HOMEOSTASIS

Homeostasis dirujuk sebagai pengekalan atau mekanisma kawal atur persekitaran dinamik dalaman (badan organisma) yang malar. Homeostasis merupakan salah satu konsep yang paling penting dalam biologi. Bidang fisiologi boleh mengklasifkasikan mekanisma homeostasis kawalatur dalaman organisma. Gerak balas homeostasis berlaku pada setiap organisma.

Terdapat 2 jenis pemalar atau keadaan mantap dalam homeostasis iaitu

1. Sistem tertutup - Keseimbangan statik o Dimana keadaan dalam yang tidak berubah seperti botol

tertutup.2. Sistem terbuka - Keseimbangan dinamik

o Dimana keadaan dalam yang malar walaupun sistem ini terus berubah contohnya seperti sebuah kolam di dasar air terjun.

Organisma mempunyai 2 persekitaran iaitu:

1. Persekitaran luar iaitu persekitaran yang mengelilingi organisma secara keseluruhan. Organisma akan hidup secara berkongsi dengan organisma-organisma(biosis) dan objek-objek yang bukan hidupan(abiosis).

2. Persekitaran dalam iaitu persekitaran dinamik dalam badan manusia yang terdiri daripada bendalir yang mengelilingi komuniti sel-sel yang membentuk badan.

Biosis ialah komponen hidupan iaitu meliputi semua organisma hidupan.Contoh komponen biosis ialah:

Manusia Tumbuhan Haiwan

Abiosis ialah komponen bukan hidupan seperti:

Suhu Nilai pH Keamatan cahaya Kelembapan Topografi Iklim

Perubahan persekitaran

Perubahan kecil dalam persekitaran dinamik dalam badan boleh

menyebabkan sel-sel mati.Contoh-contoh yang akan menyebabkan sel-sel mati walaupun dalam julat kecil ialah seperti:

Dehidrasi - Kurang air Zat makanan yang kurang Sisa toksik dikumpul dalam badan Suhu berubah dengan mendadak

[sunting] Faktor yang perlu dikawal

Setiap faktor-faktor mempunyai julat-julat tertentu yang boleh mempengaruhi persekitaran dinamik. Contoh beberapa faktor dalam bendalir yang perlu dikawal atur julatnya:

pH - 7.3pH - 7.4pH, berbeza dengan salur alimentari julat pH adalah berbeza-beza pada tempat tertentu.

Suhu - 37oC - 39oC Glukosa - 4.4 - 5.5 mmol/dm3

Urea - 3.3 - 6.6 mmol/dm3

[sunting] Kepentingan Homeostasis

Akibat daripada perubahan kecil julat ini, hal ini akan menimbulkan masalah kepada organisma yang sentiasa berada dalam persekitaran luar yang tidak tentu dan cara hidup yang kurang sihat. Maka , untuk mengadaptasi kepada perubahan ini, organ-organ tertentu dalam badan organisma berfungsi untuk mengimbangi, mengawal, mengstabilkan, menyesuaikan, dan mengekalkan persekitaran dalam supaya berada dalam keadaan yang stabil untuk sel-sel terus hidup dan berfungsi secara optimum.

Antara kepentingannya ialah:

Memboleh organisma mengadaptasi kepada persekitaran luar yang mempunyai julat dan habitat yang lebih luas.

Menyediakan keadaan dalaman (persekitaran dinamik dalam badan organisma) yang stabil supaya sel-sel dapat menjalankan hidup dengan cekap.

Membolehkan kadar metabolisma dikawal dengan cekap pada bila-bila masa.

Boleh enzim-enzim menjalan fungsinya dengan optimum.

[sunting] Mekanisme Homeostasis

Mekanisme ini dikawal oleh otak terutama hipotalamus. Hipotalamus terangsang akan merangsang koordinasi badan.Proses ini akan berterusan sehingga persekitaran dinamik dalam badan akan berada pada julat yang normal.

2 koordinasi badan yang terlibat ialah:

1. Kordinasi Kimia - Seperti Hormon2. Kordinasi Saraf - Seperti Impuls saraf

Antara proses-proses yang terlibat ialah:

1. Suap balik positif - Contoh deman, badan akan bertambah panas untuk membunuh bakteria dan virus.

2. Suap balik negatif - Contoh keadaan panas, badan akan dikawalatur untuk mengurangkan haba daripada badan.

Contoh homeostasis yang ringkas ialah

Apabila cuaca panas, sistem kulit akan bertindak balas dengan mengeluarkan peluh melalui kelenjar peluh pada epidermis kulit untuk mencegah suhu darah daripada meningkat. Salur darah pula akan mengembang untuk mengeluarkan haba ke persekitaran dan hal ini kadang kala menyebabkan kulit berwarna merah.

Apabila aras glukosa dalam darah telah habis atau berkurangan daripada julat tertentu, hati akan dirangsang oleh insulin untuk menukarkan glikogen kepada glukosa supaya dapat digunakan sebagai tenaga untuk tindakan otot.

Antara organ-organ yang terlibat dalam kawalatur homeostasis ialah:

Hati Ginjal Kulit

[sunting] Proses Pengosmokawalaturan dalam Badan Manusia

Ginjal adalah organ yang terlibat dalam proses pengosmokawalaturan. Dalam penghasilan air kencing, jumlah air yang diserap semula di liku Henle menimbulkan dua kemungkinan iaitu:

1. Apabila lebih garam dalam badan dan kurang air2. Apabila kurang garam dalam badan dan lebih air

Apabila aras garam lebih dari julat normal, darah menjadi pekat, atau dalam kata lainnya, kurang air dalam badan. Hal ini membawa kepada peningkatan

tekanan osmosis darah dan osmoreseptor pada hipotalamus akan terangsang. Seterusnya kelenjar pituitari akan dirangsang lebih aktif untuk merembeskan hormon ADH (antidiuresis) untuk meningkatkan ketelapan tubul ginjal terhadap air manakala kelenjar [[adrenal](hormon aldosteron) akan kurang dirangsang. Maka lebih banyak air diserap dan kurang ion natrium dan ion kalsium diserap semula masuk dalam badan sehingga tekanan osmosis darah akan turun. Proses ini akan berulang sehingga tekanan osmosis darah turun pada julat normal.

Apabila aras garam rendah dari julat normal dalam badan dan banyak air dalam badan, tekanan osmosis darah akan menurun. Osmoreseptor pada hipotalamus akan terangsang dan menghantar impuls kepada kelenjar pituitari. Kelenjar ini menjadi kurang dirangsang untuk merembeskan hormon ADH (antidiuresis) untuk mengurangkan ketelapan tubul ginjal terhadap air manakala kelenjar adrenal(hormon aldosteron) akan dirangsang dengan lebih aktif sehingga kurang air diserap dan kurang natrium dan kalsium diserap semula masuk dalam badan. Tekanan osmosis darah akan naik dan proses ini akan berulang sehingga tekanan osmosis darah meningkat pada julat normal.

Fungsi hormon antidiuresis ialah:

Merangsang penyerapan semula air pada tubul ginjal - Menambah ketelapan tubul ginjal terhadap air.

Fungsi hormon aldosteron ialah:

Mengekalkan keseimbangan ion natrium dan ion kalsium dalam darah - Penyerapan ion kalsium dan ion natrium pada tubul ginjal.

Memelihara keseimbangan air dan garam dalam darah

Air yang tidak diserap masuk semula dalam badan akan keluar sebagai air kencing.

[sunting] Air kencing

Proses pembentukan air kencing terdiri daripada 3 proses iaitu:

1. Proses Ultraturasan 2. Proses Penyerapan Semula 3. Proses Pembuangan Air Kencing

Diantara bahan beracun yang disalur keluar ialah:

Urea Asid urik

Ammonia Dadah - Contoh steroid

Diantara kandungan air kencing ialah:

Air Urea Asid Urik Ammonia Natrium Klorida Fosfat

[sunting] Kawalan Suhu Badan Manusia

Terdapat 2 kaedah pengawal suhu badan iaitu:

1. kaedah fizikal2. Kaedah metabolik

Semua kaedah untuk mengawal suhu badan adalah dengan bantuan koordinasi badan.

[sunting] Pengawalan suhu dengan kaedah fizikal

Kaedah ini dikenali sebagai kaedah fizikal kerana kawalaturan lebih kepada penggunaan otot-otot badan dan secara fizikal. Diantara kemungkinan yang akan berlaku ialah:

1. Suhu badan tinggi melebihi julat yang normal2. Suhu badan rendah melebihi julat yang normal

Apabila suhu badan tinggi, termoreseptor akan mengesan suhu pada kulit lalu menghantar impuls kepada otak iaitu hipotalamus yang berfungsi sebagai termostat mengesan suhu darah yang melaluinya. Mekanisme pembetulan akan diarah atau dirangsang oleh hipotalamus dengan menggunakan koordinasi badan.

Mekanisme pembetulan apabila suhu badan tinggi ialah:

1. Pemvasodilatan - Salur darah mengembang untuk berdekatan dengan kulit (iaitu persekitaran luar) yang membolehkan haba dibebaskan keluar.

2. Bulu kulit dicondongkan - Mengurangkan udara daripada terperangkap pada kulit supaya haba mudah dibebaskan kerana udara adalah penebat haba yang baik.Bulu kulit dikawal oleh otot erektor.

3. Lebih darah pada kulit (Kulit kelihatan merah) - Memudahkan haba daripada darah terbebas keluar melalui proses sinaran.

4. Berpeluh - Air peluh yang dirembes oleh kelenjar peluh mempunyai haba pendam tentu yang tinggi dapat menyerap haba yang tinggi dan terbebas ke persekitran apabila air peluh mengewap.

Haiwan seperti anjing dan kucing tidak mempunyai kelenjar peluh kecuali di tapak kaki mereka. Oleh itu, anjing akan menjelirkan lidahnya manakala kucing akan menjilat bulunya supaya suhu badan mereka berkurangan.

Apabila suhu badan rendah, perkara yang sama berlaku, di mana termoreseptor menghantar impuls kepada hipotalamus. Perbezaannya adalah bagaimana hipotalamus bertindak untuk meningkatkan suhu badan ke julat yang normal.

Mekanisme pembetulan apabila suhu badan rendah ialah:

1. Pemvasocerutan - Salur darah mengecut untuk menjauhi kulit. Dengan ini haba kurang dibebaskan ke persekitaran.

2. Bulu kulit ditegakkan - Udara akan terperangkap dicelah-celah bulu roma dan bertindak sebagai 'selimut' supaya haba sukar dibebaskan kerana udara adalah penebat haba yang baik.

3. Kurang darah pada kulit (Kulit kurang kelihatan kemerahanan atau pucat) - Kurang mengalami proses sinaran untuk mengurangkan haba daripada terbebas keluar ke persekitaran.

4. Kurang berpeluh - Apabila kurang air peluh dirembeskan oleh kelenjar peluh maka haba kurang dibebaskan melalui pengewapan air peluh.

[sunting] Pengawalan suhu dengan kaedah metabolik

Kaedah ini dikenali sebagai kaedah metabolik kerana kawalaturan lebih kepada penggunaan kimia badan daripada secara fizikal walaupun terdapat kawalaturan yang melibatkan otot-otot. Kawalan ini melibat peranan:

Otot rangka Kelenjar adrenal Kelenjar tiroid

Dalam keadaan sejuk, hipotalamus akan mengawal otot rangka untuk mengecut dan mengendur secara aktif. Hal ini akan menyebabkan seseorang mengigil dan meningkatkan suhu badan. Dalam masa yang sama, kelenjar adrenal akan merembeskan hormon adrenalina dan noradrenalina manakala kelenjar tiroid akan merembeskan hormon tiroksina, semua hormon ini bertujuan untuk meningkatkan suhu badan dengan cara meningkatkan metabolisme badan.

Dalam keadaan panas, aktiviti otot rangka akan berkurangan, begitu juga dengan perembesan hormon-hormon tertentu oleh kelenjar adrenal dan kelenjar tiroid akan berkurangan.

Hormon adrenalina dan noradrenalina bertindak dengan:

1. Meningkatkan kadar denyutan jantung dan kadar pernafasan.2. Meningkatkan tekanan darah3. Meningkatkan metabolisme badan4. Meningkatkan aras gula darah dengan merangsang penukaran

glikogen kepada glukosa.

[sunting] Kawalan Aras Gula Ringkas dalam Darah

Jika ginjal terlibat dalam pembentukan air kencing, pankreas adalah organ yang terlibat dalam pengawalaturan glukosa dalam darah. Pankreas terletak dibawah perut dan organ ini menerima bekalan darah yang banyak untuk memastikan organ ini berfungsi dengan baik. Pankreas mengandungi sel-sel Langerhans. Satu sel Langerhans terbahagi kepada dua bahagian, iaitu sel α (sel alfa) dan sel β (sel beta).

Kepekatan gula dalam darah yang normal adalah 90 mg untuk setiap 100 cm3. Dalam badan, glukosa akan ditukarkan kepada karbon dioksida, air dan tenaga (respirasi) terutama di tisu-tisu otot. Namun apabila aras glukosa terlampau banyak iaitu lebih daripada julat normal, sel-sel Langerhans beta akan merembeskan lebih banyak hormon insulin manakala sel-sel alfa akan menghentikan pengeluaran hormon glukagon. Aras glukosa dalam darah akan turun dan proses ini akan berterusan sehingga aras glukosa dalam darah berada pada julat yang normal.

Fungsi hormon insulin ialah:

Merangsang penukaran glukosa kepada glikogen untuk disimpan dalam hati.

Merangsang pengoksidaan glukosa untuk tujuan respirasi dalam sel.

Apabila aras glukosa terlampau rendah iaitu kurang daripada julat normal, sel-sel Langerhans alfa akan merembeskan lebih banyak hormon glukagon dan sel-sel beta akan berhenti merembeskan hormon insulin. Glukagon akan menukarkan glikogen kepada glukosa dan aras glukosa dalam darah akan naik. Proses ini akan berterusan sehingga aras glukosa dalam darah berada pada julat yang normal.

Fungsi hormon glukagon ialah:

Merangsang penukaran glikogen kepada glukosa dalam darah.

[sunting] Rujukan

Kursus Sains Fajar Bakti (Penerbit Fajar Bakti) (008974-T) - 1999 - Biologi STPM Jilid 1 oleh Peter Chen terjemahan oleh Liew Shee Leong dan Lim Peng Lai - ISDN 967-65-0658-3

SASBADI (139288-X) - 2004 - Master Studi Sasbadi SPM Biologi Tingkatan 4 dan 5 oleh Mah Chee Wai, Dr.Tina Lim Swee Kim, dan Nazar Shaarani - ISDN 983-59-2090-7

'K' Publishing (144639) - 2004 - KBSM Biologi Tingkatan 5 oleh Zolkofli bin Awang, Nurul Uyun binti Abdullah, Norma binti Ismail, Fathiah binti Mansoor, dan Mohd. Nazri bin Md. Saad - ISDN 983-852-379-8

Diambil daripada "http://ms.wikipedia.org/wiki/Homeostasis"Kategori: Biologi

Homeostasis (from Greek: ὅμοιος, hómoios, "similar"[1] and στάσις, stásis, "standing still";[2] defined by Claude Bernard and later by Walter Bradford Cannon in 1926[3], 1929[4] and 1932[5][6]) is the property of a system, either open or closed, that regulates its internal environment and tends to maintain a stable, constant condition. Typically used to refer to a living organism, the concept came from that of milieu interieur that was created by Claude Bernard and published in 1865. Multiple dynamic equilibrium adjustment and regulation mechanisms make homeostasis possible.

Contents

[hide]

1 Biological o 1.1 Control mechanisms

1.1.1 Positive feedback 1.1.2 Negative feedback

o 1.2 Homeostatic imbalance o 1.3 Varieties

2 Ecological 3 Biosphere 4 Reactive 5 Other fields

o 5.1 Risk o 5.2 Stress

6 Psychological 7 See also 8 References

[edit] Biological

Further information: Human homeostasis

With regards to any given life system parameter, an organism may be a conformer or a regulator. On one hand, regulators try to maintain the

parameter at a constant level over possibly wide ambient environmental variations. On the other hand, conformers allow the environment to determine the parameter. For instance, endothermic animals (mammals and birds) maintain a constant body temperature,while exothermic animals (almost all other organisms) exhibit wide body temperature variation.

Behavioral adaptations allow endothermic animals to exert some control over a given parameter. For instance, reptiles often rest on sun-heated rocks in the morning to raise their body temperature. Regulators are also responsive to external circumstances, however: if the same sun-baked boulder happens to host a ground squirrel, the animal's metabolism will adjust to the lesser need for internal heat production.

Thermal image of a cold-blooded tarantula (cold-blooded or ectothermic) on a warm-blooded human hand (endothermic).

An advantage of homeostatic regulation is that it allows an organism to function effectively in a broad range of environmental conditions. For example, ectotherms tend to become sluggish at low temperatures, whereas a co-located endotherm may be fully active. That thermal stability comes at a price since an automatic regulation system requires additional energy. One reason snakes may eat only once a week is that they use much less energy to maintain homeostasis.

Most homeostatic regulation is controlled by the release of hormones into the bloodstream. However, other regulatory processes rely on simple diffusion to maintain a balance.

Homeostatic regulation extends far beyond the control of temperature. All animals also regulate their blood glucose, as well as the concentration of their blood. Mammals regulate their blood glucose with insulin and glucagon. The human body maintains glucose levels constant most of the day, even after a 24-hour fast. Even during long periods of fasting, glucose levels are reduced only very slightly.[7] Insulin, secreted by the beta cells of the pancreas, effectively transports glucose to the body's cells by instructing

those cells to keep more of the glucose for their own use. See Dynamic equilibrium. If the glucose inside the cells is high the cells will convert it to the insoluble glycogen to prevent the soluble glucose interfering with cellular metabolism. Ultimately this lowers blood glucose levels, and Insulin helps to prevent hyperglycemia. When insulin is deficient or cells become resistant to it, diabetes occurs. Glucagon, secreted by the alpha cells of the pancreas, encourages cells to break down stored glycogen or convert non-carbohydrate carbon sources to glucose via gluconeogenesis, thus preventing hypoglycemia. The kidneys are used to remove excess water and ions from the blood. These are then expelled as urine. The kidneys perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea from the blood. These are the body's main waste products.

Another homeostatic regulation occurs in the gut. Homeostasis of the gut is not fully understood but it is believed that Toll-like receptor (TLR) expression profiles contribute to it. Intestinal epithelial cells exhibit important factors that contribute to homeostasis: 1) They have different cellular distribution of TLR’s compared to the normal gut mucosa. An example of this is how TLR5 (activated by flagellin) can redistribute to the basolateral membrane, which is the perfect place where flagellin can be detected.[8] 2) The enterocytes express high levels of TLR inhibitor Toll-interacting protein (TOLLIP). TOLLIP is a human gene that is a part of the innate immune system and is highest in a healthy gut; it correlates to luminal bacterial load.[8] 3) Surface enterocytes also express high levels of Interleukin-1 receptor (IL-1R) -containing inhibitory molecule. IL-1R are also referred to as single immunoglobulin IL-1R (SIGIRR). Animals deficient in this are more susceptible to induced colitis, implying that SIGIRR might possibly play a role in tuning mucosal tolerance towards commensal flora.[8] Nucleotide-binding oligomerisation domain containing 2 (NOD2) is suggested to have an effect on suppressing inflammatory cascades based on recent evidence.[8] It is believed to modulate signals transmitted through TLRs, TLR3, 4, and 9 specifically. Mutation of it has resulted in Crohn's disease. Excessive T-helper 1 responses to resident flora in the gut are controlled by inhibiting the controlling influence of regulatory T cells and tolerance-inducing dendritic cells.

Sleep timing depends upon a balance between homeostatic sleep propensity, the need for sleep as a function of the amount of time elapsed since the last adequate sleep episode, and circadian rhythms that determine the ideal timing of a correctly structured and restorative sleep episode.[9]

[edit] Control mechanisms

All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: The receptor is the sensing component that monitors and responds to changes in the environment.

When the receptor senses a stimulus, it sends information to a control center, the component that sets the range at which a variable is maintained. The control center determines an appropriate response to the stimulus. In most homeostatic mechanisms the control center is the brain. The control center then sends signals to an effector, which can be muscles, organs or other structures that receive signals from the control center. After receiving the signal, a change occurs to correct the deviation by either enhancing it with positive feedback or depressing it with negative feedback [10]

[edit] Positive feedback

Positive feedback is a mechanism by which an output is enhanced, such as protein levels. However, in order to avoid any fluctuation in the protein level, the mechanism is inhibited stochastically (I), therefore when the concentration of the activated protein (A) is past the threshold ([I]), the loop mechanism is activated and the concentration of A increases exponentially if d[A]=k [A]

Positive feedback mechanisms are designed to accelerate or enhance the output created by a stimulus that has already been activated.

Unlike negative feedback mechanisms that initiate to maintain or regulate physiological functions within a set and narrow range, the positive feedback mechanisms are designed to push levels out of normal ranges. To achieve this purpose, a series of events initiates a cascading process that builds to increase the effect of the stimulus. This process can be beneficial but is rarely used by the body due to risks of the acceleration's becoming uncontrollable.

One positive feedback example event in the body is blood platelet accumulation, which, in turn, causes blood clotting in response to a break or tear in the lining of blood vessels. Another example is the release of oxytocin to intensify the contractions that take place during childbirth.[10]

[edit] Negative feedback

Negative feedback mechanisms consist of reducing the output or activity of any organ or system back to its normal range of functioning. A good example of this is regulating blood pressure. Blood vessels can sense resistance of blood flow against the walls when blood pressure increases. The blood vessels act as the receptors and they relay this message to the brain. The brain then sends a message to the heart and blood vessels, both of which are the effectors. The heart rate would decrease as the blood vessels increase in diameter (or vasodilation). This change would cause the blood pressure to fall back to its normal range. The opposite would happen when blood pressure decreases, and would cause vasoconstriction.

Another important example is seen when the body is deprived of food. The body would then reset the metabolic set point to a lower than normal value. This would allow the body to continue to function, at a slower rate, even though the body is starving. Therefore, people who deprive themselves of food while trying to lose weight would find it easy to shed weight initially and much harder to lose more after. This is due to the body readjusting itself to a lower metabolic set point to allow the body to survive with its low supply of energy. Exercise can change this effect by increasing the metabolic demand.

Another good example of negative feedback mechanism is temperature control. The hypothalamus, which monitors the body temperature, is capable of determining even the slightest of variation of normal body temperature (37 degrees Celsius). Response to such variation could be stimulation of glands that produces sweat to reduce the temperature or signaling various muscles to shiver to increase body temperature.

Both feedbacks are equally important for the healthy functioning of one's body. Complications can arise if any of the two feedbacks are affected or altered in any way.

[edit] Homeostatic imbalance

Many diseases are a result of disturbance of homeostasis, a condition known as homeostatic imbalance. As it ages, every organism will lose efficiency in its control systems. The inefficiencies gradually result in an unstable internal environment that increases the risk for illness. In addition, homeostatic imbalance is also responsible for the physical changes associated with aging. Even more serious than illness and other characteristics of aging is death. Heart failure has been seen where nominal negative feedback mechanisms become overwhelmed, and destructive positive feedback mechanisms then take over.[10]

Diseases that result from a homeostatic imbalance include diabetes, dehydration, hypoglycemia, hyperglycemia, gout, and any disease caused by a toxin present in the bloodstream. All of these conditions result from the presence of an increased amount of a particular substance. In ideal circumstances, homeostatic control mechanisms should prevent this imbalance from occurring, but, in some people, the mechanisms do not work efficiently enough or the quantity of the substance exceeds the levels at which it can be managed. In these cases, medical intervention is necessary to restore the balance, or permanent damage to the organs may result.

[edit] Varieties

The Dynamic Energy Budget theory for metabolic organization delineates structure and (one or more) reserves in an organism. Its formulation is based on three forms of homeostasis:

Strong homeostasis, whereas structure and reserve do not change in composition. Because the amount of reserve and structure can vary, this allows a particular change in the composition of the whole body (as explained by the Dynamic Energy Budget theory).

Weak homeostasis, wherein the ratio of the amounts of reserve and structure becomes constant as long as food availability is constant, even when the organism grows. This means that the whole body composition is constant during growth in constant environments.

Structural homeostasis, wherein the sub-individual structures grow in harmony with the whole individual; the relative proportions of the individuals remain constant.

[edit] Ecological

Historically, ecological succession was seen as having a stable end-stage called the climax (see Frederic Clements), sometimes referred to as the 'potential biodiversity' of a site, shaped primarily by the local climate. This idea has been largely abandoned by modern ecologists in favor of nonequilibrium ideas of how ecosystems function, as most natural ecosystems experience disturbance at a rate that makes a "climax" community unattainable.

Only on small, isolated habitats known as ecological islands can the phenomenon be observed. One such case study is the island of Krakatoa after its major eruption in 1883: the established stable homeostasis of the previous forest climax ecosystem was destroyed, and all life was eliminated from the island. In the years after the eruption, Krakatoa went through a sequence of ecological changes in which successive groups of new plant or animal species followed one another, leading to increasing biodiversity and eventually culminating in a re-established climax community. This ecological

succession on Krakatoa occurred in a number of stages; a sere is defined as "a stage in a sequence of events by which succession occurs". The complete chain of seres leading to a climax is called a prisere. In the case of Krakatoa, the island reached its climax community, with eight hundred different recorded species, in 1983, one hundred years after the eruption that cleared all life off the island. Evidence confirms that this number has been homeostatic for some time, with the introduction of new species rapidly leading to elimination of old ones. The evidence of Krakatoa, and other disturbed island ecosystems, has confirmed many principles of Island Biogeography, mimicking general principles of ecological succession albeit in a virtually closed system comprised almost exclusively of endemic species.

[edit] Biosphere

In the Gaia hypothesis, James Lovelock stated that the entire mass of living matter on Earth (or any planet with life) functions as a vast homeostatic superorganism that actively modifies its planetary environment to produce the environmental conditions necessary for its own survival. In this view, the entire planet maintains homeostasis. Whether this sort of system is present on Earth is still open to debate. However, some relatively simple homeostatic mechanisms are generally accepted. For example, when atmospheric carbon dioxide levels rise, certain plants are able to grow better and thus act to remove more carbon dioxide from the atmosphere. When sunlight is plentiful and atmospheric temperature climbs, the phytoplankton of the ocean surface waters thrive and produce more dimethyl sulfide, DMS. The DMS molecules act as cloud condensation nuclei, which produce more clouds, and thus increase the atmospheric albedo, and this feeds back to lower the temperature of the atmosphere. As scientists discover more about Earth, vast numbers of positive and negative feedback loops are being discovered, that, together, maintain a metastable condition, sometimes within very broad range of environmental conditions. Environmental pressure, such as competition or change in temperature, can lead to adaptation/extinction of species over time.

[edit] Reactive

Example of use: "Reactive homeostasis is an immediate homeostasic response to a challenge such as predation."

However, any homeostasis is impossible without reaction - because homeostasis is and must be a "feedback" phenomenon.

The phrase "reactive homeostasis" is simply short for "reactive compensation reestablishing homeostasis", that is to say, "reestablishing a point of homeostasis." - it should not be confused with a separate kind of

homeostasis or a distinct phenomenon from homeostasis; it is simply the compensation (or compensatory) phase of homeostasis.

[edit] Other fields

The term has come to be used in other fields, as well.

[edit] RiskMain article: Risk homeostasis

An actuary may refer to risk homeostasis, where (for example) people that have anti-lock brakes have no better safety record than those without anti-lock brakes, because the former unconsciously compensate for the safer vehicle via less-safe driving habits. Previous to the innovation of anti-lock brakes, certain maneuvers involved minor skids, evoking fear and avoidance: now the anti-lock system moves the boundary for such feedback, and behavior patterns expand into the no-longer punitive area. It has also been suggested[citation needed] that ecological crises are an instance of risk homeostasis in which a particular behavior continues until proven dangerous or dramatic consequences actually occur.

[edit] Stress

Sociologists and psychologists may refer to stress homeostasis, the tendency of a population or an individual to stay at a certain level of stress, often generating artificial stresses if the "natural" level of stress is not enough.[citation needed]

Jean-François Lyotard, a postmodern theorist, has applied this term to societal 'power centers' that he describes as being 'governed by a principle of homeostasis,' for example, the scientific hierarchy, which will sometimes ignore a radical new discovery for years because it destabilises previously-accepted norms. (See The Postmodern Condition: A Report on Knowledge by Jean-François Lyotard)

[edit] Psychological

Author George Leonard discusses in his book Mastery how homeostasis affects our behavior and who we are. He states that homeostasis will prevent our body from making drastic changes and maintain stability in our lives even if it is detrimental to us.[11] Examples include when an obese person starts exercising, homeostasis in the body resists the activity to maintain stability.[12] Another example Leonard uses is a unstable family where the father has been a raging alcoholic and suddenly stops and the son starts up a drug habit to maintain stability in the family. Homeostasis is the main

factor that stops people changing their habits because our bodies view change as dangerous unless it is very slow. Leonard discusses this dilemma as the media today only encourages fast change and quick results. The opening of his book aptly describes his despair with the current state of the world and how it is at war with homeostasis. "The trouble is that we have few, if any, maps to guide us on the journey or even to show us how to find the path. The modern world, in fact, can be viewed as a prodigious conspiracy against mastery. We're continually bombarded with the promises of immediate gratification, instant success, and fast, temporary relief, all of which lead in exactly the wrong direction."[13]

[edit] See also

Acclimatization Aging Allostasis Apoptosis Biological rhythm Claude Bernard Climate change feedback Cybernetics Enantiostasis Gaia hypothesis Health homeodynamics Homeorhesis Lenz's law Le Chatelier's principle Milieu interieur Metabolism Osmosis Proteostasis Protobiont Self-organization Steady state

[edit] References

1. ̂ ὅμοιος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus

2. ̂ στάσις, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus

3. ̂ W. B. Cannon. ‘‘Physiological regulation of normal states: some tentative postulates concerning biological homeostatics.’’ IN: A. Pettit (ed.). A Charles Richet: ses anims, ses collegues, ses elves, p. 91. Paris: editions Medicales, 1926.

4. ̂ Cannon WB. Organization For Physiological Homeostasis. Physiol Rev. 1929; 9: 399-431.

5. ̂ Cannon WB. The Wisdom of the Body. 1932. W.W. Norton & Company, Inc., New York.

6. ̂ Karl Ludwig von Bertalanffy: ... aber vom Menschen wissen wir nichts, (English title: Robots, Men and Minds), translated by Dr. Hans-Joachim Flechtner. page 115. Econ Verlag GmbH (1970), Düsseldorf, Wien. 1st edition.

7. ̂ Bhagavan, N. V. (2002). Medical biochemistry (4th ed.). Academic Press. pp. 499. ISBN 9780120954407. http://books.google.com/?id=vT9YttFTPi0C&pg=PA499&dq=%22in+very+prolonged+fasts+the+plasma+glucose+level+decreases+very+slightly%22#v=onepage&q=%22in%20very%20prolonged%20fasts%20the%20plasma%20glucose%20level%20decreases%20very%20slightly%22.

8. ^ a b c d Ann M O'Hara, Fergus Shanahan The gut flora as a forgotten organ. EMBO reports 7, 688 - 693 (1 July 2006)

9. ̂ Wyatt, James K.; Ritz-De Cecco, Angela; Czeisler, Charles A.; Dijk, Derk-Jan (1 October 1999). "Circadian temperature and melatonin rhythms, sleep, and neurobehavioral function in humans living on a 20-h day". Am J Physiol 277 (4): R1152–R1163. Fulltext. PMID 10516257. http://ajpregu.physiology.org/cgi/content/full/277/4/R1152. Retrieved 2007-11-25. "... significant homeostatic and circadian modulation of sleep structure, with the highest sleep efficiency occurring in sleep episodes bracketing the melatonin maximum and core body temperature minimum".

10. ^ a b c Marieb, Elaine N. & Hoehn, Katja (2007). Human Anatomy & Physiology (Seventh ed.). San Francisco, CA: Pearson Benjamin Cummings.

11. ̂ http://gettingstronger.org/2010/03/george-leonards-mastery/12. ̂ http://www.procrastinationhelp.com/procrastination/why-

resolutions-fail.html13. ̂

http://sirpabs.ilahas.com/ebooks/Social%20Interactions/Mastery%20-%20The%20Keys%20To%20Success%20And%20Long-Term%20Fulfillment%20-%20George%20Leonard.pdf

[hide]v · d · eSubfields of and scientists involved in cybernetics

Subfields Polycontexturality · Second-order cybernetics · Catastrophe theory · Connectionism · Control theory · Decision theory · Information theory · Semiotics · Synergetics · Biological cybernetics · Biosemiotics · Biomedical cybernetics · Biorobotics · Computational

neuroscience · Homeostasis · Management cybernetics · Medical cybernetics · New Cybernetics · Neurocybernetics · Sociocybernetics · Emergence · Artificial intelligence

Cyberneticists

Igor Aleksander · William Ross Ashby · Anthony Stafford Beer · Claude Bernard · Ludwig von Bertalanffy · Valentin Braitenberg · Gordon S. Brown · Walter Bradford Cannon · Heinz von Foerster · Charles François · Jay Wright Forrester · Buckminster Fuller · Ernst von Glasersfeld · Francis Heylighen · Erich von Holst · Cliff Joslyn · Stuart Kauffman · Sergei P. Kurdyumov · Niklas Luhmann · Warren McCulloch · Humberto Maturana · Talcott Parsons · Gordon Pask · Walter Pitts · Alfred Radcliffe-Brown · Robert Trappl · Valentin Turchin · Jakob von Uexküll  · Francisco Varela · Frederic Vester · Charles Geoffrey Vickers  · Stuart Umpleby · John N. Warfield · Kevin Warwick · Norbert Wiener · Anthony Wilden

Retrieved from "http://en.wikipedia.org/wiki/Homeostasis"Categories: Homeostasis | Cybernetics | Physiology | Systems theory | Biology terminology | Greek loanwords

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