Feedback mechanisms in homeostasis

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Regulation and Control Assignment 1

Homeostasis is the maintaining a relatively constant level or a steady state of our bodies internal environment. Homeostasis is controlled by negative feedback. This is a corrective procedure that our bodies do called self-adjustment. In a self adjusting system a change in the internal environment, for example a spike in blood glucose would cause the receptors to acknowledge the imbalance and notify the effectors (in this case, insulin) to respond and return the body back to a suitable level. Other examples of this would be an increase of our core temperature, this would cause the effectors to cause the body to lose more heat. The opposite is true if our temperature was to drop. The diagram below is a simple negative feedback system. (KENT, Michael, 2001)

Negative feedback is not perfect and this can still allow homeostatic intervention to fail and this results in the negative feedback not working. Changes in critical areas, such as glucose or heat or salt are not intervened with corrective measures, potentially the margin of error can grow and the fault becomes bigger. This is known as positive feedback, where a small change in output can create a further change in the same direction. This means that excess volumes of a substance could become greater and create further excess or the opposite where our body is deficient in a substance and positive feedback would create a further deficiency. The diagram to the right highlights the issue.

Positive feedback is potentially very harmful due to creation of unstable conditions that can cause a spiral of negative effects such as core temperature of the body rising to a level that can threaten serious complications including death. However they are of use to body because they are designed to accelerate and enhance an output or stimulus that is already active. They push levels outside of the normal ranges by increasing the effect of the stimulus, this is very rarely used by the body as the acceleration of reactions or conditions in the body can rapidly become uncontrollable.

In contrast negative feedback mechanisms maintain and regulate the bodies functions through a set of parameters. A great example of this is the negative feedback system is temperature control. The body monitors its temperature through the hypothalamus which is capable of detecting subtle changes of the normal 37 oC temperature which would send out signals to muscles if the temperature dropped causing shivering to occur. If the body was becoming too warm, signals would be sent to stimulate the sweat glands, causing perspiration. (KENT, Michael, 2001) (JOHNSON, Allan, 2015)

One of our most important organs in the body that is actively involved in our thermoregulation and homeostasis is our skin. The diagram below shows a vertical section of mammalian skin.

The first thing to consider are the temperature receptors, these thermoreceptors detect the external temperature. They are located within the dermis layer and they're connected to the brain via nerves.

The other layer of skin is known as the epidermis and it is the top most layers, the surface contains dead cells. The epidermis is the site where sweat pores are located.

The sweat glands and sebaceous glands are located within the dermis. As the release of sweat secreted by the glands, the sweat evaporates from our bodies and this causes a cooling effect.

The hypothalamus located in our brains detects the bloods temperature.

Blood vessels are located within the dermis also play a role in thermoregulation. They have the ability to push blood to the very surface of the skin or keep the blood contained to the lower, deeper layers. This is known as vasoconstriction and vasodilatation. The diagram to the left shows how these blood vessels change their structures allowing the cold or the heat to be countered. Hairs on the skin will also aid in temperature control. When we are cold our hair erector muscle will cause the hairs to become erect, stand on end, this helps to trap air between the hair and the skin. This acts as an extra layer of insulation. (KENT, Michael, 2001) (BBC, 2015)

The diagram below demonstrates the loop of information and how excess heat or excess cooling is managed within our bodies.

Negative feedback helps to control our blood glucose levels too. Glucose is regulated by hormones which are produced in the pancreas in an area known as the islets of Langerhans.

As glucose levels in the blood rise as carbohydrates are broken down and absorbed. Specialised cells known as beta cells detect this raise of glucose which triggers a spike in production of the hormone insulin. This increase of insulin helps bring back down the levels of glucose by binding to the receptor proteins in cell membranes of which the liver, large muscles groups and fats are priority. The effect of this is that more protein channels are opened allowing more glucose to enter these cells. This combined with insulin triggering enzymes to actively convert glucose into glycogen in a process known as glycogenesis which is the way the body stores energy.

The other form of negative feedback monitoring is when there is a decrease in blood glucose, for example after exercise. Lower levels of glucose in the blood are detected by alpha cells which secrete glucagon. Glucagon's presence in the blood activates an enzyme called phosphorylase which is a catalyst involved in the breakdown of glycogen back into glucose. Gluconeogenisis is the name of this process of synthesising glucose from non-carbohydrate sources. The diagram to the right sums up the actions of insulin and glucagon working in tandem to create a negative feedback mechanism.

(KENT, Michael, 2001) (JOHNSON, Allan, 2015) (AID, Science, 2015)

The excretion process is the removal of metabolic waste.

Carbon dioxide (CO2) is very important for cell functioning as it will lower the pH of a solution which will then have a negative impact on the functioning enzymes. It also can affect the haemoglobin molecules which can cause a decrease in oxygen saturation and as oxygen is extremely vital for our survival it is very important to remove carbon dioxide and metabolic waste as they are toxic and prevent cell functions.

The removal of nitrogen wastes are also essential for our survival, these are the by-products of protein metabolism. When amino acids go through energy conversion the amino acids groups are removed. This allows the NH2 to combine with a hydrogen ion which forms ammonia, NH3.

Ammonia is very toxic and is rapidly converted by our body into a compound that our body can tolerate called urea where it is dumped into the blood and then goes onto the kidneys to become more concentrated. The body has several means to get rid of these waste products. The lungs will remove excess carbon dioxide. The liver will produce urea and uric acid, the skin will aid in the removal of excess volumes of water, salt, urea and uric acid. This is why sweat smells as it will contain urea or uric acid. Finally our kidneys will filter the blood to create urine, which is the excess volume of water, salt, urea and uric acid.

As humans we have two kidneys, they are supplied blood by the renal artery (providing oxygenated blood) and renal vein which carries by the filtered blood back to the heart. The image to the right is a cross section of a kidney.

Within the kidney there are specialised tubules which are minute in size. These are called nephrons and they all share a similar structure and function.

The Bowman's capsule which is the ultrafiltration unit of a kidney. This filters the blood and helps split up large particles from the small ones.

The proximal convoluted tubule is responsible for selective reabsorption. This allows the collection of vital and important substances eg, removing and reabsorbing glucose from urine.

The loop of Henlé is responsible for reabsorption of water by osmosis, it creates a low water potential located within the mudulla of the kidney.

The distal convoluted tubule are involved with osmoregulation by using collecting ducts they are able to control how much water they will reabsorb back into the blood.

Each of these structures are present within the nephrons of our kidneys and there are over a million nephrons in a human kidney. These structures allow the kidneys to take on their varied roles of removing waste from blood, regulating salt content, blood pressure and pH of the urine. (KENT, Michael, 2001) (AID, Science, 2015)

Ultrafiltration is one of the main roles that the nephrons provide, this occurs in the Bowman's capsule as previously mentioned. The renal artery which supplies oxygenated blood is fed into arterioles which in turn will each feed a nephron. The glomerulus is formed by a series of capillaries which form a knot like shape. This is surrounded by the Bowman's capsule. The blood enters the glomerulus at high pressure and the pressure allows filtration to occur, this is due to the arteriole leading into the glomerulus is larger than the one leaving the glomerulus. The filtering is done by endothelium and podocytes which separate the very large molecules also the capillary walls are fenestrated which allows the plasma to move through. The endothelium are the cells which line a capillary, they are very thin and flat. The podocytes are specialised cells present within the Bowman's capsule that are able to grip onto the basement membrane in which the fine filtering occurs and large molecules are not allowed to continue into the nephron. The glomerular filtrate is then allowed to pass into the proximal convoluted tube. (KENT, Michael, 2001)

The proximal convoluted tube allows for the selective reabsorption of glucose, water, amino acids, vitamins, hormones and sodium ions. Glucose in the glomerular filtrate will be absorbed into the blood by active transport. Active transport requires ATP, chemical energy, the proximal convoluted tubes are specialised cells and have vast quantities of mitochondria for the production of ATP. Sometimes however the amount of glucose is greater than what the cells can reabsorb. This level is known as the renal threshold, when this occurs glucose will be present within the urine. This is what allows for the detection of diabetes as diabetes will often have high concentrations of glucose in their urine. (KENT, Michael, 2001)

The loop of Henlé is responsible for creating the conditions needed for the final reabsorption of water. This occurs by an ion gradient, salt is added to the filtrate in the descending limb. Cells that surround this are impermeable to ions, allowing the water to move from a negative water potential to a more negative water potential. The ascending limb walls are impermeable to water. Allowing water to remain and ions to leave, creating an ion gradient in the medulla. The countercurrent exchange mechanism and multiplier enable the loop of Henlé to function. (KENT, Michael, 2001)

The kidneys themselves are controlled by both the hypothalamus and pituitary gland through a negative feedback system. This is controlled by the antidiueretic hormone, ADH, which is secreted by the pituitary gland which travels in the blood to the kidneys. In the kidneys ADH will increase how much water the distal convoluted tubules and collecting ducts will allow through by increasing the permeability. Osmoreceptors in the hypothalamus are what monitors the blood for water potential. Depending on if the water potential is high or low, will cause a variance in volumes of ADH released. The diagram to the right highlights how this works within our bodies. However the more concentrated our bloods plasma the more ADH is released causing the kidneys to reabsorb more water. This will produce a more concentrated urea presence in the urine. The opposite is true when the blood plasma is more dilute, less ADH will be secreted into the blood, allowing more water to leave the kidneys. (KENT, Michael, 2001) (BBC, 2015)

The way the body keeps track and controls itself is through homeostasis which is the maintenance of a constant internal environment. Hormones play a huge role in this along with the nervous system. Hormones are secreted by glands around the body and they have chemical effects on the body regulating processes. They control a variety of mechanisms within the body as previously discussed with ADH, this hormone directly effects the permeability of the duct cells and controls the volume of water in our body. There are two types of glands, exocrine and endocrine glands. Exocrine glands use ducts in which all their secretions are poured into, tear duct and sweat glands are a good example of exocrine glands. Endocrine glands are directly connected to the blood and are able to directly put their secretions into blood vessels. The endocrine system controls activities within the body using the chemical effects of hormones. The table below summarises a few of the endocrine systems glands and how they affect the control and coordination of our bodies activities. (BBC, 2015) (Tutor Vista, 2015)

Gland

Hormone

Hormones control and coordination effects

Adrenal Gland

Adrenalin

This produces adrenaline which is the bodies preparation for action, either fight or flight. It rapidly increases activity within the body by increasing the heart rate, the level of sugar present within the blood and diverting more blood to muscles and our brains. This is a very important control and coordination effect because this can potentially save our lives.

Ovary

Oestrogen

This controls a woman's menstrual cycle. It controls puberty and the production of luteinising hormone which triggers the egg release and the production of progesterone within the ovaries. It also acts as suppressant of follicle stimulating hormone found within the pituitary gland, which stimulates the ripening of the egg and the production of oestrogen. This again highlights how important hormones are to our control and coordination as a mix up in the hormonal balance could lead to infertility.

Pancreas

Insulin

This is produced within the beta cells of the pancreas it is a peptide hormone. It controls our blood sugar levels by using two hormones, insulin and glucagon. The pancreas is monitoring our bodies blood sugar levels constantly and when it detects a spike, for example after eating it will stimulate the beta cells to produce insulin. The insulin aids the glucose absorption rate into cells where it can be used for cellular respiration. However when there is too much glucose it will actively convert glucose into glycogen which is how the body stores energy. This process is known as gluconeogenisis. Without the system of control of blood sugar levels of the blood would continue to elevate and start to cause problems with our red blood cells, as the sugar will bind to the haemoglobin causing them to become rigid. This then can cause interference of the bloods circulatory system by allowing cholesterol to build up on our blood vessels.

Thyroid Gland

Thyroxine

Is the chemical control system for our body's metabolic rate primarily, but also causes effects on our heart, digestive system, brain development, muscle control and maintenance of large bones. It is controlled by negative feedback involving the hypothalamus, pituitary and thyroid glands. When concentrations of thyroxine and triiodothyonine increase the body detects this and the negative feedback system prevents the release of thyroid stimulating hormone. This control system prevents thyrotoxicosis which is commonly referred to as an over active thyroid. It also prevents the opposite which is called hypothyroidism which is too little of thyroxine being produced. Both of which have very serious and potentially life threatening consequences.

Testes

Testosterone

Testosterone is produced by the gonads in the testes in the Leydig cells in men. In women it is produced by the ovaries. It can also be produced by the adrenal glands however this is only in small quantities. This hormone helps the production of sperm, creation of new blood cells, muscle and bone strength and sexual libido. It is controlled by the hypothalamus and pituitary glands in a negative feedback loop. For example as testosterone levels increase in the blood the hypothalamus will stop producing gonadotrophin which will then suppress the production of luteinising hormone by the pituitary. This will prevent the stimulation of the gonads and stop the release of testosterone. When the levels of testosterone begin to fall, the negative feedback system will cause the hypothalamus to continue the release and secretion of gonadotrophi which in turn stimulates the pituitary, which stimulates the gonands to produce testosterone.

(Your Hormones Website , 2015)

Word Count: 1983 (Excluding tables, titles, references, diagrams/pictures)

Bibliography

AID, Science. 2015. Science Aid. [online]. [Accessed 28 Febuary 2015]. Available from World Wide Web: <http://www.scienceaid.co.uk/biology/humans/homeostasis.html>

BBC. 2015. BBC Bitesize. [online]. [Accessed 28 Febuary 2015]. Available from World Wide Web: <http://www.bbc.co.uk/schools/gcsebitesize/science/add_aqa_pre_2011/homeo/homeostasis5.shtml>

JOHNSON, Allan. 2015. Regulation and Control Notes. Lincoln: Lincoln College.

KENT, Michael. 2001. Advanced Biology. Oxford.

Tutor Vista. 2015. [online]. [Accessed 02 March 2015]. Available from World Wide Web: <http://www.tutorvista.com/content/biology/biology-ii/control-and-coordination/endocrine-system.php>

Your Hormones Website. 2015. [online]. [Accessed 02 March 2015]. Available from World Wide Web: <http://www.yourhormones.info/Hormones/>

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