Water
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“The forgotten nutrient”
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Adults are about 62% water
Two-thirds of this is intracellular
Rest is extracellular (blood, lymph)
You can die of dehydration long before starving to death
– Infants and elderly are most susceptible
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Functions of water in the body
Solvent for numerous molecules
Control of body temperature
Needed for metabolism--e.g., hydrolysis
Lubricant (joints) & shock absorber (brain)
pH balance
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Water and electrolytes
Na+, K+ (cations); Cl-, phosphate (anions)
Cations (+) = anions (-) in biological fluids
Na+ is main extracellular cation, K+ is main intracellular cation
– This is controlled by Na+-K+ ATPase pump in cell membranes
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Osmosis
Movement of water across a semipermeable membrane from a less to more
concentrated solution
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Osmotic pressure
The pressure exerted on a semi-permeable membrane by a solvent moving from a low
solute to a high solute concentration
There is no active transport of water in biological systems
Water balance =H2O in -H2O out
H2O in--beverages, food, metabolism
– Controlled by thirst mechanism in hypothalamus of brain
H2O out--urine, feces, sweat, exhaled breath
– Some control by pituitary hormone anti-diuretic hormone
– Losses increased by diarrhea, vomiting, diuretics, ketosis, uncontrolled diabetes,
hot weather, pregnancy, and lactation
Water requirements
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No RDA for water
Recommendation: Drink 1-1.5 mL per Calorie consumed
Example: If you eat 2000 Calories/day, you should drink 2000-3000 mL/day (think
the size of a 2- or 3-liter bottle of soda)
Drink more if you’re exercising, doing heavy physical labor, or it’s hot outside
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How to get enough water
Tap water is just as good as bottled water (and cheaper)
Juices, milk, soup, sodas (sparingly)
Coffee and tea contain caffeine, which is a diuretic (increases water excretion in urine)
Alcohol is also a diuretic
Hormones affecting fluid balance
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Anti-diuretic hormone
Renin-angiotensin system
Aldosterone
Dehydration
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A major killer worldwide
Infants and elderly most affected
Causes of dehydration
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Heat
Lack of thirst mechanism or inability to get to water
Burns
Gastrointestinal infections
– Normal bugs
– Cholera
When is dehydration fatal?
When rehydration doesn’t occur in time
– Serious dehydration cases require hospitalization
Kidneys shut down, which results in failure of other organs
death