WATER METABOLISM- facts to be known

  • Water composition of the body is not same for everybody, it depends on age, sex, muscle mass, body habitus, and fat content of each individual.
  • People with more muscle than fat have more water and thin people have less fat and more water.
  • Men contain 60% body water and women 50% water by weight.
  • Man, and woman older than 60 years have more fat and less muscle and hence contain less body water.

Where is water located within the body?

Total water found inside body can be located either inside the cells are outside the cells. Outside cell water is found in various body fluids like blood, cerebrospinal fluid (fluid found in the brain), fluid in joints, secretions like sweat, urine. Fluid containing water is also found surrounding cells and organs of body. Water in the blood is major deciding factor for our blood pressure.

What decides balance of body water?

Water content of the body is a balance of intake and excretion. Water balance is controlled by thirst, access to water, solute intake, various hormones, renal water channels, level of kidney function, and drugs.

How input and output of water is controlled?

Total Body Water is a balance of input and output. In an average adult, input approximates 1600 mL (liquids), 700 mL (foods), and 200 mL from digestion of food for a total of 2500 mL/day. Average water losses are 1500 mL (kidneys), 500 mL (skin), 300 mL (lung—respiration), and 200 mL from the gastrointestinal tract (stool) for a total of 2500 mL/day. Large losses of water (increased output) occur with excessive sweating, respiration (exercise), burns, diarrhoea, vomiting, and diuresis. Decreased water input occurs when defects in thirst and altered mental or physical function (especially in the elderly) prevent access to water.

What are the normal limits of urine output?

On a normal diet, a normal adult must excrete 0.8 to 20 L/day per day. Body builders consuming high-protein and high-carbohydrate diets require a urine output of 1.2 to 28 L/day. Alternatively, a low solute intake (starvation) with high water intake predisposes to water retention and water intoxication. This combination exists in binge beer drinkers, in whom the solute load may be only 300 mOsm /day. Low solute intake may also occur in starvation and in an elderly person on a “tea and toast diet. he ranges of urine output would drop to 0.25 to 6 L/day in such patients.

Which factors decide thirst?

A dry mouth increases thirst. Drinking and swallowing water decrease thirst due to the presence of receptors in the mouth and food passage. The floor of brain has also receptors which directs thirst when body requires water.

Are there hormones involved in controlling body water?

There are several hormones involved in water metabolism, most important is ADH is also called arginine vasopressin (AVP). Natriuretic peptides, aldosterone, angiotensin II, prostaglandins, and neurochemical

changes also affect water retention and excretion in the kidneys.

How does the kidney help in water metabolism?

The normal kidneys filter 180 L of blood each day and but 99%of it is again reabsorbed into body. To control excess or deficient water intake the kidneys play a major role, they excrete more if we drink excess water whereas in low intake conditions there is less urine excretion.

What happens when the kidneys don’t work properly?

In conditions of kidney failure, the system of water balance doesn’t work and body retains water hence patients of kidney failure are advised to take less water and salt.

What are the condition where more urine is produced?

Polyuria is a urine output greater than 3.0 L/day. Four main disorders cause polyuria: central neurogenic DI (defect in ADH secretion), nephrogenic DI (defect in ADH action on the kidney), psychogenic polydipsia (psychosis), and dipsogenic DI (defect in thirst centre). Polyuria also may occur from diabetes mellitus (glucose), recovery from renal failure (urea), and IV infusions (saline, mannitol).






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