Balancing the aqueous inputs and outputs
Water can be gained from food and drink we ingest, and mineral salts - principally sodium chloride, common salt - are also part of our diet. This varies from person to person, and at different times of our life and the season. Respiration produces water, from the oxidation of hydrogen in carbohydrates
and fats. Our other body reactions produce products which dissolve in water and enter the blood. Urea, produced in the liver as a result of deamination of excess amino acids, is an example of this.
It is important that most of these products are removed from the blood, as it is circulating to reach all the cells of the body.
The removal of metabolic products is the process of excretion
, and it also involves the loss of water, in urine or air breathed out, as well as in faeces - which biologists generally do not count as an excretory product. In hot conditions we sweat, and lose both water and salt from the body as a result.
The liquid fraction of blood - the plasma - is mostly water
: 90-92% water, and 8-10% solutes.
It is important to control the relative amounts of water and compounds dissolved in it - in order to maintain the 'osmotic balance'. This homeostatic process is called osmoregulation
. This may be described as the control of the water potential of the blood.
An animal cell surrounded by (pure/distilled) water will absorb water by osmosis and swell up, perhaps to the point of bursting (lysis). On the other hand it will lose water by osmosis and shrink in volume when surrounded by a more concentrated solution, like seawater.
In each of these cases water moves to the region with a more negative water potential, i.e. water moves down a water potential gradient
In the human body, it is important to prevent these effects. The water potential of blood is monitored within the hypothalamus, and the pituitary gland uses hormonal control to co-ordinate the action of the kidneys in removing excess water or solutes from the body. This is in addition to the excretion of body wastes e.g. urea and the removal of other substances e.g. drugs.
Typical water and salt inputs and outputs
for an adult male over a 24 hour period
There may be some variation from these figures.
|Water||food||625||expired air||400 |
|/ cm3||fluids||1875||sweat||900 |
|/ g per||faeces||0.25 |
An often recommended value for daily water intake is 3.5 litres.
Most people consume too much salt - on average 9-12 g per day.
The recommended maximum level of intake is 6 g.
- ψ or ψw
- is a term that quantifies the tendency of water to move from one area to another.
Pure water has a water potential of 0 (zero), and a solution (consisting of solute(s) dissolved in water) has a negative water potential.
Water potential is typically expressed in terms of potential energy per unit volume, so pressure units are used. See osmotic pressure opposite
In living systems water potential may be caused by a number of factors, principally osmosis, but also due to (mechanical) pressure - liquids being 'pumped' or other forms of mass transport.
These other components are often given a potential of their own: ψπ
is the solute or osmotic potential, and ψp
is the pressure potential.
The problem with salt
Salt - sodium chloride - is not poisonous, and the body needs it for fluid balance within cells.
However excess salt in our diet causes our body to retain more water, in order to balance its osmotic effects. This increases the volume of blood, raises the blood pressure and puts more stress on the heart. It may thus cause a heart attack, stroke or kidney problems.