The human water balance is essential for health. Water is taken in via fluids and solid food with intake varying according to thirst, hunger and appetite. Water is vital for human life. The proportion of body weight made up of water is dependent upon age and sex and ranges between 75% in newborn babies and around 50% in the elderly. 10% of this is found in the blood, 70% in the cells and 20% outside the cells (tissue fluid). There is a continuous exchange taking place between the fluid within the cells and that outside the cells (osmosis) so that the cells are always supplied with sufficient fresh water.