Comprises about 70-75% of the fat free mass of an adult.

Adipose tissue is about 10-40% water.

For adults over the age of 50 years the total body water comprises about 56% of the body weight of males and 47% of the body weight of women.

Intracellular water makes up 65% while extracellular fluid accounts for 35% of water.

The average 70 kg male has 29 L of intracellular water and 14 L of extracellular water.

Of the 14 L of extracellular water 11 L are in the interstitial space and 3 L are in the vascular space.

The fluid distribution is dynamic with changes in response to losses or gains in any individual department.

The transfer of fluid from the extracellular fluid compartment into the intracellular department depends in the intracellular and extracellular osmotic pressures.

The majority of water exchange between the interstitial and intravascular spaces occurs at the level of the capillaries, where transcapillary pressure determines the direction of the flow.

Body water is derived from dietary intake, retained  by the kidneys, and produced by carbohydrate, fat, and protein metabolism.

Urinary water excretion varies to match intake and metabolic generation, less insensible losses.

Metabolic water production is proportional to energy expenditure and averages 250 to 350 mL per day, a value that can rise substantially with exercise.

Most body water derives from consumption which is programmed by thirst.

44% of fluid intake is from beverages other than water.

Daily fluid consumption from all sources in adults in the US may approach 3 L, even when salt intake is low, a volume that is likely to be in excess of physiological requirements.

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