📚 How the Kidney Carries Out Excretion and Osmoregulation | 肾脏如何进行排泄与渗透调节
The kidneys are vital organs that perform two critical roles in the human body: excretion of metabolic waste products and osmoregulation of body fluids. Understanding how nephrons accomplish ultrafiltration, selective reabsorption, and hormonal fine-tuning is a key topic in Edexcel IGCSE Biology. This article explores the structure and function of the kidney, explaining how waste urea is removed and how water balance is precisely maintained.
肾脏是人体中执行两项关键功能的重要器官：排泄代谢废物以及对体液进行渗透调节。了解肾单位如何进行超滤、选择性重吸收和激素精细调节是爱德思 IGCSE 生物学的核心主题。本文探讨肾脏的结构与功能，解释代谢废物尿素如何被清除，以及水分平衡如何被精准维持。
1. The Kidney as an Excretory and Osmoregulatory Organ | 肾脏：排泄与渗透调节器官
The primary excretory function of the kidney is to remove urea, a toxic nitrogenous waste formed when the liver breaks down excess amino acids. If urea accumulates, it can disrupt enzyme function and cell activities. The kidney also eliminates other waste molecules, such as creatinine and excess ions, helping to maintain a stable internal environment.
肾脏的首要排泄功能是清除尿素——当肝脏分解过剩氨基酸时产生的一种有毒含氮废物。如果尿素积聚，会扰乱酶的功能和细胞活动。肾脏还清除其他废物分子，如肌酐和多余的离子，从而帮助维持稳定的内部环境。
Osmoregulation is the control of water and solute concentrations in the blood. This prevents cells from gaining or losing too much water by osmosis, which could cause them to swell and burst or shrivel and die. The kidney adjusts how much water is reabsorbed back into the blood, depending on the body’s hydration level, under hormonal control.
渗透调节是对血液中水分和溶质浓度的控制。这避免了细胞因渗透作用而吸收或失去过多水分，以致胀破或皱缩死亡。肾脏根据身体的水合状态，在激素调控下调节有多少水分被重吸收回血液。
2. Gross Structure of the Kidney | 肾脏的大体结构
A human has two bean-shaped kidneys located in the lower back. Each kidney receives blood through the renal artery and drains filtered blood via the renal vein. Internally, the kidney consists of an outer cortex, a middle medulla, and a central hollow pelvis that funnels urine into the ureter. The ureter carries urine to the bladder for storage before excretion.
人类有两个位于下背部的豆形肾脏。每个肾脏通过肾动脉接受血液，并通过肾静脉排出已过滤的血液。从内部看，肾脏由外层的皮质、中层的髓质和中央的肾盂组成，肾盂将尿液导入输尿管。输尿管把尿液送往膀胱储存，再排出体外。
The cortex appears granular because it contains the renal corpuscles and the convoluted tubules of nephrons. The medulla contains the loops of Henle and collecting ducts, which extend into the renal pelvis. The distinct organisation enables the kidney to create concentration gradients essential for water reabsorption.
皮质呈颗粒状外观，因为它含有肾小体以及肾单位的曲小管。髓质包含亨氏袢和集合管，这些结构延伸至肾盂。这种独特的组织方式使肾脏能够建立对水重吸收至关重要的浓度梯度。
3. The Nephron – Functional Unit of the Kidney | 肾单位——功能单位
Each kidney contains approximately one million nephrons. A nephron is a microscopic tubule that begins as a cup-shaped Bowman’s capsule surrounding a glomerulus. Together, the Bowman’s capsule and the glomerulus form the renal corpuscle, situated in the cortex.
每个肾脏约含有一百万个肾单位。肾单位是一种微观管状结构，起始于杯状的鲍曼氏囊，包裹着肾小球。鲍曼氏囊和肾小球共同构成肾小体，位于皮质中。
From the Bowman’s capsule, the nephron continues as the proximal convoluted tubule (PCT), dips into the medulla as the loop of Henle, then ascends to become the distal convoluted tubule (DCT). Several distal tubules empty into a shared collecting duct, which travels through the medulla and opens into the renal pelvis.
从鲍曼氏囊出发，肾单位延续为近曲小管，然后以亨氏袢的形式深入髓质，再上升成为远曲小管。多个远曲小管汇入一条共享的集合管，集合管穿过髓质，开口于肾盂。
4. Blood Supply and Ultrafiltration | 血供与超滤作用
Blood enters the glomerulus through a wider afferent arteriole and leaves through a narrower efferent arteriole. This difference in diameter creates high hydrostatic pressure inside the glomerular capillaries, forcing fluid out of the blood and into the Bowman’s capsule – a process called ultrafiltration.
血液经较宽大的入球小动脉进入肾小球，从较窄小的出球小动脉离开。这种口径差在肾小球毛细血管内产生高静水压，迫使液体渗出血管，进入鲍曼氏囊——这个过程称为超滤作用。
The filtration barrier consists of three layers: the fenestrated capillary endothelium, a basement membrane, and podocytes (specialised cells of the Bowman’s capsule). The basement membrane acts as the main filter, preventing large molecules such as plasma proteins and blood cells from passing through, while allowing water, glucose, ions, and urea to escape into the capsular space.
过滤屏障由三层构成：有孔的毛细血管内皮、基底膜以及鲍曼氏囊的足细胞。基底膜起主要过滤作用，阻止血浆蛋白和血细胞等大分子通过，而允许水、葡萄糖、离子和尿素进入囊腔。
5. Composition of the Glomerular Filtrate | 肾小球滤液的组成
The filtrate entering the Bowman’s capsule closely resembles blood plasma but is almost entirely free of large proteins and cells. It contains water, glucose, amino acids, urea, sodium ions, chloride ions, and other small solutes. In a healthy adult, around 125 cm³ of filtrate is produced each minute.
进入鲍曼氏囊的滤液与血浆极为相似，但几乎不含大分子蛋白和细胞。滤液中含有水、葡萄糖、氨基酸、尿素、钠离子、氯离子以及其他小分子溶质。健康成人每分钟约产生125立方厘米的滤液。
Although this seems like a massive fluid loss, the majority of the filtrate is reabsorbed further along the nephron. Without reabsorption, the body would lose essential nutrients and dehydrate rapidly. The filtrate can be thought of as a starting point that the kidney modifies through precise transport processes.
尽管这看似流失了大量液体，但绝大部分滤液会在肾单位后续部位被重吸收。若无重吸收，机体将流失必需营养物质并迅速脱水。滤液可视为一个起点，肾脏通过精确的转运过程对其进行改造。
6. Selective Reabsorption in the Proximal Convoluted Tubule | 近曲小管的选择性重吸收
Selective reabsorption is the process by which useful substances are taken back from the filtrate into the blood. In the PCT, all glucose, most amino acids, and a large proportion of ions and water are reabsorbed. The cells lining the PCT are adapted with microvilli to increase surface area and many mitochondria to provide ATP for active transport.
选择性重吸收是指将有用物质从滤液中重新吸收回血液的过程。在近曲小管中，全部葡萄糖、大部分氨基酸以及大量离子和水分被重吸收。近曲小管内衬细胞具有微绒毛以增加表面积，并有大量线粒体提供主动运输所需的ATP。
Glucose reabsorption occurs by active transport, using sodium co-transporter proteins in the cell membrane. Water follows the reabsorbed solutes by osmosis. Because all glucose is normally removed from the filtrate, a healthy person’s urine contains no glucose. Urea, a waste product, is not actively reabsorbed; only a small amount diffuses back passively, so the remainder stays in the tubule to be excreted.
葡萄糖的重吸收通过主动运输进行，利用细胞膜上的钠协同转运蛋白。水分通过渗透作用跟随被重吸收的溶质移动。由于正常情况下全部葡萄糖都被从滤液中移除，健康人的尿液中不含葡萄糖。尿素作为废物，不被主动重吸收；仅少量被动扩散回吸，其余留在小管中被排出。
7. The Loop of Henle and Water Reabsorption | 亨氏袢与水分的重吸收
The loop of Henle is a hairpin-shaped structure that extends into the medulla. Its descending limb is permeable to water but not to salts, so water leaves the filtrate by osmosis as it passes through the increasingly salty medulla. The ascending limb, however, is impermeable to water and actively transports Na⁺ and Cl⁻ out into the surrounding tissue.
亨氏袢是一种发夹状结构，延伸入髓质。其降支对水通透但对盐不通透，因此水在流经含盐量递增的髓质时，通过渗透作用离开滤液。而升支对水不通透，并主动将Na⁺和Cl⁻转运至周围组织。
This countercurrent mechanism builds a high osmotic concentration in the medulla. The salty interstitial fluid later allows the collecting duct to reabsorb water efficiently when the body needs to conserve water. Although IGCSE does not require detailed knowledge of the countercurrent multiplier, it is useful to appreciate how the kidney prepares the medulla for osmoregulation.
这种逆流机制在髓质中建立起高渗环境。此后，当身体需要保留水分时，含盐高的组织间液使集合管能高效地重吸收水分。尽管IGCSE不要求深入了解逆流倍增机制，但理解肾脏如何为渗透调节准备髓质很有帮助。
8. The Distal Tubule and Collecting Duct | 远曲小管与集合管
After the loop of Henle, the filtrate enters the distal convoluted tubule and then the collecting duct. Here, fine-tuning of urine composition occurs. The DCT reabsorbs variable amounts of Na⁺ and water, regulated by hormones. The collecting duct runs through the hypertonic medulla, and its permeability to water is the key to osmoregulation.
经过亨氏袢后，滤液进入远曲小管，之后再进入集合管。在这里，尿液的成分被精细调节。远曲小管在激素调节下重吸收数量不等的Na⁺和水分。集合管穿过高渗的髓质，其对水的通透性是渗透调节的关键。
If the collecting duct walls are permeable to water, water will move out by osmosis and be reabsorbed into the blood, producing concentrated urine. If they are impermeable, water remains in the tubule and dilute urine is excreted. The permeability is controlled by the antidiuretic hormone (ADH).
若集合管管壁对水通透，水将通过渗透作用流出并被重吸收入血液，产生浓缩尿液。若对水不通透，水分留在管腔中，排出稀尿液。这一通透性由抗利尿激素（ADH）控制。
9. Osmoregulation by Antidiuretic Hormone (ADH) | 抗利尿激素（ADH）的渗透调节作用
Osmoregulation is maintained through a negative feedback system. When the blood becomes too concentrated (low water potential), osmoreceptors in the hypothalamus detect the change. The hypothalamus stimulates the posterior pituitary gland to release more ADH into the bloodstream.
渗透调节由一个负反馈系统维持。当血液变得过浓（水势较低）时，下丘脑中的渗透压感受器检测到这一变化。下丘脑刺激垂体后叶向血液中释放更多ADH。
ADH binds to receptors on the cells of the collecting duct and DCT, triggering the insertion of aquaporin water channels into the cell membranes. This dramatically increases water permeability, allowing more water to be reabsorbed from the filtrate. As a result, the urine becomes more concentrated and its volume decreases, helping the body retain water.
ADH与集合管和远曲小管细胞上的受体结合，触发水通道蛋白（aquaporin）插入细胞膜。这极大增加了水通透性，使更多水从滤液中被重吸收。结果，尿液变得更浓缩且体积减少，帮助身体保留水分。
Conversely, if the blood is too dilute (high water potential), osmoreceptors reduce ADH secretion. With less ADH, the collecting duct becomes less permeable, so less water is reabsorbed and a large volume of dilute urine is produced. This precisely balances the body’s water content.
反之，如果血液过稀（水势高），渗透压感受器减少ADH分泌。ADH减少时，集合管通透性降低，因此重吸收的水减少，产生大量稀尿液。这就精准地平衡了体内的水分含量。
10. Integration of Excretion and Osmoregulation | 排泄与渗透调节的整合
The kidney’s dual roles are deeply intertwined. Urea is excreted by being filtered out of the blood and not being actively reabsorbed; it simply remains in the tubule fluid while water and useful solutes are reclaimed. The removal of this nitrogenous waste is an essential homeostatic function, preventing toxicity.
肾脏的双重功能是紧密交织的。尿素通过被滤出血液且不被主动重吸收的方式被排泄；当水分和有用溶质被回收时，尿素便留在小管液中。这一含氮废物的清除是维持稳态的关键功能，防止毒性积聚。
Simultaneously, osmoregulation adjusts the amount of water reabsorbed, ensuring the blood’s osmotic pressure stays within narrow limits. Whether the body is dehydrated or overhydrated, the ADH mechanism swiftly modifies collecting duct permeability to restore equilibrium. Both excretion and osmoregulation are essential for survival and are a classic example of coordination between organ systems.
与此同时，渗透调节调整水分重吸收量，确保血液渗透压维持在狭窄的范围内。无论身体是脱水还是水分过多，ADH机制都会迅速调整集合管通透性以恢复平衡。排泄和渗透调节对生存都至关重要，是器官系统间协调配合的经典实例。
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