Homeostasis- What are the factors affecting homeostasis?
Homeostasis is the body’s mechanism of maintaining the internal conditions at a level necessary for the survival of the organism.
The Cell or the organism regulates its internal conditions in accordance to changing environmental conditions using a set of controls called feedback mechanisms so that a stable system is maintained. This is critical for the efficient functioning of the body’s workhorses such as enzymes and proteins which need an optimum temperature and ph for their activity. Any serious swing away from the normal conditions unless redressed by the system, can seriously affect the functioning of the enzymes and hence the metabolic processes and cause serious problems.
Walter Cannon devised the term
Factors which are regulated: temperature, water, salt, sugar, protein, fat, calcium, oxygen and carbon dioxide
Homeostasis is a complex process involving the interactions of different organs, hormones and enzymes and neurotransmitters released by the brain. Finely tuned. Complex.The chief regulators of homeostasis are the nervous system and the hormones. They are supported by the following organs: liver, kidneys, and the brain (hypothalamus, the autonomic nervous system and the endocrine system. Hormones are biological chemical messengers which are the products of endocrine glands in the body. They are secreted in response to signals from the brain and they are responsible for modulating various processes such as the menstrual cycle, blood sugar
When the system is functioning normally, the homeostatic mechanisms operate and respond normally in response to stimuli. But disorders can throw it askew. AN inability to maintain homeostasis can lead to death and this is known as homeostatic imbalance. Homeostatic imbalance can lead to heart failures, diabetes, dehydration, hypoglycaemia, gout etc.
What happens in homeostasis?
Because of the complex nature of the homeostasis, requiring the interaction of several organs and processes, the body has developed very fine tuned mechanisms of communication and signalling mechanisms between all these different components so that signals can be sent and received. A typical homeostatic system consists of the following components:
a. Stimulus – the external change or signal or condition which brings about a change to a particular variable ( temperature, sugar, waterlevel)
b. Receptor- Its purpose is to monitor the environment for environmental changes It is usually a molecule or a structure which perceives the stimulus and transmits the information to downstream components. The information from the receptor travels by an afferent pathway to the control centre
c.Control Centere- which receives the information from the receptor and sets in chain the processes which will bring about the necessary response. This information is returned bv way of an efferent pathway to the effector
d Effector- this molecule or structure receives the information from the control centre and produces a response which balances out the original stimulus.
1. Blood sugar level control
Blood sugar- why is it important to regulate blood sugar? What will happen if there are imbalances? Which organs and tissue are involved? What are the feedback mechanisms?
Glucose or blood sugar is required by the body as one of the main energy sources for driving the various activities. The oxidation of glucose in the cells provides the energy rich molecule ATP required for all the metabolic reactions carried out by the enzymes. Blood glucose is also essential for the functioning of the nerve cells which are involved the transmission of nervous impulses. Extreme levels of glucose (hypoglycaemia or hyperglycemia) can have serious effects on the nervous system. This also leads to diabetes mellitus as well as atherosclerosis. Hypoglycemia can lead to seizures, unconsciousness and death were hyperglycemia can lead to failure of eyesight, kidney failure, heart disease and diseases affecting the nervous system.
Where is glucose obtained from? Glucose is obtained by the breakdown of complex carbohydrates like starch and glucose. The glucose transported by the blood stream to the liver, where in the hepatocytes, most of it is converted to glycogen and stored there till required. Or it ends up being oxidised to release energy for the activities of the cells or it is converted into fatty acids or amino acids.
The normal levels of glucose in the blood stream are between 3.3 and 6.1mmol/L (Guthrie and Guthrie, 2002). The levels of the blood sugar are maintained by the actions of two hormones insulin and glucagon, both of which are produced by the mixed gland pancreas. The pancreatic juice contains both digestive enzymes as well as hormones insulin, glucagon and somatostatin
Glucose homeostasis refers to the processes involved in stabilising the fluctuations in blood glucose level so that the normal functions of the body are maintained. ( de fronzo,1988) This requires the management of the uptake of glucose by the cells, conversion into storage forms in the liver and as well as during digestion of food.
Insulin is a very important hormone which is required for the uptake of sugar by most cells. Thus it is involved in functions directed to this end purpose including increasing the metabolism of intacellular glucose, glycogenesis (conversion of glucose to glycogen), increasing amino acid uptake by the cells and protein synthesis, synthesis of fats and lipids and decreases fatty acid utilisation.
When the levels of glucose in the blood has increased beyond the norm, the insulin hormone is secreted by the beta cells of the islets of langerhans. The hormone stimulates the adipose and muscle cells to take up the glucose. How it does so is as follows. Increased insulin binding to its receptors on the cell, will promote the increased production of glucose transporters by the cells. These will allow the increased uptake of glucose. There are also insulin independent methods of uptake of glucose by the cell. s. The dynamics of insulin release follows the following path with an initial rapid release of pre existing insulin followed by increase of insulin synthesised newly, in response to the blood glucose levels. Insulin may be synthesised for long term, if the glucose level does not go down. Aronoff et al., 2004; Cryer, 1992
As the glucose levels decrease , insulin levels will fal. When the sugar falls to normal levels, then insulin hormone production is reduced. If the sugar level falls too low on account of increased metabolism along with the insulin level, the hormone glucagon is released from the alpha cells of the islets of langerhans in the pancreas, which drives the utilisation of glycogen so that more glucose is available in the blood for distribution to places needing it. Glucagon mediates its action through its own receptors on the cell surface, which leads to the activation of enzymes involved in the utilisation of glycogen.
The maintenance of blood sugar by the antagonistic actions of insulin and glucagon is an example of a finely tuned homeostatic mechanism which would enables the modulation of the blood sugar levels in response to external stimuli. The control of homeostasis is by negative feedback with the high levels of each hormone, inhibiting the other. There is a bigger level of control exerted on this mechanism by the autonomic nervous system with the sympathetic part responsible for promoting the activity of glucagon while the parasympathetic system promotes that of insulin.
2. Temperature control
Another important example of homeostatic functioning is the process behind control of body temperature. This too involves the negative feedback mechanism of homeostatic regulation. Negative feedback refers to the fact that a change in a system, which shifts it away from equilibrium will initiate corrective re-dressal mechanisms which is aimed at reversing the effect of the original change and bringing back the system to equilibrium. So the system is designed in such a way that oscillations occur in either direction to bring it back to the original position, depending on the direction of the change. Thermoregulation is very important for mammals. The regulation of body temperature of mammals involves keeping the body temperature at an optimum level suitable for the functioning of the body’s enzymes and metabolic machinery, in spite of the changes in external temperature as for example extremes of heat and cold. Unlike cold blooded animals, which have to depend on behavioural mechanisms to regulate their body temperature in response to external changes, warm blooded animals such as humans have well developed, fine tuned internal mechanisms. The hypothalamus in the brain contains the thermoregulatory center. There are two kind of sensors which perceive changes in temperature. The internal sensors are present in the hypothalamus which are responsible for detecting changes in the temperature of the blood. External thermoreceptors are present in the skin which perceives changes in the external temperature. Based on the messages received from these receptors, the hypothalamus sends messages to the effectors involved to respond appropriately. There are heat centres located in the hypothalamus which responds to high or low temperatures. The thermoregulatory responses are involuntary and under the conrol of the autonomic nervous system and get activated only when basic responses to change in temperature fail.
The organs involved include the skin, the muscles, blood vessels etc. The range of temperatures which is maintained for the basal metabolic activities of the organism form the thermoneutral zone. When temperatures are low enough to disturb the body’s thermoneutral temperature , then several mechanisms are initiate to generate more heat to maintain the temperature from dipping further. These include shivering (isometric contraction of skeletal muscles) which generates heat which can be transmitted inwards into the body interiors and vasoconstriction of blood vessels which will reduce heat loss from the skin. Skin hair stands upright as the muscles contract and trap the heat.There is also an adaptation where the brown adipose tissue is utilised for generation of heat ( Ivanov 2006, heinrich 1979, Grigg et al 2004) When the temperatures rise above the thermoneutral zone, sweating responses are initiated to bring about cooling of the body through evaporation of water from the skin. Vasodilation of blood vessels also promotes heat loss. Flattening of hair to the skin allows cooling and heat loss by convection.
The master controllers of the thermoregulatory system are the nervous system and the endocrine system. Thyroid hormone is believed to regulate the heating/cooling responses in endothermic organisms like humans (Silva 2006)
3. Body’s water content
Homeostatic mechanisms also regulate the body’s fluid content i.e. the water level in the body. The chief organs involved are the kidneys, the nervous system and endocrine system. The volume of water in the cells and thus in the body, needs to be kept within an optimum range, as excessive loss of fluid from cells (excretion, sweating, urination, feces) or excessive intake of fluid via drinking can create osmotic disturbances which will affect the functioning of the cells. This is because cells are essentially membranous bags with organelles suspended in a suspension of fluids, salts, proteins and enzymes. The entry and exits of fluids and substances are tightly regulated. Therefore any kind of heavy osmotic change would dramatically affect the functioning of the cell such as if the external environment becomes too salty or too dilute. If the solution coating the cells is more salty, then fluid from within the cell will move out and when the solution is too watery, there is an influx of fluid within the cell. The former would cause shrinking of the cells, while the latter causes turgidity, which in case of animal cells, would lead to bursting.
The control of fluid volume is mediated by the ADH or Anti –Diuretic hormone or vasopressin and the homeostatic mechanism follows the typical negative feedback pattern. The osmolality of the cells determines whether the ADH would be released or not. Again the overall control is by the hypothalamus. On receiving signals from osmoreceptors communicating thirst or need for more fluid, message is sent from the hypothalamus to the pituitary gland which releases the ADH hormone. Just a 1 % change in total body water or around 400 ml out of 2500 ml of our daily intake , is enough to initiate the homeostatic mechanisms. This is distributed by the bloodstream to the kidneys where it influences the tubular reabsorption process and drives t he tubules to reabsorb more water into the blood. This it does by binding to cellular receptors and promoting increase in the permeability of the cells. As a result, the urine becomes more concentrated and is produced in smaller volume. This occurs till the volume of water in the body reaches optimum. On the other hand, when you have drunk an excess volume of water, which the body does not need, the pituitary receives the signal from the brain to reduce ADH release. The reduced ADH, prevents the re-absorption of water by the tubules and the excess water is released into urine. The urine formed is very dilute and in large amounts.
Vasopressin release is not only in response to plasma osmolality but also in response to pain and emotion and hence we feel dry or dehydrated when we feel stressed or
References:
http://anatomyandphysiologyi.com/homeostasis-positivenegative-feedback-mechanisms/
http://homeostasiseleishabiology.weebly.com/main-mechanisms-of-homeostasis.html
http://www.biologymad.com/resources/A2%20Homeostasis.pdf
http://anatomyandphysiologyi.com/homeostasis-positivenegative-feedback-mechanisms/
http://cdn.intechopen.com/pdfs-wm/23134.pdf
