Urine volume and concentration is
regulated through the same processes that regulate blood volume.
The antidiuretic hormone (ADH), produced by the pituitary gland, increases the
amount of water reabsorbed in the kidney tubules.
The concentration of urine in your
body fluids is controlled by the anti-diuretic hormone (ADH)
- which makes the kidney tubules more permeable - so more water can be
reabsorbed into the bloodstream.
In effect, the ADH controls
the permeability of the urine collecting duct in the kidney.
ADH is released into the blood by the
pituitary gland in the brain.
The brain's hypothalamus monitors the
water content of the blood via osmotic pressure changes -
osmoregulation.
If the water content is not right,
the brain instructs the pituitary gland to release or not release ADH into the
bloodstream depending on how much water is needed.
The hormone ADH makes the
collecting duct membranes of the nephrons more permeable so
that more water can be reabsorbed into the blood.
This process is controlled by a
negative feedback system so that if the water content of your body gets
too high or too low the ADH mechanism is initiated to bring the water
levels to normal.
(i) If you drink a lot of
water it may dilute the blood too much. If this happens, little, if any
ADH is released into the bloodstream. This results in less water being
reabsorbed by the nephrons in the kidney and more water passes through the tubules to
produce urine.
In passing (if you will pardon
the pun), drinking alcohol inhibits the production of ADH so you
produce much more urine, so heavy drinking of alcoholic beverages can produce dehydration.
(ii) If the blood starts to
become too concentrated it makes you feel thirsty and the brain
instructs the pituitary gland to release antidiuretic hormone ADH
into the bloodstream. ADH stimulates the nephrons in the kidneys to reabsorb water into
the blood from the urine. Reducing water loss in the urine helps to
conserve water and reduce the likelihood of dehydration.
(iii) Note that the more water
your kidneys reabsorb, the smaller the volume of water you excrete as
urine.
However, you will still excrete
all the waste products produced by the kidney, but your urine will
be more concentrated and probably a darker colour than the usual
pale straw colour.
If your water intake is in excess of what is need for a balanced water content, the
hypothalamus detects this increase 'above normal' and instructs the
pituitary gland to secrete less ADH and this instructs the kidneys
to reabsorb less water. This produces a greater volume of more
dilute urine - more water lost to restore the water content
balance in the body.
If you take in a lot of salt,
the kidney responds by excreting more salt in concentrated urine.
|
A description of a
body's homeostasis negative feedback system for controlling
water-urine content via ADH |
|
(a) The negative feedback in action for too high level of water
content in the body e.g. after drinking a large volume of water |
(b) The negative feedback in action for too low level of water
content in the body e.g. hot and thirsty due to high outside
temperature or physical exercise |
| 1. A receptor
in the brain's hypothalamus detects a stimulus that the water
level is too high
- blood solute concentrations too low - too dilute - detection is to
do with osmotic pressure (osmoregulators). |
1. A receptor
in the brain's hypothalamus
detects a stimulus that the water level is too low -
blood solute concentrations too high - too concentrated - detection
is to do with osmotic pressure (osmoregulators). |
| 2. The hypothalamus
coordination centre in the brain receives and processes the stimulus
information and then organises a response by the effector -
pituitary gland. |
2. The hypothalamus
coordination centre in the brain receives and processes the stimulus
information and then organises a response by the effector -
pituitary gland. |
| 3. The effector, the pituitary gland, responds by
releasing none or
very little ADH - so less water is reabsorbed by the blood
from the kidney tubules. This results in larger volumes of
dilute urine - pale yellow colour. |
3. The effector, the pituitary gland, responds by
releasing more ADH
into the blood stream - so more water is reabsorbed into the
blood from the kidney tubules . This results in a smaller volume
of concentrated urine - dark yellow colour. |
| 4. The
pituitary gland will carry on producing less of the ADH - the 'water reducing' response, as long as
the coordination centre is stimulated by the receptors are saying
,too much water'. Eventually the correct water balance is reached. |
4. The
pituitary gland will carry on producing the increased ADH 'water increasing' response as long
as the coordination centre is stimulated by the receptors are saying
'too little water'. Eventually the correct water balance is reached. |
| 5. If the
pituitary
effector response is more than required, and the water content becomes
too low, the receptors will detect
this, and the negative feedback will stimulate the effectors to
increase the water level (1. - 3. on the right). |
5. The
pituitary effector response is more than required, and the water
content becomes
too high, the receptors will detect
this and the negative feedback will stimulate the effectors to
decrease the water level (1. - 3. on the left). |
| This
is all automatically done by the organism's complex control
systems and enables the organism e.g. your body, to maintain as near
as possible the 'ideal' water content of your body. |
(iv) Ion concentration and the feedback
mechanism
If the concentration of sodium
ions increases in the bloodstream, the concentration of water will
fall.
Therefore, the brain detects that
the blood needs more water.
More ADH is released so more
water will be reabsorbed from the collecting ducts and the water
content of the blood will rise.
This can happen when you eat
'salty' food (NaCl salt gives sodium ions), you may feel thirsty
soon after if you haven't taken in enough water to be reabsorbed.
To much salt in your bloodstream
is bad for you. It draws water out of cells by osmosis and a high
salt diet is associated with high blood pressure.
Most experts think we have too
much salt in our diet.
(v) Sweating and dehydration
Losing a significant amount of
water can cause dehydration.
This most likely to happen in hot
weather or after vigorous exercise - both result in sweating
- loss of body fluid - mainly water containing salt (sodium
chloride).
As we have described already, the
reduction in water content is detected by receptors in the
hypothalamus (osmoregulators).
The hypothalamus registers the
change and instructs the pituitary gland to release more of the
hormone ADH.
This instructs the kidneys to
reabsorb more water so the volume of blood increases and a
smaller volume of more concentrated urine is made.
At the same time, the brain
triggers feelings of thirst when you are dehydrated.
Therefore your brain makes you
want to drink more and hence restore the water content balance in
your body.
