Water Balance System
Date: April 2006
what are the behavioral and physiological respones
responses to water balance stressors? What are the organs that are
involved in regulation and the type of feedback system the
regulation process uses?
Good question about a very important topic.
The maintenance of our bodies fluid volume and the concentration of the this
fluid (called the plasma osmolality) is regulated by our water/liquid intake
and kidney output of water and sodium.
So what we need to make this system of balance work is a detector (our
brain, kidneys and blood vessels) and effector (our kidneys). The detectors
monitor the concentration and volume of our blood. The effector (kidneys) regulate
the volume and composition of our blood.
So, to answer the first part of your question:
THIRST is the major behavioral change in response to water imbalance, and
has two major causes:
1) Increased blood concentration
Thirst occurs when areas in our brain - the detector, sense that our blood is
becoming to concentrated, this signifies that we don't have enough liquid in
our blood. This is called an increase in plasma osmolality, and it is detected
by a part of our brain called the hypothalamus. This gives us a conscious
awareness of thirst.
2) Decreased blood volume/blood pressure
Thirst also occurs when areas of our blood vessels sense (detect) that we don't
have enough blood circulating or our blood pressure is to low. Special parts
of our blood vessels send signals to our brain and help to stimulate thirst.
These special areas of the blood vessels are called Baroreceptors (Baro =
Pressure). Our kidneys dectect low blood volume and osmolality as well and
release factors that cause thirst behavior and water intake and conservation.
Our kidneys are the major controlling factor of water and blood composition.
The second part of your question deals with the physiology of thirst and
water balance. We can explain this using the principles of blood vloume/pressure
and blood concentration described above.
The major organs involved in water balance (homeostasis) are:
1 - Brain (hypothalamus): senses increased blood concentration (osmolality)
and gives us the conscious awareness of being thirsty, this causes us to seek
out water. In addition, it also responds by secreting a hormone called
Antidiuretic Hormone (ADH), this travels to the kidney and allows it to
conserve water by decreasing the water that it excretes in urine, thereby
working together with drinking water to decrease the blood osmality and
the thirst sensation.
2 - Kidneys - THE CENTRAL PLAYER: the kidney is very sensitive to blood volume
changes and responds rapidly when it thinks we don't have enough blood volume
in our system or our blood pressure is to low. When blood flow to the
kidneys is to low they secrete a hormone called Renin, which in turn is cleaved by
enzymes to Angiotensin 1 and again into Angiotensin 2.
THE KEY ----> Angiotensin 2 induces the secretion of other hormones
(aldosterone by the kidneys and ADH by the brain) that make the kidney
reclaim large amounts of water, causes thirst and also helps to increase blood
pressure by constricting blood vessels. This leads to restoration of proper blood
volume and pressure.
Sometimes these systems go awry and can lead to disease (read about
congestive heart failure).
3 - Baroreceptors - stimulate the thirst centers in the brain and ADH
Thirst and water balance are very interesting topics and are good examples
of how the body uses negative feedback to regulate important
detect an imbalance (high plasma osmolality) and cause the release and
activation of effectors (thirst sensation, drinking behavior, angiotensin 2
and ADH release) that restore proper balance. By doing so, the orignal signal
for action is eventually removed (you restore your plasma osmolality to normal)
and you are no longer thirsty.
This is homeostatsis in a nutshell. Hope this helps.
Steve Sardino - Second year medical student.
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Update: June 2012