Homeostasis change in variable is detected by

Homeostasis
can be defined as the maintenance of a constant internal environment within the
body. Sensors within our body monitor a number of things including breathing,
heart rate, body temperature and also blood sugar levels. These can also be
known as detectors, which send signals to the control centre when there is a
change, or the value has deviated from the norm. This value will then be
corrected so that the norm can be maintained.

Negative
feedback is important in homeostasis and it responds when certain conditions
change. This therefore means that receptors and effectors, i.e. muscles or
organs, carry out a series of reactions so that these conditions can remain
constant. This may also be explained by saying that a change in variable is
detected by the receptor and the information from this is sent along an
afferent pathway to the control centre. The control centre then sends the
information along an efferent pathway to the effector whereby it either opposes
or enhances the stimulus.

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In
the medulla oblongata there are chemoreceptors which are adjacent to the
respiratory centre. These chemoreceptors are sensitive to the changes of
arterial PCO2, PO2 and also pH, and send information to the medulla,
determining the nervous response depending on the changes of the variables.
Nerve impulses are therefore then sent to the respiratory muscles controlling
both the force and how often it contracts. Furthermore, this changes the rate
and depth of breathing and also ventilation. The change in ventilation brings
CO2, O2 and pH back to their norm. Nerve impulses are sent along the phrenic
nerve towards the external intercostal muscles which stimulates muscle
contraction for inspiration. Expiration occurs due to the elastic recoil of the
lungs and chest wall. This nerve firing is what gives us our resting breathing
rate of 12-15 breaths per minute. During exercise, the muscles have to
metabolise faster as they require both more oxygen and nutrients. Due to this,
the heart then pumps the blood harder and faster to keep up this demand, as the
heart is doing more work to supply this blood. This means that more oxygen is
required, meaning, the response given is breathing being increased so that
oxygen is pumped to all cells quicker. Due to homeostasis, levels of oxygen in
the blood are always being measured, ensuring oxygen, carbon dioxide and also
pH levels return to their norm. Messages that are sent to the effectors
informing them that the breathing rate has to be increased, however, will
decrease again when all activity has been stopped.

Homeostasis
also controls heart rate. The medulla which is located within the brain also
controls heart rate. It sends information or messages normally in form of
chemicals/hormones. When we are carrying out exercise the heart has to supply
oxygenated blood to the rest of the body. There is information sent to the
medulla from the muscles via the nervous system. This allows the release of
chemicals, to travel to the sinus node. The sinus node then therefore
stimulates the contractions of the heart, also increasing the force which in
turn, increases heart rate. When you are at rest, or stop exercising, another
message is sent to the medulla, which in turn releases acetylcholine, slowing
the heart rate. When engaging in more intense exercise, epinephrine and
norepinephrine is released, increasing heart rate to supply more oxygen to the
body.

There
are two pathways known as the autonomic nervous system and the parasympathetic
nervous system. During exercise the sympathetic nervous system is activated and
this increases heart rate and also the force of the contractions due to the
nerve impulses being transmitted to the heart via the sympathetic nervous
system. In comparison the parasympathetic nervous system decreases heart and rate
and therefore it returns back to the norm and this system is activated when we
are resting. The vagal nerve is what reduces heart rate.

The
sinoatrial node (SA node) acts as the body’s pacemaker. The impulses initiate
at the SA node moving a wave of electrical excitation across the atria, which
respond by contracting. The ventricles are relaxed meaning that more blood is
being pushed into them. The impulses are then passed to the atrioventricular
node (AV node), however, the AV node delays the passage of impulses to the
bundle of His and is then conducted to the  fibres. The ventricle walls will contract from
the apex working up, meaning that blood is ejected from the ventricles
efficiently sending blood to the lungs and the rest of the body.

The
level of glucose within the blood is also controlled by homeostasis. The
maintenance of the level of glucose within the blood involves both the pancreas
and the liver. Islets of Langerhans are cells located in the pancreas and these
secrete two hormones known as insulin and glucagon. Blood sugar rises after we
have ate a meal resulting in the stimulation of the pancreas cells, meaning
b-cells of Langerhans are stimulated, releasing more insulin, enabling the
sugar uptake by cells and also the storage of sugar within the liver and
muscles. As a result, blood sugar levels are decreased. If however, blood
glucose levels are low, the body will not be able to produce the sufficient
amount of ATP needed for bodily functions. Alpha cells in the pancreas are then
stimulated releasing glucagon into the blood. The liver then breaks this down
into glucose which is then released into the blood. Glucose levels in the blood
have now risen and there is no need for the release of glucagon. During
exercise there is a demand for glucose due to the contraction of the muscles
and more energy being required and so this causes an increased uptake of
glucose to working skeletal muscles which is caused by an increase in the
insulin. Normal blood glucose levels however, can be maintained during exercise
by increased glucose production and the release through the stimulation of the
breakdown of glycogen and glucose synthesis from other substances. This
increase allows the maintenance of blood sugars. When we stop exercising,
receptors send information to the liver telling it to slow down glucose
production.

There
are four different ways in which heat can be gained or lost from the body
including radiation, evaporation, convection and conduction. Radiation is when
heat from the body is given off into the atmosphere. Evaporation is when you
sweat and the evaporation from the liquid generates heat, resulting in a
cooling effect. Convection is the process of heat leaving the body via moving
air flowing by the skin. Conduction is the transfer of heat from direct contact
with another object.

The
main control centre in the brain that controls body temperature is known as the
thermoregulatory centre. When we exercise, body temperature will increase as
the body is working hard in attempt to be able to have more oxygen in the blood
which then can be delivered to the muscles providing them with energy. Change
within the temperature in the blood is detected by thermoreceptors. There are
also receptors which are in the skin and they detect changes in temperature
within the environment. Homeostasis will occur due to the negative feedback
triggering homeostatic mechanisms. The hypothalamus in the brain detects
signals and sends impulses to both blood vessels and sweat glands. Firstly the
hairs on the skin lie flat as the erector muscles are relaxed. This therefore
increases the process of heat loss by conduction and radiation. Increased
sweating also known as hyperhidrosis is due to the sweat glands releasing a
salty liquid onto the skins surface, taking heat with it. Blood vessels can
also dilate allowing more blood to flow through. The blood flows close to the
body’s surface meaning that there is increased radiation. This is a process
known as vasodilation. Also due to an increased body temperature there will also
be increased sweating, and the need to drink due to thirst. When we become too
cold however, the opposite of this happens and begin to shiver as a mechanism
to rise body temperature. Heat loss will be reduced as the hairs on the skin
stand so that they are able to trap a layer of air, acting as an insulator.

In
conclusion, homeostasis is important as it maintains the appropriate levels
within our body that our cells need to function properly and it allows us to
adapt to environmental changes. It keeps the body at a norm, however, if
conditions are at the extreme, the negative feedback mechanism will no longer
work, resulting in death, if there is no medical help.