An introduction to Homeostasis
Researched and Written by Jonjo Minns
Submitted to Biology-Online.org on February 25, 2009.
Homeostasis is defined as “the condition of equilibrium (balance) in the bodies internal environment due to the consistent interaction of the body’s main regulatory processes” Tortora and Derrickson [2009:8]. The scope of this essay is that it will describe the concept of homeostasis, in addition to the homeostatic mechanisms of which regulate heart rate, breathing rate, body temperature, and blood glucose levels. In addition to this, the importance of homeostasis in maintaining healthy functioning of the body will be explained.
The maintenance of body temperature is the responsibility of a team of structures within the body. Temperature control is vital to the maintenance of homeostasis within the body. Heat is sensed by thermo-regulators in both the skin and the hypothalamus. The difference is, internal temperature (temperature inside the body) is sensed by thy hypothalamus, and external temperature (temperature outside the body) is sensed by the skin.
When the external temperature outside is too cold, messages are sent from the many thermo-receptors located within the skin (or from thermo-receptors located either deep in the muscle or in the blood), to the cerebellum leading to the hypothalamus. The role of the cerebellum is to make the individual aware of feeling cold, of which may cause voluntary behavioural changes such as putting on more layers of clothing or a coat.
Once the message is received by the hypothalamus, a series of reactions follow. The first of which is by the hypothalamus, of which secretes thyroid releasing hormone (TRH). This hormone’s target is the anterior lobe of the pituitary gland. When the TRH reaches its target, it releases Thyroid Stimulating Hormone (TSH) of when then enters the blood stream. The target of this hormone is the thyroid gland.
Once the TSH is received by the thyroid gland, thyroxin is produced. The role of thyroxin is to increase cellular metabolism in order to generate heat. This hormone also inhibits vasoconstriction, the process whereas blood is diverted from the skin in order to conserve heat by keeping it deep within the body. Sweating is also reduced to keep the surface of the skin dry, thus preventing heat loss. In addition to all of these processes, the erector pilli muscles contract, causing the skin hairs to stand erect. This traps air between the hairs and the skin and creates a layer of insulation, therefore keeping the body warmer. In addition, the phenomenon of shivering is displayed and the bodies’ metabolic rate is increased.
One of the effects of the body becoming too cold is hypothermia. This occurs when the body’s core temperature falls from the norm, 37 degrees (98F) to an abnormal temperature below 35 degrees (95F). This is usually the response to prolonged exposure to cold temperatures. As was mentioned above, the normal response of the body in such situations is to take preventative action for example applying more layers or going out indoors. However if this is not possible, such as in hill walking, then hypothermia can ensue. When an individual is presented with a cold environment, the normal response would be shivering, vasoconstriction, and endocrine activity (where the body releases hormones in order to promote the generation of heat), however in hypothermia, these are not substantial enough to maintain the normal core temperature of the body.
There are multiple symptoms to hypothermia; these include excessive shivering, feeling cold, and lethargy, less tolerable of the cold, pale skin with any accompanying cyanosis (blue skin). These are the symptoms of a mild case of hypothermia. In the moderate case, the symptoms are as follows; extremely violent shivering of which cannot be controlled, cognitive difficulties, confusion, loss of fine motor skills, sleepiness, shallow, slow breathing rate. These are just a selection of the symptoms typically seen from a moderate case of hypothermia, of course there are more. These extend more seriously into a severe case if hypothermia symptoms include loss of gross motor skills, cessation of shivering, unconsciousness, dilated pupils, weak pulse, weak breathing rate, and cardio-respiratory arrest. There may also be a degree of cyanosis present due to the lack of blood to the superficial layers of the skin.
Hypothermia is treated by slow re-warming of the individual. This is done within the acute care setting for the moderate and severe cases. The warming of the individual’s body takes place from the inside, mainly by using warm intravenous fluids.
When the body is too warm, messages are sent in the same way as if the body is cold to the hypothalamus, this causes an increase in the amount of sweating, this is releasing heat via water, and the water on the skin evaporates, cooling the body down. Vasodilatation is also apparent, in this instance, blood is diverted to the skin in order to loose heat, the erector pilli muscles relax, allowing the skin hairs to be lowered, and the bodies’ metabolic rate is reduced. The reactions are different for each of the environmental states as the messages of which are sent are different. There is one message for cold and a different one for hot.
Water balance is another very important aspect of homeostasis of which needs to be controlled within narrow limits. The control of water balance is conducted using the following series of events. The osmoreceptors located within the hypothalamus detect the condition of fluid balance within the body. In the event of the fluid balance dropping too low, then the hypothalamus will act to bring the level back up by keeping more water within the body.
If the concentration of water within the body is too high, then the hypothalamus will react to excrete more water from the body. In the event of the hypothalamus sensing a change in fluid balance, messages are sent to the cerebellum, of where a feeling of thirst is produced, this is only when there is there is not enough water within the body. In addition, the hypothalamus sends a message also to the posterior pituitary gland to induce the secretion of ADH, the action of the ADH in this instance is to increase the permeability of the kidney’s collecting duct. Therefore, increasing the amount of water of which is reabsorbed into the body. On the contrary, if there is too much water within the body then the pituitary gland secretes no ADH, therefore more water leaves the body in the urine.
Blood glucose is another contributing factor to homeostasis. The blood glucose concentration in the blood is vital to the functioning of cells within the body and is controlled by a number of internal structures and external influence (food and drink). If too much glucose is present within the blood, then specific receptors located within the pancreas detect this. These receptors then send messages to the cerebellum, feelings of satiety (feeling full) are induced, and therefore the individual’s intake of food is decreased. Messages are also sent to the islets of Langerhans for the production of insulin to commence. Once insulin is produced, it is secreted into the capillary circulation and eventually into the systemic blood stream. The insulin has many effects, mainly consisting of increasing the intake of glucose by all the cells of the body. This action uses up surplus glucose and brings back a stable equilibrium. The insulin also aids in the conversion of glucose into a substance called glycogen in the liver, thus lowering the level of glucose in the blood and restoring equilibrium.
On the other hand, if there is not enough glucose in the bloodstream, then the very same receptors, of which are located in the pancreas detect the change. Once again, a message is sent to the cerebellum, of which brings around feelings of hunger, therefore increasing the consumption of food and drink. Messages are also sent to cells in the islets of langerhans to start the production of glucagon. This glucagon is released by the islets of langerhans into the capillary circulation. In turn the systemic blood stream, and stimulates the liver to convert stored glycogen into glucose. In addition, the liver is stimulated also to start the conversion of amino acids into glucose, therefore the levels of glucose in the bloodstream rise and equilibrium is achieved.
Homeostasis is also heavily involved with the control of the respiratory rate. In the norm, individuals are not conscious of their respiration. This is because the act of respiration is involuntary. Respiration is under involuntary control through an area of the brain termed the medulla. Within the medulla is an area known as the breathing centre. The breathing centre is composed into sections, allowing each to tackle an alternate aspect to respiration. Both the dorsal and the lateral areas assist with inspiration and provide stimulation for respiration. In addition, the ventral area increases both the depth and rate of respiration. The centre is linked with the intercostals nerves and the phrenic nerves, leading to the diaphragm. Theses routes provide a method of communication between the thorax, the respiratory system, and the medulla.
The medulla is chief in maintaining a constant rate of respiration and depth. However, both external and internal stimuli can alter the rate of respiration, making it higher or lower than the norm. The main influence to this is the level of carbon dioxide in the blood stream. If the concentration of carbon dioxide in the blood stream increases, then chemoreceptors located within both aortic and carotid bodies become aroused. This causes messages to be sent to the medulla of which send nerve impulses back down the phrenic and intercostals nerves to the intercostals muscles and the diaphragm. This causes them to contract and relax more quickly and therefore increasing the breathing rate. In order to introduce more oxygen to the blood stream and bring back equilibrium of both oxygen and carbon dioxide levels in the blood stream. This process is an example of negative feedback.
As for the control of the breathing rate, the medulla also controls the heart rate. The set process for the regulation of the heart rate is rather complex and is as follows. As an individual exercises, special receptors located within the muscles send impulses to the medulla. Once these messages are received, the medulla secretes epinephrine and norepinephrine. The combination of these two chemicals proceed through pathways within the nervous system until they reach the Sino-atrial node, located within the myocardium it acts like a pacemaker, controlling its electrical activity. These chemicals arouse the Sino-atrial node, making it produce more electrical energy, thus making the heart rate increase.
On the other hand, when exercise is ceased, the muscles send additional impulses to the medulla of which responds by secreting the hormone acetylcholine, this hormones decreases the heart rate by slowing down the electrical impulses from the Sino-atrial node and therefore, decreasing the heart rate. In addition, the medulla can also recognize other factors of which cause an increase in heart rate. These include emotional stress. In this instance, the medulla also takes information from the thalamus, which informs the medulla of the stressor. This is with the addition of information received from the nervous system. The combination of the two would enable the best response possible to be triggered.
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Tortora G. T, Derrickson B. H, 2009, Principles of Anatomy and Physiology: Volume 1: Organisation, Support, Movement, and Control Systems of the Human Body, 12th Ed, John Wiley and Sons, Pte. Ltd, Asia [Page 8]
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