2 Water in the mammalian body

In mammals water forms about 70 % of the body mass:

• about 47 % (of body mass) is inside the cells (intracellular fluid);
• about 23 % is outside the cells (extracellular fluid), in blood plasma and the other body fluids (lymph, tissue fluid, cerebrospinal fluid).

Check: 47 % + 23 % = 70 %. (Notice that the percentages are percentages of the body mass, not of the water. Of the body's water, about two-thirds is inside the cells and one-third outside.)

Other examples (approximate): a jellyfish is about 95 % water, a tomato 94 %, a human embryo 90 %, an adult man 60–65 %, an adult woman 50–55 %, bone about 22 %, and dry seeds 5–15 % (which is why dry seeds can wait for years until water arrives and germination begins).

• Problem. A student has a mass of 65 kg.
• Using the book's percentages, find the mass of water in the whole body, inside the cells, and outside the cells.

Solution. Total = 0.70 × 65 = 45.5 kg. Inside cells = 0.47 × 65 = 30.55 kg. Outside cells = 0.23 × 65 = 14.95 kg. Check: 30.55 + 14.95 = 45.5 ✔. About 45.5 litres of water, since 1 kg of water is 1 L.