-blood glucose (controlled by the endocrine system, glucagon and insulin)
-body temperature (controlled by the nervous and endocrine systems)
Source - Textbook (Mackean)
Homeostasis means ‘staying similar’. It refers to the fact that the composition of the tissue fluid in the body is kept within narrow limits. The concentration, acidity & temperature of this fluid are being adjusted all the time to prevent any big changes.
In living cells, all the chemical reactions are controlled by enzymes. The enzymes are very sensitive to the conditions in which they work. A slight fall in temperature or a rise in acidity may slow down or stop an enzyme from working & thus prevent an important reaction from taking place in the cell.
The cell membrane controls the substances which enter and leave the cell, but it is the tissue fluid which supplies or removes these substance, & it is therefore important to keep the composition of the tissue fluid as steady as possible. If the tissue fluid were too concentrated, it would withdraw water from the cells by osmosis and the body would be dehydrated. If the tissue fluid were too dilute, the cells would take up too much water from it by osmosis and the tissue would become waterlogged or swollen.
Many systems in the body contribute to homeostasis. The obvious example is the kidneys, which remove substances that might poison the enzymes. The kidney also controls the level of salts, water and acids in the blood. The composition of the blood affects the tissue fluid, which in turn, affects the cells.
Another example of a homeostatic organ is the liver, which regulates the level of glucose in the blood. The liver stores an excess glucose as glycogen, or turns glycogen back into glucose if the concentration in the blood gets too low. The brain cells are very sensitive to the glucose concentration in the blood & if the level drops too far, they stop working properly, & the person becomes unconscious and will die unless glucose is injected into the blood system. This shows how important homeostasis is to the body.
The lungs play a part in homeostasis by keeping the concentrations of oxygen and carbon dioxide in the blood at the best levels for the cells’ chemical reactions, especially respiration.
If the cells were to get too cold, the chemical reactions would become too slow to maintain life. If they became too hot, the enzymes would be destroyed.
The brain has overall of the homeostatic processes in the body. It checks the composition of the blood flowing through it and if it is too warm, too cold, too concentrated or has too little glucose, nerve impulses or hormones are sent to the organs concerned, causing them to make the necessary adjustments.
The kidneys, lungs, liver & skin all help to keep the blood composition the same (homeostasis).
Control of BGL
The endocrine system plays an important part in maintaining the composition of the body fluids
A rise in blood sugar after a meal stimulates the pancreas to produce insulin. The insulin causes the liver to remove the extra glucose from the blood & store it as glycogen. This helps to keep the concentration of blood sugar within narrow limits
The brain monitors the concentration of the blood passing through it. If the concentration is too high, the pituitary gland releases ADH (antidiuretic hormone). When this reaches the kidneys (the target organs) it causes them to reabsorb more water from the blood passing through them. If the blood is too dilute, production of ADH is suppressed & less water is absorbed in the kidneys. Thus ADH helps to maintain the amount of water in the blood at a fairly constant level.
Some of the endocrine glands are themselves controlled by hormones. EG. Pituitary hormones such as LH affect the endocrine functions of the ovaries. In some cases, the output of hormones is regulated by a process of negative feedback.
The feedback between the pituitary and the ovaries produces fluctuations which cause the menstrual cycle.
When the level of oestrogen in the blood rises, it affects the pituitary gland, suppressing it production of FSH (follicle-stimulating hormone). A low level of FSH in the blood reaching the ovary will cause it to slow down its production of oestrogen. With less oestrogen in the blood, the pituitary can resume its production of FSH, which in turn, makes the ovary start to produce oestrogen again. This cycle of events takes about 1 month and is the basis of the monthly menstrual cycle.
The oestrogen and progesterone in the female contraceptive pill act on the pituitary & suppress the production of FSH, none of the follicles in the ovary will grow to maturity and so no ovum will be released.
Source -
www.diabetes-insight.info/lwd/management/control/DI_control_bg.asp
In the short term, controlling blood glucose levels is important in order to avoid diabetic emergencies – very high or very low blood glucose levels. Both of these conditions are unpleasant and can be dangerous, so they should be avoided if at all possible. High blood glucose levels in Type 1 diabetes, if caused by a lack of insulin, can lead to a condition known as diabetic ketoacidosis or ‘DKA’ which can be fatal if it is not treated in time.
Source -
www.colorado.edu/epob/academics/web_resources/cartoons/homeo.html
Homeostasis and Negative Feedback
Homeostasis is one of the fundamental characteristics of living things. It refers to the maintenance of the internal environment within tolerable limits. All sorts of factors affect the suitability of our body fluids to sustain life; these include properties like temperature, salinity, acidity, and the concentrations of nutrients and wastes. Because these properties affect the chemical reactions that keep us alive, we have built-in physiological mechanisms to maintain them at desirable levels.
When a change occurs in the body, there are two general ways that the body can respond. In negative feedback, the body responds in such a way as to reverse the direction of change. Because this tends to keep things constant, it allows us to maintain homeostasis. On the other hand, positive feedback is also possible. This means that if a change occurs in some variable, the response is to change that variable even more in the same direction. This has a de-stabilizing effect, so it does not result in homeostasis. Positive feedback is used in certain situations where rapid change is desirable.
Source - http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/pancreas/glucagon.html
Glucagon has a major role in maintaining normal concentrations of glucose in blood, and is often described as having the opposite effect of insulin. That is, glucagon has the effect of increasing blood glucose levels.
The major effect of glucagon is to stimulate an increase in blood concentration of glucose. As discussed previously, the brain in particular has an absolute dependence on glucose as a fuel, because neurons cannot utilize alternative energy sources like fatty acids to any significant extent. When blood levels of glucose begin to fall below the normal range, it is imperative to find and pump additional glucose into blood. Glucagon exerts control over two pivotal metabolic pathways within the liver, leading that organ to dispense glucose to the rest of the body:
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Glucagon stimulates breakdown of glycogen stored in the liver. When blood glucose levels are high, large amounts of glucose are taken up by the liver. Under the influence of insulin, much of this glucose is stored in the form of glycogen. Later, when blood glucose levels begin to fall, glucagon is secreted and acts on hepatocytes to activate the enzymes that depolymerize glycogen and release glucose.
Source - www.paralumun.com
Glucagon is in fact a hormone that is made naturally in the pancreas. It releases stores of glucose from the liver causing blood glucose levels to rise.
Insulin, made naturally in the pancreas, lowers blood glucose as it helps the muscles to use glucose as energy. It is these two hormones that keep the blood glucose level stable in people without diabetes
Insulin is a protein hormone and is the main factor in controlling a persons blood glucose level. It is produced by the islet cells of the pancreas when food is eaten or if the blood glucose level is high.
In between meals the islet cells produce low levels of insulin to maintain the balance between the amount of sugar the liver produces and the use of sugar by the muscles, brain,etc. When food is eaten and the levels of insulin rise it stops the liver releasing sugar into the bloodstream and helps the liver to take up sugar to store as glycogen.
Insulin also helps the muscle and fat cells to use glucose. The effects of insulin on the liver and other tissues tend to lower the blood glucose level.
Insulin works to help a muscle cell use glucose by creating an opening to allow the glucose to pass into the cell. Insulin also activates proteins (enzymes) in the cell which help the cell either to store glucose as glycogen or use it for energy.
