📚 Internal Environment | 内环境
The internal environment of an organism refers to the conditions inside the body, including temperature, pH, water potential, and concentrations of key solutes such as glucose and ions. Maintaining a stable internal environment within narrow limits is essential for proper enzyme function and cellular activities. This concept is known as homeostasis, and it involves continuous monitoring and adjustment through feedback systems.
生物体的内环境指的是体内的条件，包括温度、pH、水势以及葡萄糖和离子等关键溶质的浓度。将内环境稳定在狭窄的范围内对于酶的正常功能和细胞活动至关重要。这一概念被称为稳态，它涉及通过反馈系统进行持续监控和调节。
1. The Concept of Homeostasis | 稳态的概念
Homeostasis is the maintenance of a constant internal environment despite changes in the external environment. Cells in multicellular organisms are bathed in tissue fluid, which provides a relatively stable medium for exchange of substances. Key physiological variables such as core temperature, blood glucose concentration, and blood pH are kept at or near set points by homeostatic mechanisms. These mechanisms rely on communication via the nervous and endocrine systems.
稳态是指尽管外部环境发生变化，仍能维持恒定的内环境。多细胞生物的细胞浸浴在组织液中，组织液为物质交换提供了一个相对稳定的介质。关键生理变量如核心体温、血糖浓度和血液pH值通过稳态机制保持或接近设定点。这些机制依赖于神经系统和内分泌系统的通讯。
2. Negative Feedback as the Core Mechanism | 负反馈作为核心机制
Negative feedback is the primary process that restores a physiological variable to its normal range. A change in a variable triggers a response that counteracts the change. For example, a rise in blood glucose stimulates the release of insulin, which lowers blood glucose back toward the set point. A receptor detects the deviation, a control centre compares this to the set point, and an effector brings about the corrective response. Negative feedback loops work to minimise fluctuations and ensure stability.
负反馈是将生理变量恢复到正常范围的主要过程。某个变量的变化会触发一个抵消该变化的反应。例如，血糖升高会刺激胰岛素释放，使血糖回落到设定点。感受器检测到偏差，控制中心将其与设定点比较，效应器产生校正反应。负反馈回路的作用是尽量减少波动并确保稳定。
3. Overview of Thermoregulation | 体温调节概述
Humans are endotherms, meaning they generate metabolic heat to maintain a core body temperature around 37 °C. Thermoregulation involves balancing heat production and heat loss. The thermoregulatory centre in the hypothalamus monitors blood temperature and receives signals from peripheral thermoreceptors. If core temperature deviates, the hypothalamus coordinates physiological and behavioural responses to adjust heat transfer. Maintaining a constant temperature ensures optimal kinetic energy for enzyme-catalysed reactions.
人类是恒温动物，这意味着他们通过代谢产热将核心体温维持在约37 °C。体温调节涉及平衡产热和散热。下丘脑中的体温调节中枢监测血液温度并接收来自外周温度感受器的信号。如果核心温度偏离，下丘脑会协调生理和行为反应来调整热传递。维持恒定的温度可确保酶促反应的最佳动能。
4. Mechanisms of Heat Loss and Conservation | 散热与保温的机制
When body temperature rises above the set point, arterioles in the skin dilate (vasodilation). More blood flows through capillaries near the skin surface, and heat is lost by radiation. Sweat glands secrete sweat, and evaporation of this water from the skin surface takes away latent heat. Hairs flatten as erector muscles relax, reducing insulation. In cold conditions, arterioles constrict (vasoconstriction) to reduce blood flow to the skin surface, conserving heat. Erector muscles contract, raising hairs to trap a layer of insulating air. Shivering involves rapid involuntary muscle contractions that generate metabolic heat.
当体温升高超过设定点时，皮肤小动脉扩张（血管舒张）。更多血液流经靠近皮肤表面的毛细血管，热量通过辐射散失。汗腺分泌汗液，汗液从皮肤表面蒸发带走潜热。立毛肌松弛使毛发平躺，减少保温作用。在寒冷条件下，小动脉收缩（血管收缩）以减少流向皮肤表面的血流量，保存热量。立毛肌收缩，竖起毛发，捕获一层隔热空气。战栗涉及快速的不自主肌肉收缩，产生代谢热能。
5. The Role of the Hypothalamus in Temperature Control | 下丘脑在体温控制中的作用
The hypothalamus acts as the body’s thermostat. It contains an anterior region that responds to heat and triggers cooling mechanisms, and a posterior region that responds to cold and initiates heat conservation and production. Thermoreceptors in the skin and central thermoreceptors in the hypothalamus itself provide input. The hypothalamus sends nerve impulses to effectors such as blood vessels, sweat glands, and skeletal muscles. It also stimulates the release of hormones like thyroxine and adrenaline, which increase metabolic rate and thus heat production over a longer timescale.
下丘脑充当身体的恒温器。它包含对热作出反应并触发降温机制的前部区域，以及对冷作出反应并启动保温与产热的后部区域。皮肤中的温度感受器和下丘脑本身的中枢温度感受器提供输入。下丘脑发出神经冲动给效应器如血管、汗腺和骨骼肌。它还刺激甲状腺素和肾上腺素等激素的释放，这些激素会提高代谢率，从而在更长的时间内增加产热。
6. Blood Glucose Regulation: Insulin and Glucagon | 血糖调节：胰岛素与胰高血糖素
The normal blood glucose concentration is tightly regulated around 4–6 mmol dm⁻³. The pancreas contains clusters of endocrine cells called the islets of Langerhans. Beta (β) cells secrete insulin in response to high blood glucose, while alpha (α) cells secrete glucagon when blood glucose is low. Insulin promotes glucose uptake by cells, especially in muscle and adipose tissue, and stimulates the conversion of glucose to glycogen in the liver. Glucagon stimulates glycogen breakdown (glycogenolysis) and the formation of glucose from non-carbohydrate sources (gluconeogenesis) in the liver.
正常血糖浓度严格维持在4–6 mmol dm⁻³左右。胰腺含有称为胰岛的内分泌细胞团。β细胞在血糖高时分泌胰岛素，α细胞在血糖低时分泌胰高血糖素。胰岛素促进细胞摄取葡萄糖，特别是在肌肉和脂肪组织中，并刺激肝脏将葡萄糖转化为糖原。胰高血糖素刺激肝糖原分解（糖原分解）和从非碳水化合物来源生成葡萄糖（糖异生）。
7. Negative Feedback Loop in Glucose Homeostasis | 葡萄糖稳态中的负反馈回路
After a meal, blood glucose rises. This is detected by β cells in the pancreas, which release insulin. Insulin lowers blood glucose by increasing cellular uptake and glycogen synthesis. As blood glucose falls back to normal, insulin secretion decreases. Between meals, blood glucose drops, stimulating α cells to secrete glucagon. Glucagon raises blood glucose by promoting glycogenolysis and gluconeogenesis. The rise in blood glucose then inhibits further glucagon secretion. This antagonistic hormone pair maintains glucose levels with minimal fluctuation.
餐后血糖升高。这被胰腺中的β细胞检测到，它们释放胰岛素。胰岛素通过增加细胞摄取和糖原合成来降低血糖。当血糖回落到正常水平时，胰岛素分泌减少。两餐之间，血糖下降，刺激α细胞分泌胰高血糖素。胰高血糖素通过促进糖原分解和糖异生来提高血糖。血糖升高随后抑制胰高血糖素的进一步分泌。这对拮抗激素以最小波动维持葡萄糖水平。
8. Osmoregulation: Water Balance | 渗透调节：水平衡
Osmoregulation is the control of water content and solute concentrations in body fluids. The water potential of blood plasma must be kept constant to prevent osmotic gain or loss of water from cells. The hypothalamus contains osmoreceptors that detect changes in blood water potential. A fall in water potential (dehydration) triggers a sensation of thirst and the release of antidiuretic hormone (ADH) from the posterior pituitary gland. ADH increases the permeability of collecting duct cells in the kidney to water, allowing more water to be reabsorbed into the blood.
渗透调节是对体液中水分含量和溶质浓度的控制。血浆水势必须保持恒定，以防止细胞因渗透作用吸水或失水。下丘脑含有渗透感受器，能检测血液水势的变化。水势下降（脱水）会触发渴觉，并促使垂体后叶释放抗利尿激素（ADH）。ADH增加肾脏集合管细胞对水的通透性，使更多水被重吸收回血液。
9. ADH Action and the Collecting Duct | 抗利尿激素的作用与集合管
ADH binds to receptors on the basolateral membrane of collecting duct cells, triggering a signalling cascade involving cyclic AMP. This causes vesicles containing aquaporin-2 water channels to fuse with the apical membrane facing the filtrate. Water moves out of the collecting duct by osmosis down a concentration gradient created by the hypertonic medullary interstitium. When blood water potential rises, ADH secretion is reduced, aquaporins are removed from the apical membrane via endocytosis, and less water is reabsorbed, producing dilute urine.
ADH与集合管细胞基底外侧膜上的受体结合，触发涉及环磷酸腺苷（cAMP）的信号级联反应。这导致含有水通道蛋白-2（aquaporin-2）的囊泡与面向滤液的上皮细胞顶膜融合。水通过渗透作用沿浓度梯度从集合管中移出，该梯度由高渗的髓质间质建立。当血液水势升高时，ADH分泌减少，水通道蛋白通过胞吞作用从顶膜移除，重吸收的水减少，产生稀释尿液。
10. The Kidney’s Role in the Internal Environment | 肾脏在内环境中的角色
Beyond water balance, the kidneys regulate the levels of ions such as Na⁺, K⁺, and Ca²⁺, as well as blood pH by excreting H⁺ or reabsorbing HCO₃⁻. The nephron is the functional unit where ultrafiltration at the glomerulus produces a filtrate that is then selectively reabsorbed and secreted along the tubule. Aldosterone from the adrenal cortex promotes Na⁺ reabsorption and K⁺ secretion in the distal convoluted tubule and collecting duct. This hormonal control adjusts salt balance and therefore blood volume and pressure, maintaining the composition of the internal environment.
除了水平衡，肾脏还调节Na⁺、K⁺和Ca²⁺等离子浓度，并通过排泄H⁺或重吸收HCO₃⁻来调节血液pH值。肾单位是功能单位，肾小球处的超滤产生滤液，然后沿着肾小管进行选择性重吸收和分泌。肾上腺皮质分泌的醛固酮促进远曲小管和集合管对Na⁺的重吸收以及K⁺的分泌。这种激素控制调节了盐平衡，从而调节血容量和血压，维持内环境的组成。
11. Disruption of the Internal Environment | 内环境的失调
Failure of homeostatic mechanisms leads to disorders. Diabetes mellitus type 1 results from autoimmune destruction of β cells, causing insulin deficiency and high blood glucose (hyperglycaemia). Type 2 diabetes involves insulin resistance. Both can lead to damage of blood vessels and nerves. Hyperthermia, such as heat stroke, occurs when thermoregulation fails and core temperature rises dangerously, denaturing proteins. Dehydration from inadequate water intake or excessive loss impairs osmoregulation, reducing blood volume and risking kidney damage. Understanding these disruptions highlights the importance of a stable internal environment.
稳态机制的失效会导致疾病。1型糖尿病是由于β细胞被自身免疫破坏，导致胰岛素缺乏和高血糖。2型糖尿病涉及胰岛素抵抗。两者都可能导致血管和神经损伤。体温过高，如中暑，发生在体温调节失败且核心温度危险升高时，会使蛋白质变性。因水分摄入不足或过度流失导致的脱水会损害渗透调节，减少血容量并有损肾脏。理解这些紊乱突显了稳定内环境的重要性。
12. Summary: Integration of Homeostatic Systems | 总结：稳态系统的整合
The internal environment is actively maintained through a network of sensors, control centres, and effectors. Negative feedback is the unifying principle that keeps temperature, glucose, water, and ion concentrations within narrow limits. The nervous system provides rapid responses, while the endocrine system provides slower, sustained adjustments via hormones. Coordination between the hypothalamus, pancreas, kidneys, and other organs ensures that cells can function efficiently despite external changes. Appreciation of these control systems is foundational to understanding physiology and disease.
内环境通过传感器、控制中心和效应器构成的网络得以积极维持。负反馈是将温度、葡萄糖、水分和离子浓度保持在狭窄范围内的统一原则。神经系统提供快速反应，而内分泌系统通过激素提供较慢但持久的调整。下丘脑、胰腺、肾脏和其他器官之间的协调确保细胞即使在外部环境变化时也能高效运作。理解这些控制系统是掌握生理学和疾病的基础。
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