The equilibrium of water and sodium is interdependent to a great extent. The total body water (TBW = total body water) is about 60% of body weight in men (with variations between 50% in obese people up to 70% in lean humans) and about 50% in women. Nearly one-third of the TBW are located in the intracellular compartment (intracellular fluid or ICF); the other third is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF in the intravascular compartment; the other 75% are interstitial fluid (fluid compartments with an average 70-kg man.).
The liquid volume and electrolyte concentration of the body are usually kept constant in spite of great variations in the food intake, metabolic activity and stress caused by the environment in a very narrow frame. The homeostasis of the body fluids is substantially maintained by the kidneys. The equilibrium of water and sodium is interdependent to a great extent. The total body water (TBW = total body water) is about 60% of body weight in men (with variations between 50% in obese people up to 70% in lean humans) and about 50% in women. Nearly one-third of the TBW are located in the intracellular compartment (intracellular fluid or ICF); the other third is extracellular (extracellular fluid, or ECF). Normally, about 25% of the ECF in the intravascular compartment; the other 75% are interstitial fluid (fluid compartments with an average 70-kg man.). Clinical Calculator: total body water in women (Watson formula) Clinical Calculator: total body water in men (Watson formula) fluid compartments with an average 70-kg man. Total Body Water = 70 kg x 0.60 = 42 l. The major intracellular cation is potassium. The most important extracellular cation is sodium, the concentrations of intracellular and extracellular cations are as follows: Clinical calculator: total body water on a weight estimate is based solely The intracellular potassium concentration 140 mmol / l. The extracellular potassium concentration is between 3.5 and 5 mEq / l. The intracellular sodium concentration of 12 mmol / l. The extracellular potassium concentration is on average 140 mEq / l. Osmotic forces is the concentration of the various solutes in water, the osmolarity (amount of solute per liter of solution), which corresponds in body fluids osmolality. (Amount of solute per kg of solution). The plasma osmolality may be laboratory-measured or estimated according to the formula below. Plasma osmolality (mOsm / kg) = serum sodium is expressed in mEq / l, and glucose and BUN (blood urea nitrogen concentration) are given in mg / dl. The osmolality in body fluids is normally from 275 to 290 mOsm / kg. Sodium is the major determinant of plasma osmolality. Apparent changes in the osmolality may be made of errors in the measurement of sodium with electrodes that are non-ion selective result (hyponatremia: Diagnostics). A osmolar gap exists when the measured osmolality exceeds the estimated osmolality of ? 10 mOsm / kg. It is caused by ungemessenenen osmotically active substances in plasma. The most common non-measurable osmotic agents are alcohols (ethanol, methanol, isopropanol, ethylene glycol), mannitol and glycine. Clinical machine: calculation of the estimated osmolarity (serum), water can pass cell membranes of zones of low solute concentration, to zones with a high concentration of solute-free. Thus, the osmolality tends to equalize within the various compartments of body fluids, which occurs primarily through the exchange of water and not on the exchange of solutes. Some solutes such. As urea, which can diffuse freely across cell membranes, have little or no effect on the fluid shift (little or no osmotic activity), whereas solutes that are mainly limited to only a liquid compartment such. Sodium and potassium, having the largest osmotic activity. Tonicity or osmolality effective reflects the osmotic activity and determines the pressure at which water flows through the various fluid compartments (osmotic pressure). The osmotic pressure can be countered by other forces. For example, plasma proteins have little osmotic effect which draws water into the plasma. This osmotic effect is removed by the vascular hydrostatic pressure of the water from the plasma, counteracted. Water absorption and excretion The average daily fluid intake is approximately 2.5 l. In healthy adults, 1-1.5, the amount needed to replace urine and other losses l / day. Nevertheless, an average young adult can survive in the short term with the inclusion of 200 ml of water per day with normal renal function to excrete nitrogenous and other waste products produced in cell metabolism. More is needed in people with a loss of concentration capacity of the kidneys. The concentration capacity of the kidneys apazität is lost in older people people with diabetes insipidus, certain kidney diseases, hypercalcemia, severe salt restriction, chronic overhydration or hyperkalemia people who ethanol, phenytoin, lithium, demeclocycline or amphotericin B accommodate people with osmotic diuresis (z. B. due to protein-rich diet or hyperglycemia) the other mandatory water losses are losses through the lungs and skin (insensible perspiration), which averages 0.4-0.5 ml / kg / h or about 650 to 850 ml / day for an adult constitute from 70 kg body weight. In fever can again 50-75 ml / day for each added ° C of the temperature increase over the normal value. The losses from the gastrointestinal tract are usually negligible, occur unless severe vomiting, diarrhea or both. The losses through sweat can be significant depending on the ambient temperature or excessive movement. The water uptake is regulated by the thirst. The feeling of thirst is receptors in the anterolateral hypothalamus, the (already on changes in the range of 2%) being responsive to an increased plasma osmolality or degraded volume of body fluids, regulated. Rare hypothalamic disorders affect the feeling of thirst. Water excretion by the kidneys is regulated primarily by vasopressin (ADH). Vasopressin is released from the posterior pituitary gland and results in an increased reabsorption of water at the distal nephron. The vasopressin release can be stimulated by the following: Increased plasma osmolality Decreased blood volume decreased blood pressure stress through some medications (. Eg ethanol, phenytoin) and a central diabetes insipidus (central diabetes insipidus), the vasopressin release affected. Water intake reduces the plasma osmolality. A low plasma osmolality inhibits vasopressin secretion, so that produce the kidneys diluted urine. The dilution capacity of healthy kidneys in young adults appears so that the maximum daily fluid intake can be up to 25 liters, larger quantities quickly lower the plasma osmolality.