Anatomy & Physiology Essay
In this assignment the concept of homeostasis will be explained and the probable
homeostatic responses to changes in the internal environment during exercise will be
discussed. Homeostasis is simply how the body keeps conditions inside the same. It is
described as the maintenance of a constant internal environment. Generally, the body is
in homeostasis when its needs are met and it’s functioning properly. Every organ in the
body contributes to homeostasis. A complex set of chemical, thermal, and neural factors
interact in complex ways, both helping the body while it works to maintain homeostasis.
In homeostasis there is the concept of Negative feedback which ensures that, in any
control system, changes are reversed and returned back to the set level. There are four
different homeostatic mechanisms for regulation these four are the heart rate, breathing
rate, body temperature and blood glucose levels.
Negative feedback system is made out of receptors to detect change, a control centre to
receive the information and process the response and effectors to reverse the change
and re-establish the original state. (Anatomy & Physiology, 2013) The autonomic nervous
system controls the heart and has two branches; the sympathetic nervous system and
the parasympathetic nervous system. When the body is undergoing muscular work, fear
or stress the sympathetic nervous system will be active. When the sympathetic nervous
system is active it will cause every heartbeat to increase in strength and heart rate. During
resting, peace and contentment the parasympathetic nervous system is active and it
calms the heart output. During periods of fright, flight and fight the sympathetic nervous
system is boosted by the hormone; adrenaline. The nerves of the adrenaline are the
cardiac nerves. A special cluster of excitable cells are supplied by the sympathetic and
parasympathetic nervous system in the upper part of the right atrium. We call this ‘the
peacemaker’ in general terms.
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A connection of impulses from the sympathetic and parasympathetic nerves acting on the
sino-atrial (‘the pacemaker’) regulates the activity of the heart to suit situations from
minute to minute, hour to hour and day to day. The sino-atrial node sends out a cluster of
nerve impulses every few seconds around the branching network of atrial muscle fibres
to cause contraction. The impulses are caught by a different group of cells forming the
atrioventricular node and relayed to a band of leading tissue made of big, modified muscle
cells called Purkinje fibres. In the atrioventricular node the transmission of impulses is
delayed slightly to enable the atria to complete their contractions and the atrioventricular
valves to start to close. The location of heart valves is on a fibrous figure-of-eight between
the atrial and ventricular muscle masses.(Aldworth and Billingham, 2010) The lowest part
of the brain is the medulla and is located above the spinal cord and is often known as the
‘brain stem’.
The two important centres for control of the heart rate are located in the brain stem. These
are called the cardiac centres. The sympathetic fibres descend through the spinal cord
from the vasomotor centre while the cardio-inhibitory centre is in charge of the origins of
the parasympathetic fibres of the vagus nerve reaching the sino-atrial node. (Aldworth
and Billingham, 2010) Baroreceptors are found in the walls of the aorta and they detect
changes in blood pressure. If in the arteries a small upward change in blood pressure
happens it often indicates that extra blood has been pumped out by the ventricles as
result of the extra blood that enters the heart on the venous or right side. When the
baroreceptors detect the change they relay the information in nerve impulses to the
cardiac centres. Movement in the vagus nerve slows the heart rate down and reduces the
high blood pressure to normal.
Thermo receptors are receptors that are sensitive to temperature and they are present in
the skin and deep inside the body. Also they relay information through nerve impulses to
the hypothalamus; this is a part of the brain which activates appropriate feedback systems.
During fear, stress and exertion, the adrenal gland releases a hormone called circulating
adrenaline. Circulating adrenaline stimulates the sino-atrial node to work faster, therefore
boosting the effect of the sympathetic nervous system. The hypothalamus activates the
sympathetic nervous system when thermo receptors indicate a rise in body temperature
to the brain. When the sympathetic nervous system is activated it causes the heart rate
to increase. Our rate of ventilation is mainly on ‘automatic pilot’ and do not notice little
variations that are the result of homeostatic regulations. We are only voluntarily controlling
our breathing when taking deep breaths, speaking or holding a breath.
Breathing rate increase slightly when metabolism produces extra carbon dioxide until this
surplus is ‘blown off’ in expiration. Also a period of forced ventilation will decrease the
carbon dioxide levels in the body and homeostatic mechanisms will slow or stop breathing
until levels return to normal. A period of forced ventilation can be for example
gasping.(Aldworth and Billingham, 2010) Internal receptors relay nervous impulses to the
brain about the status of ventilation from the degree of stretch of muscles and other
tissues when they function as stretch receptors in muscles and tissues. Changes in
chemical stimuli are detected by chemoreceptors and they supply the brain with the
information. There are to chemoreceptors; the central and peripheral. The central
chemoreceptors are located in the medulla of the brain and monitors H+ ion concentration.
When H+ ion concentration is increased it causes increase in ventilation rate. Peripheral
chemoreceptor’s increase ventilation when oxygen levels decrease. Peripheral monitors
changes in oxygen. (Aldworth and Billingham, 2010) The respiratory system has a dual
autonomic supply.
The sympathetic causes the bronchial muscle to relax and the parasympathetic causes
the bronchial muscle to contract. This causes narrowing in bronchi. Vagus means ‘a
wanderer’ and the vagus nerves is so called because it wanders all over, supplying
internal organs. Sympathetic nerves emerge from the places where nerves interconnect,
to run to the bronchi, these places are called a chain of ganglia.(Aldworth and Billingham,
2010) The upper part of the brain is called cerebral cortex; this part of the brain is
responsible for voluntary control of breathing. The respiratory centre, also called the
involuntary centre is found in the medulla and the pons. Each centre receives information
of internal receptors about the state of ventilation. The respiratory pacemaker and the
respiratory centre are similar to each other. The inspiratory and expiratory centres are
two groups of nerve cells. If one is active the other one is inhibited.
The inspiratory centre is actively sending nerve impulses to the nerve to the diaphragm,
the phrenic nerve, and the thoracic nerves are sending impulses to the intercostal
muscles which cause contraction and the contraction results in inspiration. Inspiration
stops when the stretch receptors send bursts of impulses to the inspiratory centre. These
bursts of impulses indicate that the chest and lungs are fully expanded, and the flow of
impulses subsides, releasing the expiratory centre from inhibition. The expiratory centre
then sends nerve impulses to the respiratory muscles which causes relaxation and
expiration. The information that comes from the other internal receptors, for instance the
chemoreceptors (which effects the homeostatic regulation) monitors and modifies the
cycle.