Homeostasis, the process by which an organism maintains the constant internal conditions necessary for life. The concept of homeostasis was first outlined by Claude Bernard, a 19th-century French physiologist who said, “The constancy of the internal environment is a condition of free life”. In order for an organism to be successful it must have a degree of freedom from its environment; this freedom is provided by homeostasis. The term “homeostasis” was coined by Walter Cannon in 1926 to refer to the body's capacity to regulate the composition and volume of the blood, and hence all the fluid bathing the cells of the body—the “extracellular fluid”. The word homeostasis is derived from the Greek words “homeo”, meaning same, and “stasis”, meaning position. It is now applied collectively to the many processes which prevent fluctuations in the physiology of an organism, and has even been applied to the regulation of change in various ecosystems or the universe as a whole.
Homeostasis in living organisms involves expending energy in order to maintain a position in a dynamic equilibrium. This means that, even though external conditions may be continually changing, homeostatic mechanisms ensure that the effects of these changes on the organism are minimal. If the equilibrium is upset and the homeostatic mechanisms are unable to recover it then the organism may become sick and eventually die.
Homeostasis is necessary because organisms are continually metabolizing molecules (see Metabolism), producing potentially toxic waste products and using up valuable reagents which need to be replenished. In addition to this, organisms also need to maintain a constant intracellular environment regardless of the effects of changes in their external environment.
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Homeostatic Mechanisms
Homeostasis requires the organism to be able to detect changes in the environment and to control them. A small change from the accepted level will initiate a homeostatic response which will return the environment to its desired state. Cybernetics, also known as control theory, is the study of homeostatic mechanisms or servomechanisms (the term used to describe analogous mechanisms employed by machines). In control theory, mathematical and computer models have been constructed to describe physiological control systems, although these are often somewhat crude and inadequate as there are many complex, interacting systems within an organism. Some of the mechanisms outlined in control theory are useful in understanding homeostatic mechanisms, for example, feedback loops. These usually involve the product of a mechanism acting in some way to alter the nature, speed, or efficacy of the mechanism itself and can be positive or negative in nature. In biology most feedback loops are negative feedback loops; they inhibit the mechanism or cellular process from which they were derived.
Homeostasis is necessary because organisms are continually metabolizing molecules (see Metabolism), producing potentially toxic waste products and using up valuable reagents which need to be replenished. In addition to this, organisms also need to maintain a constant intracellular environment regardless of the effects of changes in their external environment.
II
Homeostatic Mechanisms
Homeostasis requires the organism to be able to detect changes in the environment and to control them. A small change from the accepted level will initiate a homeostatic response which will return the environment to its desired state. Cybernetics, also known as control theory, is the study of homeostatic mechanisms or servomechanisms (the term used to describe analogous mechanisms employed by machines). In control theory, mathematical and computer models have been constructed to describe physiological control systems, although these are often somewhat crude and inadequate as there are many complex, interacting systems within an organism. Some of the mechanisms outlined in control theory are useful in understanding homeostatic mechanisms, for example, feedback loops. These usually involve the product of a mechanism acting in some way to alter the nature, speed, or efficacy of the mechanism itself and can be positive or negative in nature. In biology most feedback loops are negative feedback loops; they inhibit the mechanism or cellular process from which they were derived.
Homeostasis at the Cellular Level
All organisms perform homeostasis at a cellular level as the components of a cell must be maintained in a more or less uniform concentration in order for life to continue. The cell membrane is responsible for controlling which substances can enter and leave the cell; waste products must be able to leave the cell so that they do not build up to toxic levels. Substances essential to metabolism must also be taken up for use in respiration. Organisms which consist of a single cell have a more difficult homeostatic task, as the environment which surrounds them can change dramatically in any number of ways. In contrast, mutlicellular organisms facilitate each cell's task by ensuring the extracellular medium is maintained by homeostasis, thus each individual cell will not be exposed to such dramatic changes.
Homeostasis in Humans
Homeostasis occurs in all organisms, although it has been studied in greater detail in the human and other higher mammals. In such complex animals, homeostasis operates at both the level of single cells and the level of integrated cells—body fluids, tissues and organs. By maintaining constant conditions within the tissue, each individual cell is subjected to smaller variations in its own external environment. There is a constant exchange of molecules between the blood and the extracellular fluid which bathes individual cells; it is the stable composition of the blood which enables the maintenance of constancy in the extracellular fluid. The constant composition of the extracellular fluid protects the individual cells from changes in the external environment, for example if a person gets into a hot bath, the temperature of the cells in the liver, heart, gut, and pancreas remain unaffected.
Maintenance of Blood Glucose Levels
In order for humans to maintain health, the levels of glucose in the bloodstream must be maintained. Glucose is used by all cells in the body as their “fuel”, the amount of glucose each cell uses varies and depends upon the functional activity of the cell, (most cells also use fat derivative, but the brain can only metabolize glucose). Glucose enters the bloodstream when it is absorbed by the gut during digestion or from glycogen stores located mainly in the liver. The control of glucose levels in the blood is the most complicated homeostatic system known.
Normally blood glucose levels vary between 110-120 mg glucose per 100 ml blood after a meal, to 70-80 mg per 100 ml following a period without food. When levels are at their highest glucose is made into glycogen and stored. Blood glucose levels are controlled by six hormones: insulin, growth hormone, glucagon, glucocorticoids, adrenalin, and thyroxine.
Examples of Homeostasis in Other Organisms
For organisms which do not have watertight skins, one of the most crucial regulatory processes is the control of the amount of water which is gained or lost by osmosis or evaporation. Bacteria are amongst the smallest of all organisms with a high surface area to volume ratio, so they are very susceptible to dehydration. They attempt to compensate for this by having an internal osmotic pressure which is greater than the external environment, thereby reducing water loss.
Maintenance of Blood Glucose Levels
In order for humans to maintain health, the levels of glucose in the bloodstream must be maintained. Glucose is used by all cells in the body as their “fuel”, the amount of glucose each cell uses varies and depends upon the functional activity of the cell, (most cells also use fat derivative, but the brain can only metabolize glucose). Glucose enters the bloodstream when it is absorbed by the gut during digestion or from glycogen stores located mainly in the liver. The control of glucose levels in the blood is the most complicated homeostatic system known.
Normally blood glucose levels vary between 110-120 mg glucose per 100 ml blood after a meal, to 70-80 mg per 100 ml following a period without food. When levels are at their highest glucose is made into glycogen and stored. Blood glucose levels are controlled by six hormones: insulin, growth hormone, glucagon, glucocorticoids, adrenalin, and thyroxine.
Glycogenolysis, the production of glucose from glycogen stores, is stimulated by all these hormones except insulin which inhibits it; insulin stimulates glycogenesis, the production of glycogen from glucose in the blood. Insulin is produced by the pancreas when levels of glucose are high and it acts to decrease the level of glucose in the blood—an example of negative feedback. Some tissues can only take up glucose from the blood in the presence of glucose, if there are low blood glucose levels and there is no insulin, these tissues cannot use glucose at all and have to rely upon fat derivatives for the production of their energy (see Diabetes Mellitus).
For organisms which do not have watertight skins, one of the most crucial regulatory processes is the control of the amount of water which is gained or lost by osmosis or evaporation. Bacteria are amongst the smallest of all organisms with a high surface area to volume ratio, so they are very susceptible to dehydration. They attempt to compensate for this by having an internal osmotic pressure which is greater than the external environment, thereby reducing water loss.
Single celled organisms such as amoebas, especially those that live in fresh water, continually gain water from their surrounding environment by osmosis. This water is pumped into a contractile vacuole which fills with fluid and periodically fuses with the cell membrane, releasing its contents to the exterior. Thus the amount of water actively transported out of the cell is equal to the amount of water entering the cell due to osmosis and there are no variations in the tonicity of the cell. This is homeostasis at a simple level. Without the contractile vacuole, the amoeba would continue to absorb water until its cytoplasmic contents were diluted to such an extent that metabolism no longer occurred and it would die.
Fish have complex mechanisms to control the water content of their bodies; if they are freshwater fish, then they absorb water and lose salt by osmosis. They have to actively take up salt from the water passing over their gills, and they produce large amounts of dilute urine (equivalent to about 20 per cent of their body weight is produced daily). Conversely, marine fish live in water which has a higher osmotic pressure than their extracellular fluid and blood, they lose water and gain salts by osmosis. In order to maintain the correct blood composition marine fish continually swallow salt from the sea and they produce very small amounts of isotonic urine (about 4 per cent of their body weight per day). They are unable to produce urine which is more concentrated than their blood. In fish which migrate from salt to fresh water, such as salmon, the mechanisms which control the amount of water entering and leaving the body are even more complex.
In order for humans to maintain health, the levels of glucose in the bloodstream must be maintained. Glucose is used by all cells in the body as their “fuel”, the amount of glucose each cell uses varies and depends upon the functional activity of the cell, (most cells also use fat derivative, but the brain can only metabolize glucose). Glucose enters the bloodstream when it is absorbed by the gut during digestion or from glycogen stores located mainly in the liver. The control of glucose levels in the blood is the most complicated homeostatic system known.
Normally blood glucose levels vary between 110-120 mg glucose per 100 ml blood after a meal, to 70-80 mg per 100 ml following a period without food. When levels are at their highest glucose is made into glycogen and stored. Blood glucose levels are controlled by six hormones: insulin, growth hormone, glucagon, glucocorticoids, adrenalin, and thyroxine.
Glycogenolysis, the production of glucose from glycogen stores, is stimulated by all these hormones except insulin which inhibits it; insulin stimulates glycogenesis, the production of glycogen from glucose in the blood. Insulin is produced by the pancreas when levels of glucose are high and it acts to decrease the level of glucose in the blood—an example of negative feedback. Some tissues can only take up glucose from the blood in the presence of glucose, if there are low blood glucose levels and there is no insulin, these tissues cannot use glucose at all and have to rely upon fat derivatives for the production of their energy (see Diabetes Mellitus).
For organisms which do not have watertight skins, one of the most crucial regulatory processes is the control of the amount of water which is gained or lost by osmosis or evaporation. Bacteria are amongst the smallest of all organisms with a high surface area to volume ratio, so they are very susceptible to dehydration. They attempt to compensate for this by having an internal osmotic pressure which is greater than the external environment, thereby reducing water loss.
Single celled organisms such as amoebas, especially those that live in fresh water, continually gain water from their surrounding environment by osmosis. This water is pumped into a contractile vacuole which fills with fluid and periodically fuses with the cell membrane, releasing its contents to the exterior. Thus the amount of water actively transported out of the cell is equal to the amount of water entering the cell due to osmosis and there are no variations in the tonicity of the cell. This is homeostasis at a simple level. Without the contractile vacuole, the amoeba would continue to absorb water until its cytoplasmic contents were diluted to such an extent that metabolism no longer occurred and it would die.
Fish have complex mechanisms to control the water content of their bodies; if they are freshwater fish, then they absorb water and lose salt by osmosis. They have to actively take up salt from the water passing over their gills, and they produce large amounts of dilute urine (equivalent to about 20 per cent of their body weight is produced daily). Conversely, marine fish live in water which has a higher osmotic pressure than their extracellular fluid and blood, they lose water and gain salts by osmosis. In order to maintain the correct blood composition marine fish continually swallow salt from the sea and they produce very small amounts of isotonic urine (about 4 per cent of their body weight per day). They are unable to produce urine which is more concentrated than their blood. In fish which migrate from salt to fresh water, such as salmon, the mechanisms which control the amount of water entering and leaving the body are even more complex.
