Control of Water Content | 水分含量的调节

📚 Control of Water Content | 水分含量的调节

Water is the most abundant molecule in the human body, making up about 60% of body weight. It acts as a solvent, transport medium, temperature regulator and reactant in metabolic processes. Since water continuously enters and leaves the body, its content must be tightly controlled to keep the osmotic pressure of body fluids within the narrow range required for cellular function. Failure to do so would lead to cell shrinkage or lysis, enzyme dysfunction and disruption of membrane potentials. This article explores the homeostatic mechanisms that regulate water content in mammals, focusing on the role of the kidneys, osmoreceptors and antidiuretic hormone (ADH).

水是人体内含量最丰富的分子，约占体重的60%。它担任溶剂、运输介质、温度调节剂和代谢反应物的角色。由于水不断进入和排出体外，其含量必须被严格调控，以维持体液渗透压在细胞功能所要求的窄小范围内。若调节失败，将导致细胞皱缩或胀裂、酶功能紊乱以及膜电位破坏。本文探讨哺乳动物体内调节水分含量的稳态机制，重点介绍肾脏、渗透压感受器和抗利尿激素(ADH)的作用。

1. The Importance of Water Balance | 水平衡的重要性

Water balance is the state in which water intake equals water output. Intake comes from drinking, food and metabolic water produced during respiration. Output occurs via urine, faeces, sweat, evaporation from the skin and water vapour in exhaled air. Even a small mismatch can alter plasma osmolality, normally about 285–295 mOsm L⁻¹. A rise in osmolality causes water to move out of cells, impairing metabolism; a fall may cause cells to swell, particularly dangerous in the brain. Therefore, precise osmoregulation is essential for survival.

水平衡是指水摄入量与排出量相等的状态。摄入来自饮水、食物和呼吸作用产生的代谢水。排出则通过尿液、粪便、汗液、皮肤蒸发以及呼出气中的水蒸气。即使微小的不匹配也会改变血浆渗透摩尔浓度，正常值约为285–295 mOsm L⁻¹。渗透压升高导致水从细胞移出，损害代谢；下降则可能导致细胞肿胀，在大脑中尤其危险。因此，精确的渗透调节对生存至关重要。

---

2. Osmoregulation: An Overview | 渗透调节概述

Osmoregulation is the active regulation of the osmotic pressure of an organism’s body fluids. In mammals, the kidneys are the primary effectors, adjusting the volume and concentration of urine. This process is under hormonal control, chiefly by antidiuretic hormone (ADH), which alters the permeability of the collecting ducts to water. Osmoreceptors in the hypothalamus monitor the osmotic pressure of the blood and provide input to the posterior pituitary, forming a negative feedback loop that keeps water content stable.

渗透调节是对生物体体液渗透压的主动调控。在哺乳动物中，肾脏是主要的效应器，调整尿液的量和浓度。这一过程受激素调控，主要是抗利尿激素(ADH)，它改变集合管对水的通透性。下丘脑中的渗透压感受器监测血液渗透压，并向垂体后叶提供输入，形成负反馈回路，使水分含量保持稳定。

---

3. The Role of the Kidneys | 肾脏的作用

The kidneys filter around 180 litres of blood plasma per day, reclaiming most of the water and solutes and excreting waste in a small volume of urine. Each kidney contains about one million nephrons, which perform ultrafiltration, selective reabsorption and secretion. By regulating the amount of water reabsorbed, the kidneys can produce urine ranging from very dilute (50 mOsm L⁻¹) to highly concentrated (1200 mOsm L⁻¹), thereby conserving or eliminating water as required.

肾脏每天过滤约180升血浆，重吸收绝大部分水和溶质，并以少量尿液排出废物。每个肾脏含有约一百万个肾单位，它们完成超滤、选择性重吸收和分泌作用。通过调节重吸收的水量，肾脏可以产生从非常稀释(50 mOsm L⁻¹)到高度浓缩(1200 mOsm L⁻¹)的尿液，从而根据需要保存或排出水分。

---

4. The Nephron: Functional Unit | 肾单位：功能单位

A nephron consists of the renal corpuscle (Bowman’s capsule and glomerulus) and a long renal tubule. The tubule is subdivided into the proximal convoluted tubule (PCT), the loop of Henle (descending and ascending limbs), the distal convoluted tubule (DCT) and the collecting duct. The collecting duct, which receives tubular fluid from several nephrons, is the key site for controlled water reabsorption under the influence of ADH.

肾单位由肾小体（鲍曼氏囊和肾小球）和一条长的肾小管组成。肾小管可细分为近曲小管(PCT)、亨利氏袢（降支和升支）、远曲小管(DCT)和集合管。集合管接收来自多个肾单位的管液，是受ADH影响进行受控水分重吸收的关键部位。

Two types of nephrons exist: cortical nephrons with short loops of Henle, and juxtamedullary nephrons with long loops extending deep into the medulla. The long loops are responsible for generating the medullary osmotic gradient that drives water reabsorption later in the collecting duct.

存在两类肾单位：皮质肾单位具有较短的亨利氏袢，而近髓肾单位的袢较长并深入髓质。长袢负责建立髓质渗透梯度，推动集合管后续的水分重吸收。

---

5. Ultrafiltration and Selective Reabsorption | 超滤与选择性重吸收

Ultrafiltration occurs in the renal corpuscle, where high hydrostatic pressure forces water, ions, glucose, urea and other small molecules out of the glomerular capillaries into Bowman’s capsule. The filtrate is essentially protein-free plasma. The glomerular filtration rate (GFR) is kept relatively constant by autoregulation, ensuring a steady load of water and solutes for processing.

超滤发生在肾小体，此处较高的流体静压将水、离子、葡萄糖、尿素及其他小分子从肾小球毛细血管压入鲍曼氏囊。滤液基本上是去除了蛋白质的血浆。肾小球滤过率(GFR)通过自身调节保持相对恒定，确保有稳定的水和溶质负荷供后续处理。

About 65–80% of the filtered water is reabsorbed in the PCT by osmosis, following the active reabsorption of Na⁺, glucose and other solutes. This reabsorption is obligatory and not subject to hormonal control. Water moves through aquaporin-1 (AQP1) channels that are always present in the PCT cell membranes, making this segment freely permeable to water.

大约65-80%的滤出水在近曲小管通过渗透作用被重吸收，跟随Na⁺、葡萄糖和其他溶质的主动重吸收。这种重吸收是必须的且不受激素调控。水通过近曲小管细胞膜上始终存在的水通道蛋白-1(AQP1)移动，使这一段对水自由通透。

---

6. The Loop of Henle and Countercurrent Multiplication | 亨利氏袢与逆流倍增

The loop of Henle creates a hypertonic environment in the medullary interstitium through countercurrent multiplication. The descending limb is permeable to water but not to solutes; water leaves the tubule by osmosis as it encounters the increasingly hypertonic medulla. The thin ascending limb is impermeable to water but allows Na⁺ and Cl⁻ to diffuse out passively. The thick ascending limb actively transports Na⁺ and Cl⁻ out of the tubule via the Na⁺-K⁺-2Cl⁻ cotransporter, maintaining a high interstitial solute concentration.

亨利氏袢通过逆流倍增在髓质间质中形成高渗环境。降支对水通透而对溶质不通透；当管液接触到越来越高的髓质渗透压时，水通过渗透作用离开肾小管。薄升支对水不通透，但允许Na⁺和Cl⁻被动扩散出去。厚升支通过Na⁺-K⁺-2Cl⁻协同转运蛋白主动将Na⁺和Cl⁻转运出去，维持了间质的高溶质浓度。

This countercurrent arrangement multiplies the single osmotic effect, establishing a steep osmotic gradient from the cortex (≈300 mOsm L⁻¹) to the deep medulla (≈1200 mOsm L⁻¹). Without this gradient, water could not be reabsorbed efficiently in the collecting duct, even when ADH is present.

这种逆流排列将单个渗透效应倍增，建立了从皮质（约300 mOsm L⁻¹）到深层髓质（约1200 mOsm L⁻¹）的陡峭渗透梯度。若没有这一梯度，即使存在ADH，水也无法在集合管被有效重吸收。

---

7. Osmoreceptors and the Hypothalamus | 渗透压感受器与下丘脑

Osmoreceptors are specialised neurones in the hypothalamus, primarily in the organum vasculosum of the lamina terminalis (OVLT) and the supraoptic nucleus. When plasma osmolality rises, water moves out of these neurones by osmosis, causing them to shrink. This mechanical change increases their firing rate, sending signals to the supraoptic and paraventricular nuclei to stimulate ADH synthesis and release from the posterior pituitary.

渗透压感受器是位于下丘脑的特化神经元，主要分布在终板血管器(OVLT)和视上核。当血浆渗透压升高时，水通过渗透离开这些神经元，导致细胞皱缩。这种机械性改变增加其放电频率，向视上核和室旁核发送信号，刺激ADH的合成并从垂体后叶释放。

Conversely, when plasma osmolality falls (e.g. after drinking a large volume of water), osmoreceptor cells swell, firing rate decreases, and ADH release is inhibited. The threshold for ADH release is typically around 280–285 mOsm L⁻¹, making the system highly sensitive to small changes in hydration status.

相反，当血浆渗透压下降时（例如大量饮水后），渗透压感受器细胞膨胀，放电频率降低，ADH的释放受到抑制。ADH释放的阈值通常在约280–285 mOsm L⁻¹，这使得系统对水合状态的微小变化高度敏感。

---

8. Antidiuretic Hormone (ADH) | 抗利尿激素(ADH)

ADH, also known as vasopressin, is a peptide hormone synthesised in the cell bodies of hypothalamic neurones and transported down their axons to the posterior pituitary, where it is stored in vesicles. Upon stimulation, it is secreted into the bloodstream. The hormone acts primarily on V2 receptors in the principal cells of the collecting ducts and distal tubules, initiating a signalling cascade that increases water reabsorption.

ADH，也称血管加压素，是一种肽类激素，在下丘脑神经元的胞体中合成，并沿轴突运输到垂体后叶，储存在囊泡中。受到刺激时，它被分泌进入血液。该激素主要作用于集合管和远曲小管主细胞上的V2受体，启动信号级联反应以增加水分重吸收。

ADH binding → V2 receptor → G-protein → adenylyl cyclase → cAMP → PKA → AQP2 vesicle fusion

ADH结合 → V2受体 → G蛋白 → 腺苷酸环化酶 → cAMP → PKA → AQP2囊泡融合

---

9. Effect of ADH on Collecting Duct | ADH对集合管的作用

The collecting duct is normally impermeable to water because its principal cells lack aquaporins in the apical membrane. ADH triggers the insertion of aquaporin-2 (AQP2) water channels into the apical membrane via cAMP-dependent protein kinase A (PKA). Water then enters the cells through AQP2, moves across the cell and exits through basolateral aquaporin-3 and aquaporin-4, driven by the medullary osmotic gradient. Consequently, large volumes of water can be reabsorbed, producing a small volume of concentrated urine.

集合管通常对水不通透，因为其主细胞顶膜缺乏水通道蛋白。ADH通过cAMP依赖性蛋白激酶A(PKA)触发水通道蛋白-2(AQP2)插入顶膜。随后，水在髓质渗透梯度的驱动下通过AQP2进入细胞，穿过胞体并通过基底外侧的AQP3和AQP4离开细胞。因此，大量水可被重吸收，产生少量浓缩尿。

When plasma osmolality is low, ADH secretion falls, AQP2 channels are removed from the apical membrane by endocytosis, and the collecting duct becomes watertight. This results in a large volume of dilute urine, rapidly restoring normal osmotic balance.

当血浆渗透压较低时，ADH分泌减少，AQP2通道通过内吞作用从顶膜移除，集合管变为不透水状态。这导致产生大量稀释尿液，迅速恢复正常的渗透平衡。

---

10. Negative Feedback Control | 负反馈调节

The control of water content is a classic example of negative feedback. A rise in plasma osmolality (dehydration) stimulates osmoreceptors, increasing ADH release, which enhances water reabsorption. The resulting decrease in plasma osmolality feeds back to reduce osmoreceptor activity and ADH secretion. The loop operates continuously, making fine adjustments to urine concentration in response to fluid intake and loss.

水分含量的控制是一个典型的负反馈实例。血浆渗透压升高（脱水）刺激渗透压感受器，增加ADH释放，从而增强水分重吸收。由此引起的血浆渗透压下降反馈减少渗透压感受器活性和ADH分泌。该回路持续运作，根据液体摄入和丢失对尿液浓度作出精细调整。

Additionally, thirst provides behavioural control. Dehydration increases plasma osmolality and blood volume reduction stimulates thirst centres in the hypothalamus, prompting water intake. Together, the endocrine and behavioural responses ensure precise water homeostasis.

此外，口渴提供行为控制。脱水导致血浆渗透压升高，血容量减少刺激下丘脑的口渴中枢，促使人饮水。内分泌和行为反应协同作用，确保精确的水分稳态。

---

11. Diabetes Insipidus: A Case Study | 尿崩症：案例研究

Diabetes insipidus illustrates the consequences of failed water regulation. In central diabetes insipidus, the posterior pituitary fails to secrete sufficient ADH, often due to head injury or tumour. In nephrogenic diabetes insipidus, the collecting ducts are unresponsive to ADH, sometimes caused by mutations in the V2 receptor or AQP2 gene. Both forms result in polyuria (excretion of large volumes of dilute urine) and polydipsia (excessive thirst). Patients can lose 10–20 L of water per day, risking severe dehydration unless fluid intake is maintained.

尿崩症展示了水分调节失败后的后果。中枢性尿崩症中，垂体后叶未能分泌足够的ADH，常见原因是头部损伤或肿瘤。肾性尿崩症中，集合管对ADH无反应，有时由V2受体或AQP2基因突变引起。两种形式均导致多尿（排出大量稀释尿）和烦渴（极度口渴）。患者每天可丢失10-20升水，若不维持液体摄入，会面临严重脱水的风险。

Treatment for central diabetes insipidus involves ADH analogues such as desmopressin, while nephrogenic forms are managed by dietary salt restriction and thiazide diuretics, which paradoxically reduce urine volume by enhancing proximal water reabsorption.

中枢性尿崩症的治疗使用ADH类似物如去氨加压素，而肾性尿崩症则通过限制食盐摄入和噻嗪类利尿剂处理，后者通过增强近端水分重吸收而反常地减少尿量。

---

12. Factors Affecting Water Balance | 影响水平衡的因素

Several external and internal factors influence water content regulation. Alcohol inhibits ADH release from the posterior pituitary, leading to increased urine output and potential dehydration. Caffeine also acts as a mild diuretic by increasing glomerular filtration and reducing tubular reabsorption. Diuretics prescribed for hypertension, such as loop diuretics, block Na⁺ reabsorption in the thick ascending limb, reducing the medullary gradient and thereby impairing water reabsorption.

若干内外因素影响水分调节。酒精抑制垂体后叶释放ADH，导致排尿量增加和潜在脱水。咖啡因通过增加肾小球滤过和减少肾小管重吸收，也起到温和的利尿作用。治疗高血压的利尿剂，如袢利尿剂，阻断厚升支的Na⁺重吸收，降低髓质梯度，从而削弱水分重吸收。

Environmental temperature and exercise increase water loss through sweating and evaporation. High protein diets increase urea production, requiring more water for its excretion. All these factors alter the set point at which ADH is released, highlighting the dynamic nature of water balance.

环境温度和运动通过出汗和蒸发增加水分丢失。高蛋白饮食增加尿素生成，需要更多水进行排泄。所有这些因素改变ADH释放的调定点，凸显了水平衡的动态特性。

---

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）