Water in the Human Body

If you could remove all the water from an adult human, you’d take away more than half their body weight. Water is the most abundant molecule in you by far — by number of molecules, about 99 % of the molecules in your body are water. It isn’t just filler. Almost every reaction in your cells happens in water, and water’s unusual chemistry helps control your temperature, carry nutrients, cushion organs and shape proteins. Here’s why this small molecule matters so much.

How much water are you?

In a typical adult, water makes up roughly 50–60 % of body mass. The exact figure depends on age and body composition:

• Newborn babies: about 75 %.
• Adult men: around 60 % on average.
• Adult women: around 50–55 % on average, because of a typically higher proportion of body fat.
• Older adults: lower, partly because of reduced muscle mass.

Different tissues hold very different amounts. Muscle is about three-quarters water, blood plasma over 90 %, the brain around three-quarters, while fat tissue is only about 10–20 % water and bone about a fifth to a quarter.

Where the water is

Body water is divided into compartments separated by membranes:

• Intracellular fluid (inside cells): about two-thirds of body water.
• Extracellular fluid (outside cells): about one-third, made up of:
  • interstitial fluid bathing the cells (the larger part)
  • blood plasma
  • small amounts in special places, such as the fluid around the brain and in joints.

Water moves between these compartments by osmosis: it flows across membranes towards the side with a higher concentration of dissolved particles. The main dissolved particles differ: inside cells, potassium ions dominate; outside, sodium and chloride dominate (see sodium and potassium). Cells pump sodium out and potassium in constantly, using ATP, to keep this balance — which is also how nerves create their electrical signals.

Why water? The chemistry of a special molecule

Water, H₂O, is small (molar mass 18.02 g mol⁻¹), but its structure gives it unusual properties:

• The O–H bonds are polar: oxygen is much more electronegative than hydrogen (3.44 vs 2.20 on the Pauling scale), so oxygen carries a partial negative charge and each hydrogen a partial positive charge.
• The molecule is bent (about 104.5°), so the charges don’t cancel: water is a polar molecule.
• Each water molecule can form up to four hydrogen bonds with neighbours (see hydrogen bonding).

These features explain water’s roles in the body.

1. The universal solvent of life

Polar water molecules surround ions and polar molecules, pulling them apart and keeping them in solution. Sodium, potassium and chloride ions, glucose, amino acids, vitamins and many hormones all travel dissolved in water. Water-soluble waste such as urea is carried away in the same way.

Non-polar substances such as fats don’t dissolve in water. That’s actually useful: it’s why cell membranes form. The fatty tails of phospholipids cluster away from water while their polar heads face it, creating a barrier that separates the inside of a cell from the outside.

2. A reactant, not just a solvent

Water takes part directly in many biochemical reactions:

• Hydrolysis reactions use water to split molecules — digestion of starch, proteins and fats depends on it (see the chemistry of digestion).
• Condensation reactions release water when building molecules such as proteins and glycogen.
• Water is a product of aerobic respiration: the electron transport chain reduces oxygen to water. This “metabolic water” amounts to roughly 250–350 cm³ a day in an adult.
• Water is split in photosynthesis in plants, releasing the oxygen we breathe.

3. Temperature control

Water has a very high specific heat capacity — about 4.18 J g⁻¹ K⁻¹ — because energy is needed to break hydrogen bonds before molecules can move faster. A body made mostly of water therefore heats up and cools down slowly, protecting cells from sudden temperature changes.

Water also has a very high enthalpy of vaporisation. Evaporating sweat carries away a lot of heat: evaporating 1 g of water at skin temperature absorbs roughly 2.4 kJ. That’s why sweating is such an effective cooling system (see what happens chemically when you exercise).

4. Transport, lubrication and cushioning

Blood plasma carries nutrients, hormones, carbon dioxide (mostly as hydrogencarbonate ions) and heat around the body. Water-based fluids lubricate joints, keep the eyes moist, help food slide down the oesophagus, and cushion the brain and spinal cord. The fluid in the eyeball helps keep its shape.

5. Shaping proteins and DNA

Proteins fold partly because of water. Hydrophobic amino acids tend to be buried away from water in the middle of a protein, while hydrophilic ones face outward (see protein folding). The DNA double helix also depends on water: the charged phosphate backbone faces outward into the surrounding water, while the bases stack inside.

Water balance: in and out

An average adult in a mild climate takes in and loses roughly 2.5 litres of water a day, although this varies hugely with climate, activity and diet.

Water in (approximate):

• drinks: 1.5 L
• food: 0.7 L (fruit and vegetables are often 80–95 % water)
• metabolic water: 0.3 L

Water out (approximate):

• urine: 1.5 L
• skin (sweat and evaporation): 0.5 L
• breath (water vapour): 0.35 L
• faeces: 0.15 L

How the body controls water

The body keeps the concentration of its fluids within tight limits. The key sensor measures osmolality — the concentration of dissolved particles in blood.

When you haven’t drunk enough, blood becomes slightly more concentrated. Sensors in the hypothalamus of the brain detect this and:

1. Trigger thirst.
2. Release antidiuretic hormone (ADH, vasopressin) from the pituitary gland.

ADH acts on the kidneys, inserting water channels (aquaporins) into the walls of the collecting ducts. More water is reabsorbed back into the blood, so urine becomes smaller in volume and darker. When you’ve drunk plenty, less ADH is released and the kidneys produce more, paler urine. (Alcohol suppresses ADH, which is why it makes you urinate more — see how the body metabolises alcohol.)

A separate system, involving the hormone aldosterone, controls how much sodium the kidneys keep. Because water follows sodium by osmosis, this adjusts the total volume of body fluid and so blood pressure.

Too little and too much

• Dehydration: losing even 1–2 % of body mass as water can reduce physical performance and concentration. Severe dehydration, from heat, illness or diarrhoea, can be dangerous. Oral rehydration solutions contain glucose and sodium together because glucose and sodium are absorbed together by the gut’s SGLT1 transporter, pulling water with them — a simple piece of chemistry that has saved millions of lives.
• Overhydration (hyponatraemia): drinking very large amounts quickly, especially during long endurance events without replacing salt, dilutes blood sodium. Water moves into cells by osmosis, including brain cells, which can swell dangerously.

Common misconceptions

• “Everyone must drink eight glasses of water a day.” Needs vary with size, climate and activity, and food provides a lot of water. Thirst is a good guide for most healthy people.
• “Tea and coffee don’t count.” For regular drinkers, they contribute to fluid intake (see what caffeine does in the body).
• “Clear urine is the goal.” Pale straw-coloured urine is a reasonable sign of good hydration; completely clear urine may mean you’re drinking more than you need.
• “Water is just a solvent.” It’s also a reactant, a temperature buffer and a structural partner for proteins and DNA.

Key takeaways

• Water is about 50–60 % of adult body mass, with two-thirds inside cells.
• Its polarity and hydrogen bonding make it an excellent solvent, a temperature buffer and a cooling agent.
• Water is a reactant in hydrolysis and a product of respiration.
• The kidneys, under the control of ADH, keep body water and blood concentration steady.

For more on the elements dissolved in your body’s water, read elements in the human body and minerals in nutrition.