System Functions, Structures and Control Mechanisms in the Human Body.

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Grace Wemyss

Section C: System Functions, Structures and Control Mechanisms


Homeostasis is defined as – ‘The ability of the body or a cell to seek and maintain a condition of equilibrium or stability within its internal environment when dealing with  changes.’ (16) Homeostasis is the bodies’ control mechanisms which are designed to keep the body’s internal environment constant, an example of this will be temperature. Most processes within the human body only work efficiently within a narrow range. Because of this, the body is constantly monitoring the internal environment and makes alterations where necessary. Homeostasis is achieved by a process called negative feedback.

Negative Feedback:


Negative feedback is when systems respond in an opposite way to stabilise a system as it is able to counterbalance what is affecting the body. Without this the body may, for example, over heat which can be fatal. Negative feedback can reduce abnormally high activity or increase low activity returning the body back to its desired state of equilibrium. Each system that uses negative feedback will have a sensor which constantly monitors the status of that system and changes its status when necessary. Without this constant monitoring the body may get too cold or over heat which without urgent medical attention can result in death. An example of negative feedback is body temperature. The internal body temperature must stay within narrow barriers despite its environmental temperature. If the body goes outside these narrow barriers the body’s state of equilibrium will be unbalanced and the hypothalamus will have to work hard to re-obtain its state of equilibrium. When the body temperature rises above its narrow barriers the body sweats. The body does not sweat out water; sweat is made out of toxins and other waste products. In addition to sweating blood vessels dilate, because of this blood runs close to the skins surface and heat is lost, this causes the body temperature to drop back down to within its normal range. The part of the body that controls our body temperature is the hypothalamus. The hypothalamus is located in the brain; it is pea sized and controls the autonomic nervous system (ANS), including the sympathetic division of the central nervous system. If the hypothalamus detects the temperature of the blood flow is below normal the hypothalamus sends out a signal that triggers a sympathetic reaction. The sympathetic reaction defers the blood flow away from the skins surface by constricting the blood vessels close to the skin surface, as this reduces the amount of heat lost through the skin as the vital organs in the bodies core need to be protected and maintained at a regular temperature. This causes the skin to appear paler in colour and feel cold because the amount of blood close to the skins surface is minimal, when a person is hot and blood flow is close to the skins surface the person will appear flushed. The blood is deferred away from the skin surface as it is able to keep the internal organs warmer; limbs are able to tolerate a lower temperature. Another response in aid to keep the human body warm the hairs on the skin surface become erect. This creates a layer of insulation as the hairs help trap warm air. The last involuntary response by the sympathetic system is muscle contractions, also known as shivers; these contractions speed up as the person becomes colder. The movement of the muscle contracting generate heat making the body warm up slightly. Homeostasis also controls heart rate but, unlike temperature, heart rate is not held at a constant rate. This is because our blood does not need to be kept at a high pressure all the time. If it was kept so high all the time it could be too tiring for our hearts as our heart will be over working. The heart rate fluctuates throughout the day to meet the demands of the body. Without this change in heart rate we will not be able to carry out simple activities such as running for the bus as if our heart rate was kept low minimal amount of oxygen would reach our muscles. The heart rate increases supplying the body with an increased amount of oxygen and blood sugar for cellular respiration. The initial heart beat is caused by a group of cells in the upper right atrium of the heart; these cells are called muscle cells. This group of muscle cells are called the sinoatrial node (SAN). This is the hearts natural pacemaker which initiates the hearts every beat by sending out many electrical impulses down through the atrium to the atrioventricular node (AVN). At the AVN the electrical impulse is delayed slightly by the non-conductive tissue separating the atrium from the ventricles. After this short delay the electrical impulse travels down the septum to the Apex of the heart via the purkyne fibres. The heart then contracts from the bottom up forcing blood up and out of the heart. As a person exercises these contractions happen more regularly increasing blood flow otherwise the person would not be capable of exercise as not enough oxygen and sugar will be supplied to the body. The sympathetic division of the autonomic system is responsible for the rise in blood pressure because this is the system that stimulates the adrenal glands to release a hormone known as adrenaline. Adrenaline is a powerful hormone which is the main part of the human body's acute stress response system; this term can also be known as the "fight or flight" response. Adrenaline works by increasing the heart rate within the human body, dilating air passages and contracting blood vessels – these processes increases blood flow to the muscles and oxygen to the lungs. (18) Once adrenaline is released into the blood it then circulates around the body until it reaches the heart. Once it reaches the heart the heart senses its presence and stimulates the sinoatrial node to increase the amount of heart beats per minute. Increasing the amount of heart beats per minute will supply an efficient amount of oxygen and sugar to respiring cells. On the other hand, when the heart rate needs to be slowed down by the parasympathetic system of the autonomic system. This nervous system uses the vagus nerve to do so. It releases a neurotransmitter called acetylcholine; this substance is released as it slows down the number of heart beats per minute. This neurotransmitter runs from the top of the spinal cord to the heart. Once the heart senses the neurotransmitter acetylcholines presence the sinoatrial node is instructed to reduce the number of beats per minute. This is because that high amount of oxygen and sugar is no longer needed. This maintains a low heart rate of between 60 – 70 bpm. Events that cause a high blood pressure are; exercise, stress or an emergency. Events that cause a low blood pressure can be anger, fear or anxiety. The respiration centre of the brain detects the level of carbon dioxide in the blood. If carbon dioxide levels were not detected they will not be able to be accurately maintained. If it rises above a certain level this can be extremely dangerous so the negative feedback mechanisms work to reduce this. The negative feedback mechanism at work is the sympathetic division of the ANS as the sympathetic nervous system increases the activity within the body. Firstly, the bronchi, in the lungs, dilate allowing faster airflow out of the lungs and effectively removes the carbon dioxide. As this happens the tidal volume increases, and also the rate of ventilation increases which means more oxygen can be breathed into the body. The cardiovascular rate is then increased by the sympathetic system steeply increases the transport of carbon dioxide and oxygen around the body as the oxygen is urgently needed by cells and muscles.  This continues to happen until the brain detects lower normal levels of carbon dioxide in the body. Once the low levels of carbon dioxide are detected the brain senses it is safe to reduce the breathing rate again back to normal. Ventilation and circulation is then brought back down to normal by the parasympathetic division as the parasympathetic nervous system is in charge of maintaining and decreasing activities in the body.

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Circulatory System:

The main components of the circulatory system structure are; the human heart, arteries, arterioles, capillaries, venules, veins and the blood. The heart consists of 4 chambers which are each perfectly adapted to their function. The four chambers are called the left atria, right atria, left ventricle and the right ventricle. As you can see from figure 1. the left ventricular wall is significantly thicker than the    right ventricular wall; it can be up to three times thicker. The left ventricle has a thicker muscular wall as it needs to be able to pump the blood out ...

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