1.反馈控制系统（feedback control system）

是一种“闭环”系统，即控制部分发出信号，指示受控部分活动，而受控部分的活动可被一

定的感受装置感受，感受装置再将受控部分的活动情况作为反馈信号送回到控制部分，控制部分可以根据反馈信号来改变自己的活动，调整对受控部分的指令，因而能对受控部分的活动进行调节。可见，在这样的控制系统中，控制部分和受控部分之间形成一个闭环联系。在反馈控制系统中，反馈信号对控制部分的活动可发生不同的影响，从而实现对受控部分活动的调节。如果经过反馈调节，受控部分的活动向和它原先活动相反的方向发生改变，这种方式的调节称为负反馈（negative feedback)调节；相反，如果反馈调节使受控部分继续加强向原来方向的活动，则称为正反馈（positive feedback)调节。在正常人体内，绝大多数控制系统都是负反馈方式的调节，只有少数是正反馈调节

而正反馈控制系统则仅有很少几个，例如，在血液章中将会讲到，血液凝固是正反馈控 制。当一处血管破裂时，各种凝血因子相继激活，最后形成血凝块，将血管破口封住（见第三章）。又如，在正常分娩过程中，子宫收缩导致胎儿头部下降并牵张子宫颈，子宫颈部受牵张时可进一步加强子宫收缩，再使胎儿头部进一步牵张子宫颈，子宫颈牵张再加强子宫收缩，如此反复，直至整个胎儿娩出。在第二章中将会讲到，神经细胞产生动作电位的过程中，细胞膜钠通道的开放和钠离子内流互相促进，也是正反馈控制。  在病理情况下，则会有许多正反馈的情况发生。例如，在大量失血时，心脏射出的血量减少， 血压明显降低，冠状动脉的血流鼻就减少，使心肌收缩力减弱，心脏射出的血量就更少，如此反 复，最后可导致死亡。在这个过程中，心脏活动减弱，经过反馈控制，使心脏活动更弱，所以是正 反馈。这类反馈控制过程常称为恶性循环（vicious circle）

当一个系统的活动处于某种平衡或稳定状态时，如果因某种外界因素使该系统的受控部分活动增强，则该系统原先的平衡或稳定状态遭受破坏。在存在负反馈控制机制的情况下，如果受控部分的活动增强，可通过相应的感受装置将这个信息反馈给控制部分；控制部分经分析后，发出指令使受控部分的活动减弱，向原先的平衡状态的方向转变，甚至完全恢复到原先的平衡状态。反之，如果受控部分的活动过低，则可以通过负反馈机制使其活动增强，结果也是向原先平衡状态的方向恢复。所以，负反馈控制系统的作用是使系统的活动保持稳定。机体的内环境和各种生理活动之所以能够维持稳态，就是因为体内许多负反馈控制系统的存在和发挥作用。举例来说，脑内的心血管

活动中枢通过交感神经和迷走神经控制心脏和血管的活动，使动脉血压维持在一定的水平。当由于某种原因使心脏活动增强、血管收缩而导致动脉血压高于正常时，动脉压力感受器就立即将这一信息通过传人神经反馈到心血管中枢，心血管中枢的活动就会发生相应的改变，使心脏活动减弱，血管舒张，于是动脉血压向正常水平恢复。在另一些情况下，例如当人体由卧位转变为立位时，体内有一部分血液滞留在下肢静脉内，使单位时间内流回心脏的血量减少，动脉血压降低；此时动脉压力感受器传人中枢的神经冲动立即减少，使心血管中枢活动发生改变，其结果是心脏活动加强，血管收缩，动脉血压回升至原先的水平。在后面的各章中，将会讲到许多负反馈调节的例子。许多内分泌细胞也受到各种负反馈机制的调控，使其活动能够维持在一定的水平（见第十一、十二章）。 

    体内许多负反馈调节机制中都设置了一个“调定点”（set point），负反馈机制对受控部分活动的调节就以这个调定点为参照水平，即规定受控部分的活动只能在靠近调定点的一个狭小范围内变动。在上述动脉血压的负反馈调节机制中，就有一个动脉血压的调定点（见第四章）。假如正常情况下动脉血压的调定点设置在100mmHg,则当各种原因使血压偏离这个水平时，上述的负反馈机制就会使血压重新回到接近100mmHg的水平。在不同的条件下，调定点是可以发生变动的。例如，在原发性高血压病人中，血压的调定点被设置在较高的水平，因此动脉血压就保 持在一个高于正常的水平。生理学中将调定点发生变动的过程称为重调定（resetting）

alpha cells α细胞

胰腺胰岛中产生高血糖素的细胞，高血糖素有升高血糖的作用

pancreatic isles(胰岛)

 insulin 胰岛素

glucagon   胰高血糖素,起着增加血糖的作用

 

 

 

内环境指的是细胞外液——血浆、组织液（又称细胞间隙液）和淋巴。

内环境理化性质的相对恒定，理化性质包括：温度、pH、渗透压、化学组成等。目前，稳态的概念扩大到泛指体内从细胞和分子水平、器官和系统水平到整体水平的各种生理功能活动在神经和体液等因素调节下保持相对稳定的状态.

稳态是内环境恒定概念的引伸与发展。内环境恒定概念是19世纪法国生理学家贝尔纳（Claud Bernard）所提出。他认为机体生存在两个环境中，一个是不断变化的外环境，一个是比较稳定的内环境。内环境是围绕在多细胞动物的细胞周围的细胞外液。内环境的特点是其理化特性及其组成分的数量和性质，处于相对恒定状态，为细胞提供一适宜的生活环境，也是维持生命的必要条件。“内环境恒定是（机体）自由和独立生存的首要条件”，这是贝尔纳对生命现象的高度概括。稳态即相似的状态，是美国生理学家坎农（W.B.Cannon）于本世纪20年代末提出的，是内环境恒定概念的引伸和发展。在坎农时期，稳态主要指内环境是可变的又是相对稳定的状态。稳态是在不断运动中所达到的一种动态平衡；即是在遭受着许多外界干扰因素的条件下，经过体内复杂的调节机制使各器官、系统协调活动的结果，这种稳定是相对的，不是绝对的，一旦稳态遭破坏，就导致机体死亡。

随着控制论和其他生命科学的发展，稳态已不仅指内环境的稳定状态，也扩展到有机体内极多的保持协调、稳定的生理过程，例如生命活动功能以及正常姿势（直立以及行路姿势）的维持等；也用于机体的不同层次或水平（细胞、组织器官、系统、整体、社会群体）的稳定状态；以及在特定时间内（由几毫秒直至若干万年）保持的特定状态。稳态不仅是生理学，也是当今生命科学的一大基本概念。它对控制论、遗传学（基因的稳态调节）、心理学（情绪稳态等）、病理学、临床医学等多种学科都有重要意义。

组成人和动物体的细胞数以亿计，其中绝大多数细胞不能直接与外界环境接触。那么，这些与外界环境隔离的体内细胞生活在什么样的环境中？它们是怎样与外界环境进行物质交换的呢？下面以人体为例来说明。

内环境 人体内含有大量的液体，这些液体统称为体液。体液可以分为两大部分：存在于细胞内的部分，叫做细胞内液；存在于细胞外的部分，叫做细胞外液。细胞外液主要包括组织液(组织间隙液的简称)、血浆(血液的液体部分)和淋巴等。人体内的细胞外液，构成了体内细胞生活的液体环境，这个液体环境叫做人体的内环境。

体液的各个部分之间既是彼此隔开的，又是相互联系的。细胞浸浴在组织液中，在细胞内液与组织液之间只隔着细胞膜，水分和一切能够透过细胞膜的物质，都可以在细胞内液与组织液之间进行交换(如图)。在组织液与血浆之间只隔着毛细血管壁，水分和一切能够透过毛细血管壁的物质，都可以在两者之间进行交换。组织液还可以渗入毛细淋巴管形成淋巴。因此，人体内的细胞就可以通过内环境，与外界环境之间间接地进行物质交换了。具体地说，就是由呼吸系统吸进的氧和消化系统吸收的营养物质先进入血液，然后再通过组织液进入体内细胞；同时，体内细胞新陈代谢所产生的废物和二氧化碳，也要先进入组织液，然后再进入血液而被运送到泌尿系统和呼吸系统，排出体外。由此可见，体内的细胞只有通过内环境，才能与外界环境进行物质交换。

内环境是体内细胞生存的直接环境。细胞与内环境之间、内环境与外界环境之间不断地进行着物质交换，因此，细胞的代谢活动和外界环境的不断变化，必然会影响内环境的理化性质，如pH、渗透压、温度等。那么，内环境的理化性质会不会经常发生剧烈的变化呢？下面以内环境的pH为例来说明。血液是内环境的重要组成部分。人体在新陈代谢过程中，会产生许多酸性物质，如乳酸、碳酸；人的食物(如蔬菜、水果)中往往含有一些碱性物质，如碳酸钠。这些酸性和碱性的物质进入血液，就会使血液的pH发生变化。但是，通过实际测定发现，正常人血液的pH通常在7.35～7.45之间，变化范围是很小的。这是什么原因呢？原来，血液中含有许多对对酸碱度起缓冲作用的物质——缓冲物质，每一对缓冲物质都是由一种弱酸和相应的一种强碱盐组成的，如H2CO3/NaHCO3、NaH2PO4/Na2HPO4等。当机体剧烈运动时，肌肉中产生大量的乳酸、碳酸等物质，并且进入血液。乳酸进入血液后，就与血液中的碳酸氢钠发生作用，生成乳酸钠和碳酸。碳酸是一种弱酸，而且又可以分解成二氧化碳和水，所以对血液的pH值影响不大。血液中增多的二氧化碳会刺激控制呼吸活动的神经中枢，促使增强呼吸活动，增加通气量，从而将二氧化碳排出体外。当碳酸钠进入血液后，就与血液中的碳酸发生作用，形成碳酸氢盐，而过多的碳酸氢盐可以由肾脏排出。这样，由于血液中缓冲物质的调节作用，可以使血液的酸碱度不会发生很大的变化，从而维持在相对稳定的状态。内环境的其他理化性质，如温度、渗透压、各种化学物质的含量等，也都能够维持在一个相对稳定的状态。生理学家把正常机体在神经系统和体液的调节下，通过各个器官、系统的协调活动，共同维持内环境的相对稳定状态，叫做内环境的稳态。

内环境稳态的生理意义 机体的新陈代谢是由细胞内很多复杂的酶促反应组成的，而酶促反应的进行需要温和的外界条件，例如温度、pH等都必须保持在适宜的范围内，酶促反应才能正常进行。可见，内环境的稳态是机体进行正常生命活动的必要条件。当内环境的稳态遭到破坏时，就会引起细胞新陈代谢紊乱，并导致疾病。例如，当血液中钙、磷的含量降低时，会影响骨组织的钙化，这在成年人表现为骨软化病，在儿童则表现为骨质生长障碍、骨化不全的佝偻病。血钙过高则会引起肌无力等疾病。

当人体剧烈运动时，内环境稳态不会遭到破坏，这是因为人体血液中存在着缓冲物质。

Homeostasis — also spelled homoeostasis or homœostasis (from Greek: ?μοιος, "hómoios", "similar",[1] and στ?σις, stásis, "standing still"[2]) — is the property of a system in which variables are regulated so that internal conditions remain stable and relatively constant. Examples of homeostasis include the regulation of temperature and the balance between acidity and alkalinity (pH). It is a process which maintains the stability of the human body‘s internal environment in response to changes in external conditions.在一个系统内，各个变量有条不絮，内部环境稳定，持续，例如体温和ph值。

The concept was described by Claude Bernard in 1865 and the word was coined by Walter Bradford Cannon in 1926,[3] 1929[4] and 1932.[5][6] Although the term was originally used to refer to processes within living organisms, it is frequently applied to automatic control systems such as thermostats恒温器. Homeostasis requires a sensor to detect changes in the condition to be regulated, an effector mechanism which can vary that condition; and a negative feedbackconnection between the two.

 

 

Examples from technology[edit]

The following are all examples of familiar homeostatic mechanisms:

• A Thermostat operates by switching heaters or air-conditioners on and off in response to the output of a temperature sensor.

• Cruise control adjusts a car‘s throttle in response to changes in speed.

• An Autopilot operated the steering controls of an aircraft or ship in response to deviation from a pre-set compass bearing or route.

• Process control systems in a chemical plant or oil refinery maintain fluid levels, pressures, temperature, chemical composition, etc. by controlling heaters, pumps and valves.

• The centrifugal governor of a steam engine, as designed by James Watt in 1788, reduces the throttle valve in response to increases in the engine speed, or opens the valve if the speed falls below the pre-set rate.

Biological[edit]

All living organisms depend on maintaining a complex set of interacting metabolic chemical reactions. From the simplest single-celled organisms to the most complex plants and animals, internal processes operate to keep the conditions within tight limits to allow these reactions to proceed. Homeostatic processes act at the level of the cell, the tissue, theorgan, as well as for the organism as a whole.所有生物存活依靠复杂化学新陈代谢系统

Principal Homeostatic processes include the following:

• "Warm blooded" (endothermic) animals (mammals and birds) maintain a constant body temperature, while ectothermic animals (almost all other organisms) exhibit 表现wide bodytemperature variation巨大温差变化. [7] An advantage of temperature regulation is that it allows anorganism to function effectively in a broad range of environmental conditions.  For example,ectotherms tend to become sluggish 迟缓的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.[8] If the temperature rises, the body loses heat by sweating or panting喘气, via the latent heat of evaporation. If it falls, this is counteracted 对抗，抵消by increased metabolic action, by shivering, and - in fur- or feather-coated creatures - by thickening the coat.热血（内温）动物（哺乳动物和鸟类保持恒定体温），外温（冷血）动物体温变化很大.在各种环境下，一个高级温度调节可以让有机物有效工作。例如冷血动物在低温下，行为迟缓，外温动物可以完全发挥。高级温度调节机制需要消耗额外能量。如果温度上升，身体流汗或喘气散发热量。

• Regulation of the pH of the blood at 7.365 (a measure of alkalinity 碱度，碱性and acidity).人血液的恒定ph值是7.365

• All animals also regulate their blood glucose萄糖concentration血糖浓度. Mammals regulate their blood glucose with insulin 胰岛素and glucagon胰高血糖素. 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.[9] Insulin, secreted by the beta cells β细胞 f the pancreas胰, 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 from interfering with cellular metabolism. Ultimately this lowers blood glucose levels, and insulin helps to preventhyperglycemia. When insulin is deficient or cells become resistant to it, diabetes occurs.当胰岛素缺乏或膝部产生抗体，糖尿病发生。 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.肾用于移除血液里多余水和离子 These are then expelled as urine就像尿液方式排出体外. The kidneys perform a vital role in homeostatic regulation in mammals, removing excess water, salt, and urea尿素 from the blood.肾在稳态中极为重要，它能移除多余水，盐，尿素。（排尿原因）

• If the water content of the blood and lymph淋巴 fluid falls, it is restored in the first instance by extracting water from the cells. The throat and mouth become dry, so that the symptoms of thirst motivate the animal to drink.如果血液和淋巴液的水分流出，将从细胞提取水分，口腔咽喉干燥，就会出现口渴状态，然后去喝水。（口渴原因）

• If the oxygen content of the blood falls, or the carbon-dioxide concentration increases, blood flow is increased by more vigorous heart action and the speed and depth of breathing increases.

• Sleep timing depends upon a balance between homeostatic sleep propensity倾向，习性，癖好，偏爱 ，he need for sleep as a function of the amount of time elapsed since the last adequate sleep episode时期, and circadian rhythms 

  昼夜节律that determine the ideal timing of a correctly structured and restorative有恢复健康作用的sleep episode.

• 睡眠事件取决于一个平衡,即稳态的睡眠偏好和昼夜节律之间平衡。稳态的睡眠偏好是自从最近的睡眠时期，他需要睡眠的时间。昼夜节律决定了正确的和有恢复作用的睡眠时间框架。今天翻译了稳态，明白了熬夜就是慢性自杀。熬夜破坏了稳态，稳态破坏后人生命会有危险。

Control mechanisms控制机制[edit]

All homeostatic control mechanisms have at least three interdependent 互相依靠的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刺激物, 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. The control center then sends signals to an effector受动器;感受器

, 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偏离;by depressing 压低it with negative feedback.

为了变量可以被调节，所有稳态控制机制有至少三个互相依靠的成分。接收器是一个感应成分，它可以监控和回应环境的变化。当接收器感应到刺激物，就会把信息传输到控制中心。控制中心负责设置变量值可取范围。针对刺激物，控制中心决定一个合适回应。控制中心发送信号到感受器。感受器可以是肌肉，器官，或者能够从控制中心接收信号的结构。接收器收到信号后，依靠negativefeedback 压低它，矫正偏差.Control mechanisms中，大脑充当控制中心角色。就是说大脑存储一个字典dict，字典的keys是身体各个器官，values是器官活动正常值范围。大脑是所有器官的枢纽中心，例如血压升高，血管会把信息传递给大脑，大脑决定一个合理值（心脏速度减慢）传递给心脏，心脏跳动减慢后，血压恢复正常，达到人体稳态。

2组成

负反馈

正反馈

反馈控制系统由控制器、受控对象和反馈通路组成。在反馈控制系统中，不管出于什么原因（外部扰动或系统内部变化），只要被控制量偏离规定值，就会产生相应的控制作用去消除偏差。因此，它具有抑制干扰的能力，对元件特性变化不敏感，并能改善系统的响应特性。 

Negative feedback[edit]

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 thebrain. 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 直径(known asvasodilation). 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.
负反馈机制用于减少输出或活动。活动是任何器官或系统的正常发挥范围。例子，血压，血压升高后，血管能感应血流对墙的抗体。血管充当接收器，把信号传递给大脑。大脑发送信号到心脏和血管（接受器）。血管心脏跳动速度就会降低。心脏跳动速度改变会让血压返回到正常值。相反，如果血压降低，血管会收缩。

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.另一个例子是禁食。人体重新设置新陈代谢值（低于正常值）。这会导致尽管饥饿，人体以较低节奏继续活动。我们用节食方式减肥，起初效果很明显，到后来效果越来越差。因为人体可以自动调节自身，当供应能量减少后，人体以一个较低新陈代谢速度继续存活。运动后，新陈代谢加快，改变这个效果。同时节食和运动，减肥效果并不好？？？

Another good example of negative feedback mechanism is temperature control. The hypothalamus, which monitors the body temperature, is capable of determining even the slightest variation of normal body temperature (37 degrees Celsius). Response to such variation could be stimulation of glands that produce 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.

Homeostatic imbalance[edit]

Many diseases involve a disturbance of homeostasis.

As the organism ages, the efficiency in its control systems becomes reduced. The inefficiencies gradually result in an unstable internal environment that increases the risk of illness, and leads to the physical changes associated with aging.[11]

Certain homeostatic imbalances, such as high core temperature, a high concentration of salt in the blood, or low concentration of oxygen, can generate homeostatic emotions (such as warmth, thirst or breathlessness) which motivate behavior aimed at restoring homeostasis (such as removing a sweater, drinking or slowing down).[12]

Ecological[edit]

The concept of homeostasis is central to the topic of Ecological Stoichiometry. There it refers to the relationship between the chemical composition of an organism and the chemical composition of the nutrients it consumes. Stoichiometric homeostasis helps explain nutrient recycling and population dynamics.

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.

Biosphere[edit]

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, it is sometimes claimed that when atmospheric carbon dioxide levels rise, certain plants are able to grow better and thus act to remove more carbon dioxide from the atmosphere[dubious – discuss]. However, warming has exacerbated droughts, making water the actual limiting factor on land. When sunlight is plentiful and atmospheric temperature climbs, it has been claimed that the phytoplankton of the ocean surface waters may 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. However, rising sea temperature has stratified the oceans, separating warm, sunlit waters from cool, nutrient-rich waters. Thus, nutrients have become the limiting factor, and plankton levels have actually fallen over the past 50 years, not risen. 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.

Reactive[edit]

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.

Other fields[edit]

The term has come to be used in other fields, for example:

Risk[edit]

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.

Stress[edit]

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)

Psychological[edit]

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.[13] Examples include when an obese person starts exercising, homeostasis in the body resists the activity to maintain stability.[14] Another example Leonard uses is an 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." 



 外温动物又叫冷血动物，变温动物。体温随着外界温度改变而改变的动物，叫做外温动物；如鱼、蛙、蛇等。外温动物又称冷血动物,地球上的动物大部分都是 冷血动物.冷血动物并不是需要寒冷,而是其体温与其所生活的环境类似,例如蚯蚓的体温等于所住的土壤 的温度;鱼的体温等于其四周的水温.这一类动物的体 温是随著环境温度的改变而改变,所以,以外温动物的专有名词来形容其可变的体温,最能叙述其真义.

所有的动物,除了鸟类和哺乳类外,都是外温动物,它们的体温是随著环境而改变.此意并非说它们绝不能控制它们的体温,它们能藉由寻找凉爽或温暖的环境来改变自己的体温,而不能直接的控制自己的体温,即 它们缺乏维持一定体温的生理机能.

鱼类，两生类及爬虫类等，因未具体温调节机制，故体温不易保持一定而会随环境的改变而改变，称为外温动物(Poikilothermic animal)，也称「外温动物」 (ectotherms)，其意为体温来自外界之意。这类动物不仅代谢率较低，其身体之隔热效果亦差。故其体温的维持并不是靠著代谢所获取之能量。

外温动物通常在外界温度的改变时，常靠调整自己的行为来适应环境，而不会任凭环境摆布。这种行为方式称为行为体温调节(behavioral temperature regulation)。例如蜥蜴到处移动寻找温暖地方栖身;必要时，以最大体表面积暴露於太阳下，以吸收红外线的热量;或靠著已烤热的岩石，藉传导获得热量。

蚊子为冷血动物，因此它的新陈代谢过程、生活史、寿命以及生殖营养周期，均受制於环境的温度。它们无法控制自身的温度，但在低温之下，却可减缓其代谢过程而生存。大多数蚊种其发育之平均最适温，约在25~27度。在10度之低温或超过40度之高温，其发育完全停止，且死亡率甚高。蚊子的呼吸器官为气管系统。因此，一般对干燥特别敏感，所以室内的蚊子常集中於有足够湿气的地方，而外栖性之蚊子，多停留於近地面的植物上。因此，适当的雨量及日照将加速外栖性蚊子的大量孳生。由此可知，蚊子的生长环境、温度、雨量均是影响蚊子生存的主要因素。登革热必须靠病媒蚊才能将病毒传播出去，而埃及斑蚊与白线斑蚊是台湾地区传播登革热病毒的主要祸首。换句话说，只要有埃及斑蚊与白线斑蚊的地方，我们就将深受登革热的威胁，因此，要消除登革热就必须先了解埃及斑蚊与白线斑蚊的型态特征、生态事项与孳生环境。


逆热流交换机制 产热 局部异温性