Water is essential for living things to survive because all the chemical reactions within a body take place in a solution of water. An animal’s body consists of up to 80% water. The exact proportion depends on the type of animal, its age, sex, health and whether or not it has had sufficient to drink. Generally animals do not survive a loss of more than 15% of their body water.
In vertebrates almost 2/3rd of this water is in the cells (intracellular fluid). The rest is outside the cells (extracellular fluid) where it is found in the spaces around the cells (tissue fluid), as well as in the blood and lymph.
Diagram 12.1 - Water in the body