With reference to specific examples and mechanisms assess the significance of homeostasis to the human body.
WITH REFERENCE TO SPECIFIC EXAMPLES AND MECHANISMS ASSESS THE SIGNIFICANCE OF HOMEOSTASIS TO THE HUMAN BODY
What is homeostasis?
According to the 'Oxford Colour Medical Dictionary, Third Edition' homeostasis is the "the physiological process by which the internal systems of the body (e.g. blood pressure, body temperature, acid-base balance) are maintained at equilibrium, despite variations in the external conditions"
Homeostasis is the maintenance of a stable internal environment within tolerance limits, this is the restricted range of conditions where cellular operations effectively work at a consistent rate and maintain life.
These conditions include temperature, blood glucose levels, pupil diameter control and many more.
Homeostasis actually means 'unchanging', but that is not a true description of biological systems. DYNAMIC EQUILIBRIUM is a more accurate description. (1)
"An amoeba, a single celled organism, needs to be able to take in oxygen, food and nutrients and to excrete waste products. It needs a constant state of hydration and a controlled temperature for a happy life. Man is complex and multicellular but each cell has the same needs as the amoeba and we have developed complex mechanisms to provide each cell with all that it needs" (2)
The human animal is a very complex multi-cellular organism in which the maintenance of life depends upon various physiological and biochemical activities.
The body is made up of many cells (e.g. brain cells). Many specialised cells group up to form a tissue (e.g. blood). Tissues group up to form organs (e.g. the heart), these organs can then connect to form organ systems (e.g. the digestive system). The body is therefore made up of many specialised systems. In general the combined activities of the specialised systems provide the optimum environment for individual cells. This means the preservation of a constant composition of the fluid that bathes the body cells i.e. tissue fluid. This situation is for health and is called homeostasis.
Source: http://www.sirinet.net/~jgjohnso/intro.html
"Maintaining homeostasis is more complicated than it appears at first glance. Virtually every organ system plays a role in maintaining the constancy of the internal environment". (3)
Adequate blood levels of essential nutrients must be continuously present, and heart activity and blood pressure must be continuously monitored and adjusted so that the blood is propelled to all body tissues. Wastes must not be allowed to accumulate, and body temperature must be precisely regulated (3)
The internal environment and homeostasis
The external environment surrounds the body and provides the oxygen and nutrition required by all body cells. Waste products of cell activity such as carbon dioxide are eventually excreted into the external environment (tissue fluid).
The internal environment provides chemical substances produced by specialised cells. All living cells in the body are bathed in fluid called 'tissue fluid'. Oxygen, nutritional materials, chemicals produced by the body, and waste products, pass through the tissue fluid between the cells and internal transport systems. The composition of the internal environment is maintained within narrow ...
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The internal environment and homeostasis
The external environment surrounds the body and provides the oxygen and nutrition required by all body cells. Waste products of cell activity such as carbon dioxide are eventually excreted into the external environment (tissue fluid).
The internal environment provides chemical substances produced by specialised cells. All living cells in the body are bathed in fluid called 'tissue fluid'. Oxygen, nutritional materials, chemicals produced by the body, and waste products, pass through the tissue fluid between the cells and internal transport systems. The composition of the internal environment is maintained within narrow limits, this is homeostasis. When the balance is threatened or lost, there is a serious risk to the well-being of the individual. (4)
Homeostasis adjusts to the changing external environment, e.g. a cold climate; it is a self-adjusting system (auto-regulation), which involves biological feedback. Hormones and the nervous system achieve this. This feedback allows for the maintenance of cells, tissues and the body to be in a state of equilibrium, which results in the sustenance of life. The body has particular features such as the skin and circulatory system, which helps its survival against the environment. The metabolism is able to adjust to internal and external changes that occur.
BIOLOGICAL FEEDBACK SYSTEMS IN HOMEOSTASIS
Negative Feedback
According to G.W. Stout and N.P.O Green in 'Work out Biology, A level' negative feedback opposes any detected tendency away from the optimal level and returns it to the optimal level.
Homeostasis is maintained by control systems, which detect and respond to changes in the internal environment. Analogy is often made with domestic heating and cooling systems where a fall in temperature will trigger the thermostat, fire up the boiler and heat up the radiators until the temperature rises and then the thermostat will switch off. A rise in temperature will trigger the thermostat to switch off the boiler until the temperature is normal again. A control system has three basic components:
A detector (receptors)-detects the stimulus disturbing the body, for example baroreceptors will detect blood pressure.
A 'norm' control centre-determines the limits within which the variable factor should be maintained. It receives an input from the detector, and integrates the incoming information.
An effector-when the incoming signal indicates that an adjustment is needed the effector responds and the input is changed (corrective response) e.g. this could be the release of a hormone that lowers blood glucose levels for example i.e. insulin. (5)
Source: http://www.biology-online.org/4/1_physiological_homeostasis.ht
This type of homeostatic mechanism is known as negative feedback where the body opposes the departure of a controlled variable from the normal range and restores the variable to its normal range. This is the most common type of regulation in the body. The outcome is to maintain a relatively constant or stable environment.
An example could be in thermal homeostasis, which maintains normal cell function and normal metabolism where during cold stress (below body core temperature) which could potentially slow down chemical reactions and slow down neural signals, the body would engage in many activities to reduce this stress, these include:
-Stimulation of skeletal muscles by the brain's hypothalamus (control centre) to contract and shiver to produce body heat
-Stimulation of smooth muscle tissue (effectors) in blood vessels in skin by the brain (control centre) to contract to reduce blood flow to skin and slow down loss of heat by skin's surface (response).
In heat stress (above normal core temperature), which disrupts structure of body proteins, the hypothalamus (control centre) stimulates sweat glands (effectors) in skin to release watery fluid; water evaporation cools the body (response).
The brain (control centre) also causes smooth muscle (effector) in blood vessels in skin to relax; this dilates blood vessels in skin so that blood in skin can lose excess heat to skin's surface (response). (6)
Behavioural mechanisms can also aid homeostasis, such as putting a jacket on.
Source: Marieb EN, 2003, Human Anatomy & Physiology, sixth edition, San Francisco, Pearson Benjamin Cummings, page 988
Positive feedback
"In positive feedback mechanisms, the result or response enhances the original stimulus so that the activity (output) is accelerated". (7)
This feedback mechanism is labelled as 'positive' due to the fact that the change it causes happens in the same direction as the original stimulus, it deviates it further from its range. These control systems also have an end-point. An example is the mechanism of blood clotting in a blood vessel. Once a blood vessel has been damaged, platelets immediately begin to cling to the damaged site and release chemicals that attract more platelets. This rapidly growing pileup of platelets initiates the sequence of events that finally forms a clot. (8)
Source: Marieb EN, 2003, Human Anatomy & Physiology, sixth edition, San Francisco, Pearson Benjamin Cummings, page 12
Homeostatic imbalance in this case could be fatal, for example if the clotting mechanism fails on the large central artery that supplies the brain, the brain will be short of oxygen (hypoxia) leading to a stroke and if not treated immediately it will result in death.
Another examples of positive feedback include labour contractions during birth and lactation from the mother's nipple. The importance of homeostasis is again demonstrated in these mechanisms.
Communication
Communication within the body is crucial for homeostasis. Communication is accomplished mainly by the nervous and endocrine systems, which use electrical impulses delivered by nerves or blood-borne hormones respectively as information carriers. (3)
Examples of homeostatic mechanisms
Blood glucose levels
To carry out normal metabolism, body cells need a continuous supply of glucose, their major fuel for producing cellular energy, or ATP. Blood sugar levels are normally maintained around 90 milligrams (mg) of glucose per 100 millilitres (ml) of blood. If blood glucose levels rise, the ?-cells of Langerhans are stimulated and insulin is released into the blood, and blood glucose levels decline.
If blood glucose levels fall, the ?-cells of Langerhans are stimulated and glucagon is released into the bloodstream and blood glucose levels rise again.
Source: Marieb EN, 2003, Human Anatomy & Physiology, sixth edition, San Francisco, Pearson Benjamin Cummings, page 11
The effect of homeostatic imbalance in this case is 'diabetes mellitus-type 1', which is hyposecretion of insulin. As an intervention insulin injections are given to sufferers. It is at these times that we appreciate the importance of homeostasis.
Pupil diameter control caused by changes in light intensity
Pupil size influences accommodation by controlling the amount of light entering the eye. Pupil constriction occurs when there is bright light while pupil dilation happens in dim light.
If pupil dilation occurred during bright light, too much light would enter the eye and damage the very sensitive retina. On the other hand if pupil constriction happened during dim light, insufficient light would go into the eye to activate the photosensitive pigments in the rods and cons, which stimulate nerve endings in the retina.
The iris contains one layer of circular muscle and one layer of radiating smooth muscle fibres. Contraction of the circular fibres constricts the pupil, and contraction dilates it. The size of the pupil is regulated by the autonomic nervous system and it demonstrates an example of a nervous-control mechanism. Sympathetic stimulation causes dilation while parasympathetic stimulation causes constriction of the pupil. (9)
Source: Marieb EN, 2003, Human Anatomy & Physiology, sixth edition, San Francisco, Pearson Benjamin Cummings, page 566
Significance of homeostasis
Homeostasis is so important that most disease is regarded as a result of its disturbance, a condition called homeostatic imbalance.
As we get older, the body's internal environment becomes progressively less stable. As a result we are more vulnerable to illnesses and they produce the changes we associate with ageing.
Homeostatic imbalance takes place when the usual negative feedback mechanisms are overwhelmed and the destructive positive feedback mechanisms take over. An example of this phenomenon is heart failure. Illnesses such as hypertension, diabetes are all a result of failed homeostatic mechanisms, this demonstrates the importance of homeostasis to an individual. (10)
Put simply, it is the difference between health and illness.
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