This section will help you answer questions such as: How does the body control its water content? Why is it important for the body to
control its water content? How do the kidneys work? Why are the
kidneys important? What can you do about kidney failure?
Homeostasis control in the human body
- regulating water content
The first thing to realise is that water is a very
important molecule in the body (around 60% of body mass), acting as a solvent to dissolve substances for transportation, acts as
reaction medium and maybe a reactant molecule in the
metabolic chemistry of cells.
Homeostasis is a word that is
sometimes used to describe your bodily functions that try to maintain a
stable constant internal environment including the factors listed above.
Know that internal conditions that are
controlled in the body include water control, ion and urea concentrations.
Know and understand that water and
ion content, body temperature and blood glucose levels must be kept within
very narrow ranges.
Know and understand that waste products
that have to be removed from the body include:
Carbon dioxide, produced by
respiration and removed via the lungs when we breathe out.
Urea, produced in the liver
by the breakdown of amino acids and removed by the kidneys in the urine,
which is temporarily stored in the bladder.
The body's control of water content
and ion concentrations is called osmoregulation.
It is most important that the
water content of the blood is controlled to keep cells functioning
Body cells are surrounded by
Tissue fluid usually has a
different water potential to the fluid inside cells (mainly the
Tissue fluid moves out of the
blood capillaries to supply all the needs of cells.
If the water potentials are
different, then water will move in or out of cells by osmosis.
If the water concentration in the
tissue fluid is too high, water is diffuses into the cells by
osmosis because of the higher water potential of the tissue fluid -
this is because the concentration of dissolved substances in the
cells is greater than that in the tissue fluid.
If too much water is absorbed the cells can burst and die
quite easily (the process is called lysis) - they don't have rigid cell walls like plants, which
can withstand much greater changes in water concentration changes.
If the water concentration of the
blood is too low, water is lost from cells by osmosis causing the
cells to shrink and possibly malfunctioning.
In other words if the water
potential of the cells is higher than the surrounding fluid,
water will diffuse out of the cells by osmosis - this is because
the concentration of dissolved substances in the tissue fluids
is greater than that in the cells.
If the water potential of the
cell fluids equals the water potential of the surrounding fluids
(similar concentrations of dissolved substances), there is no net
transfer of water and the cells are quite stable, won't change size
and can function normally.
Therefore it is obviously important to keep the water content of the body at the right level
to keep the cells in a 'healthy' state and functioning correctly.
Even the loss of a few % of
our body's weight in water can make us feel thirsty and
uncomfortable - mild dehydration.
With ~5% water loss we lose
our ability to function properly e.g. doing our job at work.
If the water loss of the
body's weight reaches over 10%, the situation is life
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