Water_metabolism.docx

WATER BALANCE Distribution of water: Water constitutes about 73% of lean body mass. It is distributed in extracellular and intracellular compartments. The extracellular fluid, which comprises a little more than one-third of total body water, is made up of (i) blood plasma, (ii) interstitial fluid (iii) lymph and (iv) transcellular fluid. The blood plasma and interstitial fluids are fairly uniform in composition throughout the body. The transcellular fluids are heterogeneous group of fluids that are the product of the secretion of various epithelial membranes. Transcellular fluids are the cerebrospinal fluid (C.S.F.), the aqueous humor of the eye, various secretions of gastrointestinal tract and the contents of the renal tubules and urinary passages. The remainder of the body water is the intracellular fluid which may vary markedly in composition from tissue to tissue. Functions of water in body: Water is the most abundant constituent of the body. Its major functions are as below; 1. Water provides the medium in which all of the metabolic processes of the body take place. 2. It acts as solvent for many biomolecules. 3. It provides a specific H+ ion concentration and mineral ions like Na+, K+ and Cl- in the biological fluids. 4. It also acts as a lubricant in the body so as to prevent friction in joints, pleura, peritoneum and conjunctiva. Water balance (Water metabolism): The body's content of water is maintained constant on the average, by the maintenance of balance between intake and output. Intake of Water Most of our daily intake of water enters by oral route, approximately two-third is in the form of pure water or some other beverage and remainder is in the food that is eaten. A small amount of water is also synthesized in the body as the result of oxidation of hydrogen in the food. This quantity of metabolic water ranges between 150-200 ml per day, depending on the rate of metabolism. The normal intake of fluid, including that synthesized in the body, averages about 2400 ml per day. Output of Water Water is normally lost from body through the lungs and skin and in the feces and urine. Approximately 1400 ml of 2400 ml of water intake, is lost in the urine, 100 ml is lost in the sweat and 200 ml in the feces. The remaining 700 ml is lost in the evaporation through the lungs or by diffusion through the skin.

Insensible water loss: Loss of water by diffusion through the skin and by evaporation from the lungs is known as insensible water loss because the person does not know that he is actually losing water at the time that it is leaving the body. These losses are not ordinarily measurable. When intake and output are measured experimentally or clinically, the measured intake exceeds output, the difference representing most of the insensible losses. All air that enters the lungs becomes totally saturated with moisture before being exhaled. Water lost by this route varies directly with the alveolar ventilations and inversely with the absolute humidity of the expired air. The atmospheric vapour pressure normally, decreases with decreasing temperature so that loss is greatest in very cold and least in warm weather. This explains the dry feeling in the respiratory passages in cold weather. Regulation of water balance: 1. Hypothalamus (Thirst mechanism): A thirst centre is located in the hypothalamus. Dehydration leads to osmoconcentration of plasma, which stimulates the thirst centre. This tends to draw water from intracellular compartment and provokes to drink required amount of water. 2. Solutes in the body: Osmotic forces are the principal factors which control the amount of fluid in various compartments. These osmotic forces are maintained by the solutes. Organic substances of larger molecular size, mainly the proteins, effect the transfer of fluid from one compartment to another and do not effect the total body water. The inorganic electrolytes -Na+ and K+ are most important, both in the distribution and in the retention of body water. 3. Vasopressin or antidiuretic hormone: Vasopressin exerts an antidiuretic effect. The hormone affects the renal tubules and enhances facultative reabsorption of water. 4. Aldosterone: Aldosterone increases the reabsorption of sodium and chloride by the renal tubules. Accompanying the retention of sodium, potassium excretion is increased as a result of exchange of intracellular potassium with extracellular sodium. 5. Renin-angiotensin system: Renin results in the production of angiotensin-II, which acts directly on the aldosterone producing cells of adrenal cortex, consequently increasing the extracellular sodium and water. 6. Kinins: Action of kinins is opposite of antidiuretic hormone. Kinins (bradykinin and kallidin) are produced in the kidney and increase salt and water excretion. Prostaglandins modulate the activity of kinins in kidney.

Dehydration: Dehydration is a state in which loss of water exceeds intake resulting into reduced water body content. In the body, negative water balance Types of dehydration: Primary dehydration: In primary dehydration there is purely water depletion and no salt depletion. It occurs due to deprivation of water as in; desert travelling, in mental patients who refuse to drink water, in water/fluid restricted 'fast, during fever or in the high temperature environment, excessive water loss due to vomiting, prolonged diarrhoea, gastroenteritis and excretion of large quantities of urine or sweat. When the blood becomes hypertonic, it lowers the urinary output and also makes the urine concentrated which leads to acidosis and eventually coma. Death occurs in man due to renal failure, acidosis, intracellular hyperosmolality, circulatory collapse or neural depression, when body water falls by 20%. Secondary dehydration: The decrease or increase in the total electrolytes is accompanied by a corresponding increase or decrease in the volume of body water. Secondary dehydration is due to decrease in total electrolyte content of body fluids. Dehydration due to injection of hypertonic solution: When a highly concentrated solution of sugar or salt is injected into the body of an individual, the osmotic pressure of blood will increase which results in the flow of fluid from the tissues into the blood until equilibrium is reached. Consequently, blood volume increases. This increased blood volume soon returns to normal by the loss of excess material through excretion which finally causes a net loss of body water producing dehydration. Effects of dehydration: The consequences of dehydration are disturbance in acid base balance, loss of body weight, rise in nonprotein nitrogen of blood, dryness, wrinkling and looseness of skin, elevation of plasma protein concentration and chloride, rise in the body temperature, increased pulse rate, reduced cardiac output, exhaustion and collapse. Management of dehydration: 1. Ordinarily, sodium chloride solution may be given parenterally to compensate the loss. 2. In several disorders like diarrhoea, gastroenteritis pancreatic or biliary fistulas etc., a mixture of two-thirds isotonic saline solution and one-third sodium lactate solution (M/6) should be administered intravenously.