Controlling the internal environment

An essay i did over easter break!

hope it helps!

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  • Created on: 24-04-11 20:07
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Control of the internal environment
The maintenance of a steady internal state in the body almost regardless of changes in either the
external or internal conditions is known as homeostasis. This involves a high level of control and
coordination, involving positive and negative feedback systems. Homeostasis plays an important role
during exercise as conditions in the body are rapidly changing and so are the demands of different
One element of homeostasis is adjusting the heart rate. This is controlled by electrical messages as well
as chemical ones. Firstly this essay will explore how nervous control (electrical) of the heart. The
cardiovascular centre controls the heart rate and is found in the medulla of the brain. Chemical and
stretch receptors in the lining of the vessels and chambers of the heart send impulses to the
cardiovascular centre. Then two types of nerves carry these impulses to the heart, controlling at which
rate it beats. Most of the nervous control of the heart is done by the autonomic nervous system which is
involuntary. This consists of the sympathetic nervous system which is excitatory and so will speed up the
heart rate and also the parasympathetic nervous system which is inhibitory and so will slow down the
heart rate. Nervous impulses travel down the sympathetic nerve from the cardiovascular centre in the
brain to the SAN in the heart to stimulate it. This increases the frequency of the signals from the hearts
pacemaker which in turn makes the heart beat more quickly. Whereas when a nerve impulse travels
down the parasympathetic nervous system, the SAN in inhibited and so the heart rate slows down. There
is also hormonal control (chemical) over the heart rate. Our hearts beat faster when we feel nervous,
excited or frightened etc. This occurs because when one is stressed, our bodies release the hormone
adrenaline. This affects the SAN and speeds up the frequency of excitation and so increasing the heart
rate. When exercising homeostasis plays and important role in regulating the heart beat to match the
demands of the body. The main control system that regulates the heart rate involves a negative
feedback response to the pressure of blood in the circulatory system. As the atria fill with blood, stretch
receptors in the muscle wall respond by sending nervous impulses to the cardiovascular control centre.
At the start of exercise, more blood than usual returns to the heart as big muscle blocks work harder. As
more blood flows into the atria, the receptors are stretched more than usual and so send impulses to the
brain. This in turn sends more nerve impulses along the sympathetic nerve to the SAN, causing an
increase in heart rate. The increased stretching of the hearts atria muscles also make the heart muscles
contract harder. This increased the volume of blood expelled at every contraction. Baroreceptors found
in the sinuses of the carotid arteries also play a part in homeostasis and are an important part of the
feedback control of heart rate. As blood pressure increases in the arteries, the baroreceptors are
stretched. This causes them to send impulses to the cardiovascular centre which then send impulses via
the parasympathetic nerve to slow down the rate and to cause vasodilatation. Both of these actions
lower the blood pressure. This occurs at the end of exercise.
Another element of homeostasis is controlling the temperature of the body. The temperature in our
bodies is regulated within a very narrow range. The main source of our bodies heat in from metabolism
but we still need effective methods of losing and conserving heat. The major homeostatic organ involved
in thermoregulation is the skin. The skin has a huge surface area which can be modified either to
conserve or loose heat. A rich supply of capillaries runs near the surface of the skin and heat loss occurs
here due to radiation, convection and conduction to the environment. This heat loss is controlled by the

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When someone is exercising, the shunt is closed which allows more blood to flow
through the capillaries at the surface of the skin. This is known as vasodilatation and is why when people
are exercising they appear red and flushed. As a result, more heat is lost. Also when exercising, the
erector pili muscles are relaxed and so the hair lie flat against the body, this minimising the insulating
layer of air that is trapped next to the skin.…read more


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