Total body water

In physiology, body water is the water content of an animal body that is contained in the tissues, the blood, the bones and elsewhere. The percentages of body water contained in various fluid compartments add up to total body water (TBW). This water makes up a significant fraction of the human body, both by weight and by volume. Ensuring the right amount of body water is part of fluid balance, an aspect of homeostasis.

By weight, the average human adult male is approximately 60% water and the average adult female is approximately 55%. There can be considerable variation in body water percentage based on a number of factors like age, health, weight, and sex. In a large study of adults of all ages and both sexes, the adult human body averaged ~65% water. However, this varied substantially by age, sex, and adiposity (amount of fat in body composition). The figure for water fraction by weight in this sample was found to be 48 ±6% for females and 58 ±8% water for males. The body water constitutes as much as 73% of the body weight of a newborn infant, whereas some obese people are as little as 45% water by weight. This is due to how fat tissue does not retain water as well as lean tissue. These statistical averages will vary with factors such as type of population, age of people sampled, number of people sampled, and methodology. So there is not, and cannot be, a figure that is exactly the same for all people, for this or any other physiological measure.

Most of animal body water is contained in various body fluids. These include intracellular fluid; extracellular fluid; plasma; interstitial fluid; and transcellular fluid. Water is also contained inside organs, in gastrointestinal, cerebrospinal, peritoneal, and ocular fluids. Adipose tissue contains about 10% of water, while muscle tissue contains about 75%.

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