📚 Internal Environment: Homeostasis in Mammals | 内环境：哺乳动物的稳态
The internal environment of a mammal is the fluid environment that surrounds every cell. It consists mainly of tissue fluid, blood plasma and lymph. Cells obtain oxygen and nutrients from this environment and release carbon dioxide and other wastes into it, so its composition must be kept stable.
哺乳动物的内环境是指围绕每个细胞的液体环境，主要由组织液、血浆和淋巴组成。细胞从这一环境中获取氧气和营养物质，并向其中释放二氧化碳和其他代谢废物，因此内环境的成分必须保持稳定。
In A-Level Biology, the study of the internal environment focuses on homeostasis: how mammals regulate temperature, blood glucose, water potential and the removal of metabolic wastes. Negative feedback is the fundamental control mechanism.
在 A-Level 生物中，内环境的学习重点是稳态：哺乳动物如何调节体温、血糖、水势以及代谢废物的清除。负反馈是基本的调控机制。
1. The Internal Environment and Tissue Fluid | 内环境与组织液
Tissue fluid is the liquid that directly bathes the cells. It is formed when blood enters the capillaries under pressure from the heart, forcing water and small solutes such as glucose, amino acids and oxygen out through the capillary walls. Plasma proteins are too large to leave the capillaries in significant amounts.
组织液是直接浸泡细胞的液体。当血液在心脏压力下进入毛细血管时，水和小分子溶质如葡萄糖、氨基酸和氧气被压出毛细血管壁，形成组织液。血浆蛋白因分子太大，通常不能大量离开毛细血管。
The composition of tissue fluid is constantly changing because cells take up oxygen and nutrients and release carbon dioxide, urea and other wastes. Excess tissue fluid drains into the lymphatic system and is returned to the blood, preventing the accumulation of fluid in tissues.
组织液的成分不断变化，因为细胞吸收氧气和营养物质，同时释放二氧化碳、尿素和其他废物。多余的组织液排入淋巴系统并返回血液，防止液体在组织中积聚。
2. Why Homeostasis Is Essential | 为何稳态至关重要
Homeostasis is the maintenance of a constant internal environment despite changes in external or internal conditions. In mammals, the core conditions that must be controlled include blood glucose concentration, core body temperature, water potential of blood, pH and carbon dioxide concentration.
稳态是指尽管外部或内部条件发生变化，仍能维持恒定的内环境。在哺乳动物中，必须控制的核心条件包括血糖浓度、核心体温、血液水势、pH 和二氧化碳浓度。
These conditions are kept within narrow limits because enzyme activity and cell membrane function depend on them. For example, enzymes have an optimum temperature and pH; if body temperature rises too far above 37 °C, enzymes may denature and metabolic reactions fail.
这些条件必须维持在狭窄范围内，因为酶活性和细胞膜功能都依赖于它们。例如，酶有最适温度和最适 pH；如果体温远高于 37 °C，酶可能变性，代谢反应会受到破坏。
3. Negative Feedback: The Core Mechanism | 负反馈：核心调节机制
Most homeostatic control systems operate by negative feedback. A change from the normal set point is detected by receptors, the information is processed by a control centre, and effectors carry out corrective responses that reverse the change.
大多数稳态控制系统通过负反馈运作。偏离正常调定点的变化被感受器检测到，信息由控制中心处理，效应器执行纠正反应，使变化逆转。
For example, when the concentration of carbon dioxide in the blood rises, chemoreceptors detect the change and the breathing centre increases ventilation rate. This removes excess carbon dioxide, and the stimulus is reduced until breathing returns to normal.
例如，当血液中二氧化碳浓度升高时，化学感受器检测到变化，呼吸中枢提高通气速率。这清除了多余的二氧化碳，刺激减弱，呼吸恢复到正常水平。
Negative feedback is essential because it prevents overcorrection. Unlike positive feedback, which amplifies a change, negative feedback brings the internal environment back towards its set point.
负反馈至关重要，因为它能防止矫正过度。与放大变化的正反馈不同，负反馈使内环境回到调定点附近。
4. Blood Glucose Regulation: Insulin and Glucagon | 血糖调节：胰岛素与胰高血糖素
Blood glucose concentration is normally kept between about 4 and 6 mmol dm⁻³. The pancreas detects changes in blood glucose and releases hormones from the islets of Langerhans to restore normal levels.
血糖浓度通常维持在约 4 至 6 mmol dm⁻³ 之间。胰腺检测血糖变化，并从胰岛释放激素，使血糖恢复到正常水平。
After a carbohydrate-rich meal, blood glucose rises. Beta cells in the islets secrete insulin, which travels in the blood to target cells in the liver, muscles and adipose tissue. Insulin increases the permeability of these cells to glucose and activates enzymes for glycogenesis, converting glucose into glycogen for storage.
进食高碳水化合物食物后，血糖升高。胰岛中的 β 细胞分泌胰岛素，胰岛素随血液到达肝细胞、肌细胞和脂肪细胞。胰岛素增加这些细胞对葡萄糖的通透性，并激活糖原生成酶，将葡萄糖转化为糖原储存。
Between meals, blood glucose falls below the set point. Alpha cells secrete glucagon, which stimulates the liver to break down glycogen into glucose by glycogenolysis. The liver can also synthesise new glucose from amino acids and glycerol by gluconeogenesis, and release it into the blood.
在两餐之间，血糖降至调定点以下。α 细胞分泌胰高血糖素，刺激肝脏通过糖原分解将糖原分解为葡萄糖。肝脏还可以通过糖异生作用从氨基酸和甘油合成新的葡萄糖，并将其释放入血。
5. Diabetes Mellitus: When Regulation Fails | 糖尿病：调节失灵时
Diabetes mellitus is a condition in which blood glucose cannot be controlled effectively. In type 1 diabetes, the body’s own immune system destroys the beta cells in the pancreas, so little or no insulin is produced.
糖尿病是一种血糖无法被有效控制的疾病。在 1 型糖尿病中，人体自身免疫系统破坏胰腺中的 β 细胞，因此几乎不产生胰岛素。
Type 2 diabetes is more common and is associated with insulin resistance, where target cells do not respond properly to insulin, or with insufficient insulin secretion. Risk factors include obesity, poor diet and lack of exercise.
2 型糖尿病更为常见，与胰岛素抵抗有关，即靶细胞对胰岛素反应不正常，或胰岛素分泌不足。风险因素包括肥胖、不良饮食和缺乏运动。
In both types, blood glucose remains abnormally high after meals. Glucose is then excreted in urine, drawing water with it by osmosis, which causes excessive urination and thirst. Type 1 diabetes is treated with insulin injections, while type 2 is managed by diet, exercise and medication.
在这两种类型中，餐后血糖持续异常升高。葡萄糖随后随尿液排出，并通过渗透作用带走水分，导致多尿和口渴。1 型糖尿病通过注射胰岛素治疗，2 型则通过饮食、运动和药物控制。
6. Thermoregulation: Heat Gain and Heat Loss | 体温调节：产热与散热
Mammals are endotherms, meaning they generate heat internally from metabolic reactions, especially respiration in the liver and muscles. Core body temperature is maintained at about 37 °C by balancing heat gain and heat loss.
哺乳动物是内温动物，它们通过体内的代谢反应，尤其是肝脏和肌肉中的呼吸作用产生热量。通过平衡产热和散热，核心体温维持在约 37 °C。
The hypothalamus is the body’s thermostat. It monitors the temperature of the blood flowing through it and receives nerve impulses from thermoreceptors in the skin, which detect the external temperature. It then coordinates nervous and hormonal responses to restore normal temperature.
下丘脑是人体的恒温器。它监测流经它的血液温度，并接收来自皮肤温度感受器的神经冲动，这些感受器检测外部温度。随后它协调神经和激素反应以恢复正常体温。
7. Responses to Cold: Conserving and Generating Heat | 寒冷时的反应：保存和产生热量
When core temperature falls below the set point, the hypothalamus activates heat-conservation and heat-production mechanisms. Arterioles in the skin constrict, a response called vasoconstriction. This reduces blood flow to the skin surface, so less heat is lost by radiation and conduction.
当核心体温降到调定点以下时，下丘脑启动保温和产热机制。皮肤小动脉收缩，这一反应称为血管收缩。它减少流向皮肤表面的血流量，从而减少通过辐射和对流损失的热量。
Shivering is triggered by the hypothalamus through involuntary skeletal muscle contractions. The increased muscle activity raises the rate of respiration, generating heat. Hair erector muscles also contract, raising hairs to trap a layer of insulating air, although this is much less effective in humans than in fur-covered mammals.
下丘脑通过不自主的骨骼肌收缩引发颤抖。肌肉活动增加提高了呼吸速率，产生热量。立毛肌也收缩，使毛发竖起以捕捉一层绝缘空气，不过这在人类中的效果远不如在毛皮覆盖的哺乳动物中明显。
Metabolic rate also increases in the long term because the hormones adrenaline and thyroxine are released. These hormones stimulate mitochondrial activity and raise heat production in tissues.
从长期来看，代谢率也会因肾上腺素和甲状腺素的释放而增加。这些激素刺激线粒体活动，提高组织中的产热量。
8. Responses to Heat: Increasing Heat Loss | 炎热时的反应：增加散热
When core temperature rises above the set point, the hypothalamus triggers vasodilation of skin arterioles. More warm blood flows close to the skin surface, and heat is lost by radiation, conduction and convection.
当核心体温升高到调定点以上时，下丘脑引起皮肤小动脉血管舒张。更多温热血液流至皮肤表面附近，热量通过辐射、传导和对流散失。
Sweat glands secrete sweat onto the skin surface. As sweat evaporates, it absorbs latent heat from the skin, cooling the body. Hair erector muscles relax so hairs lie flat, reducing insulation.
汗腺将汗液分泌到皮肤表面。汗液蒸发时从皮肤吸收潜热，使身体降温。立毛肌舒张，使毛发平贴，减少隔热作用。
In hot conditions, metabolic rate may decrease to reduce internal heat production. Behavioural responses, such as seeking shade, moving less and drinking more water, also help maintain core temperature.
在炎热环境中，代谢率可能降低，以减少体内产热。行为反应，如寻找阴凉处、减少活动和多喝水，也有助于维持核心体温。
9. Osmoregulation and ADH | 渗透调节与抗利尿激素
Osmoregulation is the control of water potential in the blood and tissue fluid. When the body is dehydrated, the blood becomes more concentrated, so its water potential falls. Osmoreceptors in the hypothalamus detect this change and send impulses to the posterior pituitary gland.
渗透调节是对血液和组织液水势的控制。当身体脱水时，血液变得更浓缩，水势下降。下丘脑中的渗透压感受器检测到这一变化，并向垂体后叶发送神经冲动。
The posterior pituitary releases antidiuretic hormone (ADH) into the blood. ADH travels to the collecting ducts of the kidney nephrons, where it increases the permeability of the collecting duct walls to water by causing the insertion of aquaporin proteins into the cell membranes.
垂体后叶将抗利尿激素（ADH）释放入血。ADH 到达肾单位集合管，通过使水通道蛋白插入细胞膜，增加集合管壁对水的通透性。
As a result, more water is reabsorbed from the filtrate into the surrounding medulla and then into the blood. The urine becomes more concentrated and its volume decreases. When blood is dilute, ADH secretion is inhibited, the collecting ducts become less permeable, and excess water is lost in dilute urine.
结果是更多的水从滤液中被重吸收到周围髓质，再进入血液。尿液变得更浓缩，尿量减少。当血液稀释时，ADH 分泌受到抑制，集合管通透性降低，多余水分随稀释尿液排出。
10. The Liver and Homeostasis | 肝脏与稳态
The liver plays a central role in maintaining the internal environment. It stores glucose as glycogen, regulates blood glucose under the influence of insulin and glucagon, and synthesises plasma proteins that help maintain blood water potential and transport functions.
肝脏在维持内环境方面发挥着核心作用。它以糖原形式储存葡萄糖，在胰岛素和胰高血糖素的作用下调节血糖，并合成血浆蛋白，有助于维持血液水势和运输功能。
Excess amino acids cannot be stored, so the liver deaminates them. Deamination removes the amino group, which is converted first to ammonia and then to urea. Urea is released into the blood and excreted by the kidneys, preventing the toxic accumulation of ammonia.
多余的氨基酸不能被储存，因此肝脏将其脱氨基。脱氨基作用去除氨基，氨基先转化为氨，再转化为尿素。尿素释放入血并由肾脏排出，防止有毒氨的积累。
The liver also detoxifies harmful substances such as alcohol and drugs, breaks down old red blood cells, and converts haemoglobin breakdown products into bilirubin, which is excreted in bile.
肝脏还解毒酒精和药物等有害物质，分解衰老红细胞，并将血红蛋白分解产物转化为胆红素，随胆汁排出。
11. Excretion and the Internal Environment | 排泄与内环境
Excretion is the removal of metabolic waste products from the body. The main excretory products in mammals are carbon dioxide from respiration, urea from the breakdown of amino acids, and excess water and salts.
排泄是将代谢废物排出体外的过程。哺乳动物的主要排泄物包括呼吸作用产生的二氧化碳、氨基酸分解产生的尿素，以及多余的水和盐。
These wastes must be removed because they can alter the pH, water potential or solute concentration of the internal environment. Carbon dioxide forms carbonic acid in blood plasma, so its removal by the lungs is vital for pH balance.
这些废物必须被清除，因为它们会改变内环境的 pH、水势或溶质浓度。二氧化碳在血浆中形成碳酸，因此肺排出二氧化碳对 pH 平衡至关重要。
The kidneys excrete urea and regulate water and salt balance by adjusting urine concentration. The skin excretes small amounts of urea, salts and water in sweat. Together these organs ensure that the internal environment remains stable despite metabolic activity.
肾脏排出尿素，并通过调节尿液浓度来调控水和盐的平衡。皮肤通过汗液排出少量尿素、盐和水。这些器官共同作用，确保内环境在代谢活动中保持稳定。
12. Exam Focus: Linking Systems and Terminology | 考试重点：系统联系与术语
In Cambridge A-Level exams, you should be able to explain homeostasis using named examples such as blood glucose regulation, thermoregulation and osmoregulation. Always identify the receptor, control centre and effector in a negative feedback loop.
在剑桥 A-Level 考试中，你应能使用血糖调节、体温调节和渗透调节等具体例子解释稳态。在负反馈回路中，要始终指出感受器、控制中心和效应器。
Use precise scientific language. For example, insulin does not break down glucose; it lowers blood glucose by increasing cell uptake, stimulating glycogenesis and promoting conversion to fat. Thermoregulation is coordinated by the hypothalamus, not by the skin alone.
使用精确的科学语言。例如，胰岛素并不分解葡萄糖；它通过增加细胞摄取、促进糖原生成和转化为脂肪来降低血糖。体温调节由下丘脑协调，而不是仅由皮肤控制。
When interpreting data, describe trends in blood glucose or urine output after meals and during dehydration, and link these changes to insulin, glucagon or ADH. This will help you answer structured and data-response questions confidently.
在解读数据时，要描述餐后和脱水期间血糖或尿量的变化趋势，并将这些变化与胰岛素、胰高血糖素或 ADH 联系起来。这将帮助你自信地回答结构化试题和数据分析题。
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