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A significant fraction of the human body is water. This body water is distributed in different compartments in the body. Lean muscle tissue contains about 75% water. Blood contains 83% water, body fat contains 25% water and bone has 22% water.
In diseased states where body water is affected, the compartment or compartments that have changed can give clues to the nature of the problem.
Body water is regulated by hormones, including anti-diuretic hormone (ADH), aldosterone and atrial natriuretic peptide.
There are many methods that can be used to determine body water. One way to get a simple estimate is by calculation.
Calculation of body water:
In individuals of normal weight, water is abundant in most parts of the body, except in adipose tissue (fat). These calculations are for adults of average build, and are inappropriate for obese or overly muscular people. These proportions are very simplified and use round numbers for quick calculation.
In males about 60% of the body mass is water. This value is about 55% in women due to a higher proportion of body fat. This is the total body water.
Of this water, two-thirds of it is in the cells, and is called intracellular water. The other third is extracellular water.
The extracellular water is made up of blood plasma, which is one-fifth of extracellular water, and extracellular fluid constitutes the other four-fifths.
Water exists in other places, (such as in the eye) but this can be ignored as being of insignificant quantity in quick calculation.
Measurement of body water:
Total body water can be determined using Flowing afterglow mass spectrometry FA-MS measurement of deuterium abundance in breath samples from individuals. A known dose of deuterated water (Heavy water, D2O) is ingested and allowed to equilibrate within the body water. The FA-MS instrument then measures the deuterium-to-hydrogen (D:H) ratio in the exhaled breath water vapour. The total body water is then accurately measured from the increase in breath deuterium content in relation to the volume of D2O ingested.
An average adult body is 50 to 65 percent water -- that's roughly 45 quarts. Men are more watery than women. A man's body is 60 to 65 percent water, compared to 50 to 60 percent for a woman. In infants, the figure is a whopping 70 percent according to statistics compiled by the International Bottled Water Association.
``That's mostly for cushioning,'' says Felicia Busch, a spokesperson for the American Dietetic Association and specialist on the nutritional effects of water. ``Because of the birth process, they need a lot of cushioning.''
Water content differs throughout the body. Blood is made up of 83 percent water, bones are 22 percent water, and muscle is 75 percent water.
Men have more water in their bodies because they generally have more muscle mass than women, says Busch. There is no water in fat, she says, and women generally have a higher percentage of body fat than men.
Water plays several crucial roles in the body. It helps regulate temperature, carries nutrients and oxygen, and removes waste. It also cushions joints and organs.
``Water is the most important nutrient,'' says Busch. ``It really is the basis of a healthy diet, even though it's not included in the food guide pyramid.''
It's important, she says, to drink at least eight ounces of water a day, and more if you're overweight, exercising, or spending time in rooms with heated, recirculated air. Planes are especially drying. It's estimated that you can lose up to two pounds of water during a three-hour flight. Some airlines advise that passengers drink a glass of water for every hour in the air.
Fruit juice, milk, and soup count as water intake, says Busch, but anything with caffeine or alcohol doesn't. Caffeine and alcohol are diuretics, which cause urination and water loss.
Don't wait until you're thirsty to drink water. By the time you notice the symptoms of dehydration -- dry mouth, dark urine, lightheadedness -- you're already very dehydrated.
Water content isn't measured directly. Because physicians know the amount of water in different types of tissue, they can estimate the total water content in a person's body by measuring the percentage of fat and muscle and then using calculations based on those measurements.