compartments, resulting primarily from movement of water, not solutes. Solutes such as urea that freely diffuse across cell membranes have little or no effect on water shifts (little or no osmotic activity), whereas solutes that are restricted primarily to one fluid compartment, such as Na and K, have the greatest osmotic activity. Tonicity, or effective osmolality, reflects osmotic activity and determines the force drawing water across fluid compartments (the osmotic force). Osmotic force can be opposed by other forces. For example, plasma proteins have a small osmotic effect that tends to draw water into the plasma; this osmotic effect is normally counteracted by vascular hydrostatic forces that drive water out of the plasma.
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
People with diabetes insipidus, certain renal disorders, hypercalcemia, severe salt restriction, chronic overhydration, or hyperkalemia
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/h 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 C of temperature elevation above normal. GI 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 by ADH (vasopressin). ADH is released by the posterior pituitary and results in increased water reabsorption in the distal nephron. ADH release is stimulated by any of the following:
Increased plasma osmolality
Decreased blood volume
ADH release may be impaired by certain substances (eg, ethanol, phenytoin
) and by central diabetes insipidus (see Central Diabetes Insipidus).
Water intake decreases plasma osmolality. Low serum osmolality inhibits ADH secretion, 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 serum osmolality.