📚 Homeostatic Control in A-Level Biology | A-Level 生物：稳态控制
Homeostatic control is a central concept in A-Level Biology because it explains how mammals and plants maintain stable internal conditions despite continuous external and internal disturbances. Understanding the components of feedback loops, the roles of hormones and the responses of effector organs is essential for Cambridge examinations.
稳态控制是A-Level生物的核心概念，因为它解释了哺乳动物和植物如何在持续的外界和内部干扰下维持稳定的内部条件。理解反馈环路的组成部分、激素的作用以及效应器官的反应对剑桥考试至关重要。
1. The Meaning of Homeostasis | 稳态的含义
Homeostasis is the maintenance of a relatively constant internal environment within narrow limits, even when the external environment changes. In mammals, the internal environment is represented mainly by blood plasma and tissue fluid, which surround the cells and provide the conditions needed for enzyme activity and cell function.
稳态是指在外部环境发生变化时，仍将内部环境维持在相对恒定的狭窄范围内。在哺乳动物中，内部环境主要由血浆和组织液构成，它们包围着细胞，为酶活性和细胞功能提供所需条件。
The key variables controlled by homeostasis include core body temperature, blood glucose concentration, blood water potential, blood pH and carbon dioxide concentration. Each variable has a normal range or set point, and any significant deviation from that set point triggers a corrective mechanism.
受稳态控制的关键变量包括核心体温、血糖浓度、血液水势、血液pH和二氧化碳浓度。每个变量都有一个正常范围或调定点，任何明显偏离该调定点的情况都会触发纠正机制。
CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻
Blood pH is maintained at about 7.4 partly by the carbonic acid-hydrogencarbonate buffer system shown above. The lungs and kidneys also remove acid or base to keep the hydrogen ion concentration stable.
血液pH维持在约7.4，部分依赖于上述碳酸-碳酸氢盐缓冲系统。肺和肾脏也会排出酸或碱，以保持氢离子浓度稳定。
2. The Principle of Negative Feedback | 负反馈原理
Negative feedback is the fundamental control mechanism in homeostasis. In a negative feedback loop, a change in a controlled variable is detected and a response is produced that opposes the change, returning the variable to its set point.
负反馈是稳态的基本控制机制。在负反馈环路中，受控变量的变化被检测到后，会产生一个对抗该变化的反应，使变量回到调定点。
- Receptor detects the deviation from the set point. | 感受器检测与调定点的偏差。
- Control centre coordinates the information and sends signals to effectors. | 控制中心整合信息并向效应器发送信号。
- Effector produces a response that returns the variable to normal. | 效应器产生使变量恢复正常的反应。
For example, if blood glucose rises after a meal, the pancreas releases insulin, which lowers blood glucose. As the glucose level falls back to the set point, insulin secretion is reduced, so the correction is switched off.
例如，餐后血糖升高时，胰腺释放胰岛素降低血糖。当血糖水平回落到调定点时，胰岛素分泌减少，因此纠正作用被关闭。
3. Thermoregulation: Cooling the Body | 温度调节：身体降温
Core body temperature in humans is maintained at about 37 °C. The hypothalamus acts as the central thermoregulatory centre, monitoring blood temperature and receiving nerve impulses from peripheral thermoreceptors in the skin.
人体核心体温维持在约37 °C。下丘脑作为体温调节中枢，监测血液温度，并接收来自皮肤外周温度感受器的神经冲动。
When the core temperature rises above the set point, the hypothalamus triggers heat-loss responses. Arterioles in the skin dilate, increasing blood flow to the skin surface so that more heat is lost by radiation and convection. Sweat glands also secrete more sweat, and the evaporation of sweat removes latent heat from the body.
当核心体温高于调定点时，下丘脑启动散热反应。皮肤小动脉扩张，增加流向皮肤表面的血流量，从而通过辐射和对流散失更多热量。汗腺也分泌更多汗液，汗液蒸发时从体内带走潜热。
In addition, hair erector muscles relax, so body hair lies flat and traps less insulating air. Behavioural responses, such as seeking shade or removing clothing, also help lower body temperature.
此外，立毛肌松弛，体毛平躺，减少隔热空气的滞留。寻找阴凉处或脱掉衣物等行为反应也有助于降低体温。
4. Thermoregulation: Warming the Body | 温度调节：身体升温
When core temperature falls below the set point, the hypothalamus initiates heat-conservation and heat-production responses. Arterioles in the skin constrict, reducing blood flow to the surface and therefore reducing heat loss from the skin.
当核心体温低于调定点时，下丘脑启动保温和产热反应。皮肤小动脉收缩，减少流向体表的血流量，从而减少皮肤散热。
Shivering occurs when skeletal muscles undergo rapid, involuntary contractions that generate heat by increased respiration. Hair erector muscles contract, causing body hair to stand upright and trap a thicker layer of insulating air.
颤栗是骨骼肌进行快速、不自主的收缩，通过增强呼吸作用产生热量。立毛肌收缩使体毛竖立，从而捕获更厚的隔热空气层。
Longer-term responses include increased secretion of thyroxine from the thyroid gland and adrenaline from the adrenal glands. Both hormones raise the metabolic rate of many tissues, increasing heat production.
长期反应包括甲状腺增加甲状腺素分泌和肾上腺增加肾上腺素分泌。这两种激素都能提高许多组织的代谢率，从而增加产热。
5. Blood Glucose Regulation: Insulin | 血糖调节：胰岛素
Blood glucose concentration is normally kept at about 5 mmol dm⁻³. After a carbohydrate-rich meal, glucose is absorbed from the gut into the blood, causing blood glucose concentration to rise. This is detected by β cells in the islets of Langerhans in the pancreas.
血糖浓度通常维持在约5 mmol dm⁻³。摄入富含碳水化合物的食物后，葡萄糖从肠道吸收进入血液，导致血糖浓度升高。这一变化被胰腺胰岛中的β细胞检测到。
β cells respond by secreting insulin into the blood. Insulin binds to specific receptors on the cell membranes of target cells, especially liver and muscle cells, and increases the permeability of these cells to glucose. It also activates enzymes involved in glycogenesis, the conversion of glucose into glycogen for storage.
β细胞通过向血液分泌胰岛素作出反应。胰岛素与靶细胞（尤其是肝细胞和肌细胞）细胞膜上的特异性受体结合，增加这些细胞对葡萄糖的通透性。它还激活参与糖原生成的酶，即将葡萄糖转化为糖原储存。
Insulin also stimulates the uptake of glucose into adipose tissue and increases the synthesis of fat. As a result, blood glucose concentration falls back to the normal range, and insulin secretion decreases.
胰岛素还刺激脂肪组织摄取葡萄糖并增加脂肪合成。结果，血糖浓度回落到正常范围，胰岛素分泌随之减少。
6. Blood Glucose Regulation: Glucagon | 血糖调节：胰高血糖素
When blood glucose concentration falls below the set point, α cells in the islets of Langerhans secrete glucagon. The main target organ for glucagon is the liver, although it also affects adipose tissue under fasting conditions.
当血糖浓度低于调定点时，胰岛中的α细胞分泌胰高血糖素。胰高血糖素的主要靶器官是肝脏，但在禁食条件下也会影响脂肪组织。
Glucagon binds to liver cell receptors and activates glycogen phosphorylase, which catalyses glycogenolysis, the breakdown of stored glycogen into glucose. The glucose is then released into the blood, raising blood glucose concentration.
胰高血糖素与肝细胞受体结合，激活糖原磷酸化酶，该酶催化糖原分解，即将储存的糖原分解为葡萄糖。葡萄糖随后释放入血，使血糖浓度升高。
Glucagon also promotes gluconeogenesis, the synthesis of glucose from non-carbohydrate sources such as amino acids and glycerol. This is important during prolonged fasting, when glycogen stores become depleted.
胰高血糖素还促进糖异生，即从氨基酸和甘油等非碳水化合物来源合成葡萄糖。这在长时间禁食、糖原储备耗尽时尤为重要。
| Feature | 特征 | Insulin | 胰岛素 | Glucagon | 胰高血糖素 |
|---|---|---|
| Secreted by | 分泌细胞 | β cells | β细胞 | α cells | α细胞 |
| Stimulus | 刺激 | High blood glucose | 高血糖 | Low blood glucose | 低血糖 |
| Main effect | 主要作用 | Glycogenesis and glucose uptake | 糖原生成和葡萄糖摄取 | Glycogenolysis and gluconeogenesis | 糖原分解和糖异生 |
7. Control of Blood Water Potential: ADH | 血液水势控制：抗利尿激素
Blood water potential is monitored by osmoreceptors in the hypothalamus. When blood water potential falls, meaning the blood is more concentrated, water moves out of the osmoreceptors by osmosis and the cells shrink, generating nerve impulses.
血液水势由下丘脑中的渗透压感受器监测。当血液水势下降，即血液变得更浓时，水通过渗透作用从渗透压感受器流出，细胞收缩并产生神经冲动。
The hypothalamus sends impulses to the posterior pituitary gland, which releases antidiuretic hormone (ADH) into the blood. ADH travels to the distal convoluted tubules and collecting ducts of the kidney nephrons.
下丘脑向垂体后叶发送冲动，后者将抗利尿激素释放入血。ADH随血液到达肾单位的远曲小管和集合管。
ADH binds to receptors on the collecting duct cells and causes vesicles containing aquaporin-2 proteins to fuse with the cell surface membrane. This makes the collecting duct walls more permeable to water, so more water is reabsorbed into the blood by osmosis. The urine becomes more concentrated and water potential of the blood rises.
ADH与集合管细胞上的受体结合，使含有水通道蛋白-2的囊泡与细胞表面膜融合。这使得集合管壁对水的通透性增加，因此更多的水通过渗透作用被重吸收回血液。尿液变得更浓，血液水势升高。
8. The Role of the Kidney in Homeostasis | 肾脏在稳态中的作用
The kidney is the main excretory and homeostatic organ in mammals. It removes nitrogenous waste such as urea and regulates the water potential, pH and ion concentration of the blood. Each kidney contains about one million nephrons, the functional units of filtration and selective reabsorption.
肾脏是哺乳动物主要的排泄和稳态器官。它排出尿素等含氮废物，并调节血液的水势、pH和离子浓度。每个肾脏约含一百万个肾单位，这些肾单位是过滤和选择性重吸收的功能单位。
In the Bowman's capsule, ultrafiltration of blood occurs under high hydrostatic pressure. Water, glucose, salts, amino acids and urea pass into the glomerular filtrate, while blood cells and large plasma proteins remain in the blood.
在鲍曼囊中，血液在较高的静水压下发生超滤。水、葡萄糖、盐、氨基酸和尿素进入肾小球滤液，而血细胞和大分子血浆蛋白留在血液中。
In the proximal convoluted tubule, useful substances are selectively reabsorbed, including all glucose, most water and many ions. The loop of Henle creates a concentration gradient in the medulla, allowing the collecting duct to reabsorb water under the influence of ADH.
在近曲小管中，有用的物质被选择性重吸收，包括全部葡萄糖、大部分水和许多离子。亨氏袢在髓质中建立浓度梯度，使集合管能在ADH的作用下重吸收水。
9. Cell Signalling in Homeostasis | 稳态中的细胞信号传导
Hormones involved in homeostasis are signalling molecules that travel in the blood and affect only target cells with complementary receptors. Many water-soluble hormones, such as glucagon and adrenaline, cannot cross the plasma membrane, so they bind to receptors on the cell surface.
参与稳态的激素是信号分子，它们随血液运输，只影响具有互补受体的靶细胞。许多水溶性激素，如胰高血糖素和肾上腺素，不能穿过质膜，因此与细胞表面受体结合。
Binding of glucagon to its receptor activates a G protein, which in turn activates the membrane enzyme adenylyl cyclase. This enzyme converts ATP into cyclic AMP (cAMP), which acts as a second messenger inside the cell and activates protein kinase A. This leads to the phosphorylation of target enzymes and the cell response.
胰高血糖素与其受体结合后激活G蛋白，G蛋白又激活膜上的腺苷酸环化酶。该酶将ATP转化为环磷酸腺苷，后者在细胞内充当第二信使，激活蛋白激酶A。这导致靶酶磷酸化并引起细胞反应。
This second messenger model explains how a small amount of hormone at the cell surface can produce a large intracellular response, because each step activates many enzyme molecules.
第二信使模型解释了为何细胞表面少量的激素能产生较大的胞内反应，因为每一步都会激活许多酶分子。
10. Positive Feedback: A Contrast | 正反馈：一种对比
Positive feedback is less common in homeostasis because it amplifies a change rather than reversing it. In positive feedback, the output of a process stimulates further production of that output, moving the variable further away from the set point.
正反馈在稳态中较少见，因为它放大变化而不是逆转变化。在正反馈中，某一过程的输出会刺激该输出进一步产生，使变量更加偏离调定点。
Examples of positive feedback include the release of oxytocin during childbirth. Uterine contractions push the baby's head against the cervix, stretching it and sending nerve impulses to the hypothalamus, which stimulates more oxytocin release. This increases the strength and frequency of contractions until birth occurs.
正反馈的例子包括分娩时催产素的释放。子宫收缩将胎儿头部推向宫颈，使宫颈扩张并向大脑发送神经冲动，刺激更多催产素释放。这增强了收缩的强度和频率，直到分娩完成。
Blood clotting is another example, because thrombin produced in the clotting cascade activates factors that produce even more thrombin, rapidly sealing a wound. Unlike negative feedback, positive feedback must be terminated by an external event, such as birth or full clot formation.
血液凝固是另一个例子，因为凝血级联中产生的凝血酶会激活更多产生凝血酶的因子，从而迅速封闭伤口。与负反馈不同，正反馈必须有外部事件来终止，例如分娩完成或血栓完全形成。
11. Homeostatic Imbalance: Diabetes | 稳态失衡：糖尿病
Diabetes mellitus is a condition in which blood glucose homeostasis is impaired, leading to prolonged hyperglycaemia. In type 1 diabetes, the body's immune system destroys the β cells of the pancreas, so little or no insulin is produced. Type 1 diabetes usually begins in childhood and requires regular insulin injections.
糖尿病是一种血糖稳态受损的疾病，导致持续高血糖。在1型糖尿病中，人体免疫系统破坏胰腺β细胞，因此胰岛素分泌极少或完全不分泌。1型糖尿病通常始于儿童期，需要定期注射胰岛素。
In type 2 diabetes, insulin is produced but target cells become less responsive to its effects, a condition called insulin resistance. Type 2 diabetes is often associated with obesity, poor diet and lack of exercise, and it can sometimes be controlled by diet and lifestyle changes.
在2型糖尿病中，胰岛素能够产生，但靶细胞对其作用变得不敏感，这种状态称为胰岛素抵抗。2型糖尿病通常与肥胖、不良饮食和缺乏运动有关，有时可通过饮食和生活方式改变加以控制。
Untreated diabetes can lead to glucose appearing in the urine when blood glucose exceeds the renal threshold of about 10 mmol dm⁻³. This is because the kidney cannot reabsorb all the filtered glucose, causing osmotic diuresis and excessive urine production.
未治疗的糖尿病会导致血糖超过约10 mmol dm⁻³的肾糖阈时出现糖尿。这是因为肾脏不能重吸收全部滤过的葡萄糖，引起渗透性利尿和尿量过多。
12. Plant Homeostasis: Stomatal Control | 植物稳态：气孔控制
Plants also use homeostatic mechanisms to balance gas exchange and water loss. Stomata in the leaf epidermis open to allow carbon dioxide to enter for photosynthesis, but open stomata also allow water vapour to escape through transpiration.
植物也利用稳态机制来平衡气体交换和水分损失。叶表皮上的气孔打开以允许二氧化碳进入进行光合作用，但开放的气孔也会让水蒸气通过蒸腾作用逸出。
Guard cells control stomatal aperture by changing their turgor. When guard cells accumulate potassium ions by active transport, their water potential falls, water enters by osmosis and the cells become turgid, causing the stomatal pore to open. When potassium ions leave the guard cells, water follows by osmosis, the cells become flaccid and the pore closes.
保卫细胞通过改变膨压控制气孔开度。当保卫细胞通过主动转运积累钾离子时，其水势下降，水通过渗透作用进入，细胞变得膨大，气孔打开。当钾离子离开保卫细胞时，水分随之通过渗透作用流出，细胞变得松弛，气孔关闭。
Under water stress, the plant hormone abscisic acid (ABA) is produced. ABA stimulates guard cells to lose potassium ions and close the stomata, reducing transpiration and helping the plant conserve water.
在水分胁迫下，植物激素脱落酸产生。ABA刺激保卫细胞丢失钾离子并关闭气孔，从而减少蒸腾作用，帮助植物保存水分。
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