The basic definition of homeostasis is the process of keeping things the same. However a little more detailed explanation will clarify its meaning – Williams (1996) p251 defines it as: ‘Homeostasis is the tendency of the internal environment of the body to remain constant in spite of varying external conditions’. As Williams (1996) goes on to explain, it does this by using a variety of automatic self-regulating feedback mechanisms operating within different systems of the body. Many hormones participate in the role of homeostasis and many things are monitored including for example blood pressure, body temperature, water-balance and glucose levels. If these are operating within normal parameters then the body operates more effectively. If for any reason these systems fail to self-regulate then what is known as homeostatic imbalance occurs and we can become ill, the body can degenerate or even die eg untreated diabetes.
The BBC web page spells out the 3 essential components of homeostasis: ‘To keep things the same in spite of changing external conditions we need at least 3 components.
• First, we need receptors to detect when things such as temperature change.
• Then we need a processing centre to receive this information and coordinate our response.
• Finally, we need effectors to produce a response eg for temperature to ensure our body temperature stays at 37°C.’
Youngson (1995) describes how the processing or control centre has a range of normal operating parameters and if the receptor readings are within those parameters then there is no reaction, however if they are outside normal ranges then a corrective response must be made until the level of the factor returns to normal and the response ends (no further signal is sent). This process is called negative feedback and is the more general way that the body regulates itself. However positive feedback does occur from time to time and the best example of this would be what happens when blood clots. (see example ii).
i) Jane is beginning to get very cold because she is not dressed appropriately for the weather. One thing that Jane could do to help herself is to start doing some exercises – just running on the spot or jumping up and down, even stamping her feet or rubbing her hands together will exercise her muscles and produce heat which will help to warm her. If she is just standing still then her core body temperature will begin to fall below the normal temperature (37°C) and thus trigger a homeostatic response. Siegfried (2002) explains that the detection of temperature happens in two ways a) Blood flows through receptors in the hypothalamus which detect a decrease in core temp/temp of blood. b) Skin receptors detect temperature changes in external environment.
Once the body detects that it is becoming cold then several things begin to happen. The blood vessels near the body’s surface begin to constrict (vasoconstriction )so that less blood flows to the skin’s surface and so less heat is lost by radiation. Blood is kept deep within the body and around vital organs. The sweat glands close and we get ‘goose bumps’ as a way of trying to raise our body hair to try and achieve a layer of trapped air to insulate us from the external coldness – fairly ineffective even in the hairiest of humans! The body metabolism increases, especially in the liver, resulting in heat being produced.
When really cold we start to shiver – an involuntary contraction and relaxation of muscles which generates a small amount of heat. If Jane is outside for a long time then hypothermia could result where the body temperature falls dangerously below normal.
BiologyGuide : When this happens ‘Body temp falls dangerously below normal.Heat energy is lost from body more rapidly than it can be produced.Brain is affected first → person becomes clumsy and mentally sluggish. As body temp falls, metabolic rate falls as well. Makes body temp fall even further, causing a positive feedback. Temp is taken further away from the norm. Death when core body temp is below ≈25°C / by ventricular fibrillation / normal beating of the heart is replaced by uncoordinated tremors’
ii) Pushkar has cut his hand, but although he is still bleeding it is slowing down. Blood loss is potentially dangerous to the body and where there is a problem in preventing this from happening such as in haemophilia it can be potentially dangerous. Normally the outflow of blood washes any debris and microbes away and fills the injured area immediately. The blood flow starts to slow down so that you don’t lose too much blood through the cut. Siegfried (2002) p125 ‘A component of blood called platelets stick to the collagen fibres that make up the blood vessel wall and thus patch the blood vessel with a platelet plug’.
Siegfried (2002) explains further that as this is happening the body is releasing enzymes called clotting factors which make fibrin threads that wrap around the platelet plug forming a overlapping meshlike structure which traps the red blood cells and forms a clot which slows down blood loss. The red blood cells on the out side of the skin dry and oxidise and turn into a protective scab.
Underneath the scab the body tries to repair the damage – Siegfried p125 (2002) ‘cells called fibroblasts spur on the creation of new cells to regenerate the tissues in the damaged layers.’ until the blood vessels have regenerated and repaired themselves.
Ehow: ‘Positive feedback mechanisms are less common in homeostatic processes, but they do sometimes occur. Blood clotting is one example of a homeostatic positive feedback loop. Bleeding activates the clotting process, triggering the release of a clotting enzyme. The enzyme triggers more enzyme production, self accelerating the process and causing clotting to occur more rapidly, until the bleeding has stopped’.
If Pushkar’s bleeding was not stopping then eventually he could have acute blood loss and need an infusion, but let’s investigate Pushkar’s friend Vicky to further explore this.
iii) Vicky is bleeding badly. She is very pale, sweating profusely and feeling very dizzy and nauseous. To all intents and purposes it looks like Vicky has lost a lot of blood and is going into heamorrhagic shock where blood loss exceeds the body’s ability to compensate and provide adequate tissue perfusion and oxygenation. This is very serious and needs urgent medical treatment.
Garrioch 2004 p74 ‘Important physiological parameters to aid recognition and treatment of major haemorrhage are mental status, respiratory rate, peripheral perfusion, pulse rate, blood pressure and urine output. These parameters form the basis of the four classes of haemorrhage recognized in a 70-kg adult.’
According to Siegfried (2002) the body responds to blood loss by several methods – when there is only a small blood loss then the only effect is likely to be a decrease in urine as the body tries to conserve its fluids. As blood loss increases heart and breathing rates will increase to try and keep all the organs oxygenated and vasoconstriction (similar to i) above will occur so that blood will be kept around the main core body organs. Blood pressure will tend to drop as there is a lower volume of blood to be pumped round the system although the body may try and compensate by increasing the diastolic pressure. If there is no intervention at this stage then organ damage can start to occur as the organs fail to receive enough blood and oxygen. Kidney and brain damage can occur as well as heart attacks and death would be the inevitable outcome.
4) Jen has been working all day and neglecting to eat and is now feeling shaky and has a throbbing headache. These are some of the first signs of hypoglycemia – where the body’s glucose levels are too low. Jen is probably feeling hungry, but is ignoring or over-riding this in order to carry on working. Siegfried (2002) explains that hunger would be the natural homeostatic response – telling your body to eat, so that your body will regain its balance of energy stores. Energy from food is stored as glycogen and fat, but when these stores are used up they must once again be replenished with food. When you aren’t eating, your blood glucose starts to fall and this signals the production of a hormone called glucagon, which tells the body to release the glycogen and convert it into glucose for immediate energy. This released glucose then raises your blood sugar to a normal level until the glycogen stores are exhausted. The hunger is your body’s way of saying the balance needs to be restored and a snack or drink containing sugar will raise the blood glucose level leading to an immediate improvement in symptoms. If, as seems likely, Jen is readily susceptible to hypoglycaemia then she should really be eating regular balanced meals with low GI foods.Untreated, hypoglycemia can lead to loss of consciousness and coma.

BBC Web page http://www.bbc.co.uk/schools/gcsebitesize/science/add_ocr/homeostasis/homeostasisrev1.shtml accessed 30/6/11

BiologyGuide http://www.biologyguide.net/human/homeostasis.htm accessed 30/6/11

Ehow http://www.ehow.com/info_8602311_homeostasis-processes.html
accessed 7/7/11

Garrioch, M.A (2004) The body’s response to blood loss Vox Sanguinis 87 (Suppl. 1) S74–S76 2004 Blackwell Publishing http://onlinelibrary.wiley.com/doi/10.1111/j.1741-6892.2004.00435.x/pdf accessed 30/6/11

Siegfried, D. R, (2002) Anatomy and Physiology for Dummies, New York, Hungry Minds Inc

Williams, Tom (1996) Complete Chinese Medicine, Shaftesbury, Element Books Ltd

Youngson R.M. (1995) The Royal Society of Medicine Encyclopedia of Family Health London,Bloomsbury Publishing PLC