📚 A-Level Biology: Kidney Structure and Nephron Function | A-Level 生物：肾脏结构与肾单位功能
The kidney is a vital excretory organ that maintains homeostasis by filtering blood, removing metabolic waste, and regulating water and solute balance. In CIE A-Level Biology, understanding the structural organisation of the kidney and the detailed functioning of the nephron is essential for answering exam questions on osmoregulation and excretion.
肾脏是重要的排泄器官，通过过滤血液、清除代谢废物以及调节水和溶质平衡来维持机体内环境稳定。在 CIE A-Level 生物考试中，理解肾脏的结构层次以及肾单位的详细功能，是回答关于渗透调节和排泄考题的关键基础。
1. Gross Structure of the Kidney | 肾脏的大体结构
Each kidney is surrounded by a tough fibrous capsule. A longitudinal section reveals three distinct regions: the outer renal cortex, the inner renal medulla, and the funnel-shaped renal pelvis, which collects urine and funnels it into the ureter.
每个肾脏外覆一层坚韧的纤维囊。纵切面可观察到三个明显区域：外侧的肾皮质、内侧的肾髓质，以及漏斗状的肾盂。肾盂收集尿液并将其导入输尿管。
Blood enters the kidney through the renal artery, which branches into smaller arterioles and capillaries within the kidney tissue. The renal vein returns filtered blood to the circulatory system. The functional unit of the kidney is the nephron, and each human kidney contains approximately one million nephrons.
血液经肾动脉进入肾脏，在肾组织内分支为更细的小动脉和毛细血管。肾静脉将过滤后的血液送回循环系统。肾脏的功能单位是肾单位，每个人肾脏约含一百万个肾单位。
- Renal cortex – outer region containing Bowman’s capsules, glomeruli, proximal and distal convoluted tubules.
- Renal medulla – inner region containing loops of Henle and collecting ducts.
- Renal pelvis – central cavity continuous with the ureter.
- 肾皮质——外层区域，包含鲍曼囊、肾小球、近曲小管和远曲小管。
- 肾髓质——内层区域，包含髓袢和集合管。
- 肾盂——与输尿管相连的中央腔室。
2. The Nephron: Structure Overview | 肾单位：结构总览
The nephron is composed of several specialised segments, each with distinct cellular adaptations that enable specific functions. These segments include the Bowman’s capsule, proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting duct.
肾单位由若干特化节段组成，每个节段具有独特的细胞适应性结构，从而执行特定功能。这些节段包括鲍曼囊、近曲小管（PCT）、髓袢、远曲小管（DCT）和集合管。
There are two types of nephrons. Cortical nephrons have shorter loops of Henle that extend only slightly into the medulla, while juxtamedullary nephrons have long loops that penetrate deep into the medulla, enabling the production of concentrated urine.
肾单位分为两类。皮质肾单位的髓袢较短，仅略微伸入髓质；而髓旁肾单位的髓袢较长，深入髓质内部，从而能够产生浓缩尿液。
Blood flow route: renal artery → afferent arteriole → glomerulus → efferent arteriole → peritubular capillaries / vasa recta → renal vein
血流路径：肾动脉 → 入球小动脉 → 肾小球 → 出球小动脉 → 管周毛细血管 / 直小血管 → 肾静脉
3. The Bowman’s Capsule and Glomerulus: Ultrafiltration | 鲍曼囊与肾小球：超滤作用
Ultrafiltration occurs at the renal corpuscle, which consists of the glomerulus (a network of capillaries) surrounded by the Bowman’s capsule. Blood pressure forces fluid through three filtration barriers: the capillary endothelium, the basement membrane, and the podocyte filtration slits.
超滤作用发生在肾小体处。肾小体由肾小球（毛细血管网）及其外包绕的鲍曼囊组成。血压驱动流体穿过三层滤过屏障：毛细血管内皮、基底层以及足细胞滤过裂隙。
The capillary endothelium has fenestrations (pores) approximately 100 nm in diameter, which prevent blood cells from passing. The basement membrane is a meshwork of collagen and glycoproteins that acts as the main filter, blocking plasma proteins larger than about 68,000 Da. Podocytes wrap around capillaries with interdigitating foot processes, forming narrow filtration slits.
毛细血管内皮具有直径约 100 nm 的窗孔，可阻止血细胞通过。基底膜是胶原蛋白和糖蛋白构成的网状结构，是主要的滤过屏障，可阻挡大于约 68,000 Da 的血浆蛋白。足细胞通过相互交错的足突包裹毛细血管，形成狭窄的滤过裂隙。
The filtrate in the Bowman’s capsule is essentially plasma minus proteins and blood cells. It contains water, glucose, amino acids, urea, inorganic ions (Na⁺, K⁺, Cl⁻, HCO₃⁻), and small peptides below the molecular weight cut-off.
鲍曼囊中的滤液本质上是不含蛋白质和血细胞的血浆。滤液中含有水、葡萄糖、氨基酸、尿素、无机离子（Na⁺、K⁺、Cl⁻、HCO₃⁻）以及低于分子量截断阈值的小分子肽。
| Substance | Relative concentration in filtrate | Relative concentration in plasma |
| Water | Same | Same |
| Glucose | Same | Same |
| Urea | Same | Same |
| Plasma protein | Absent | Present |
| Red blood cells | Absent | Present |
| 物质 | 滤液中相对浓度 | 血浆中相对浓度 |
| 水 | 相同 | 相同 |
| 葡萄糖 | 相同 | 相同 |
| 尿素 | 相同 | 相同 |
| 血浆蛋白 | 无 | 有 |
| 红细胞 | 无 | 有 |
4. The Proximal Convoluted Tubule: Selective Reabsorption | 近曲小管：选择性重吸收
The PCT is lined with cuboidal epithelial cells bearing dense microvilli on their apical surface, which greatly increase the surface area for reabsorption. These cells also contain numerous mitochondria, providing ATP for active transport processes.
近曲小管由立方上皮细胞衬里，其顶端表面密布微绒毛，大大增加了重吸收的表面积。这些细胞还含有大量线粒体，为主动转运过程提供 ATP。
Approximately 65-70% of filtered Na⁺ and water, and virtually 100% of filtered glucose and amino acids, are reabsorbed in the PCT. Glucose reabsorption occurs via sodium-dependent secondary active transport. The Na⁺ gradient driving this is maintained by Na⁺/K⁺ ATPase pumps on the basolateral membrane.
近曲小管重吸收约 65%-70% 的滤过 Na⁺ 和水，以及几乎 100% 的滤过葡萄糖和氨基酸。葡萄糖的重吸收通过钠依赖性继发性主动转运实现。维持该过程所需的 Na⁺ 梯度由基底外侧膜上的 Na⁺/K⁺ ATPase 泵来维持。
Urea also undergoes passive reabsorption in the PCT, but to a lesser extent. Water follows solutes passively via osmosis through aquaporins and the paracellular pathway. The fluid leaving the PCT is still isotonic to plasma.
尿素在近曲小管中也会发生被动重吸收，但程度较低。水通过水通道蛋白和细胞旁途径，以渗透方式被动地跟随溶质移动。离开近曲小管的液体与血浆仍保持等渗。
5. The Loop of Henle: Countercurrent Multiplier | 髓袢：逆流倍增系统
The loop of Henle is a U-shaped tubule that extends into the renal medulla. It consists of a descending limb and an ascending limb, each with different permeability characteristics. Together, they establish a high solute concentration in the medullary interstitial fluid, which is essential for enabling the collecting duct to reabsorb water and produce concentrated urine.
髓袢是伸入肾髓质的 U 形小管，由降支和升支组成，两支的渗透性特征不同。它们共同在髓质组织间液中建立高溶质浓度，这是集合管能够重吸收水分并产生浓缩尿液的必要条件。
The descending limb is permeable to water but impermeable to solutes such as Na⁺ and Cl⁻. As filtrate descends into the increasingly concentrated medulla, water leaves by osmosis, concentrating the tubular fluid. The ascending limb, in contrast, is impermeable to water but actively transports Na⁺, K⁺, and Cl⁻ out via the Na⁺-K⁺-2Cl⁻ cotransporter in the thick segment.
降支对水有通透性，但对 Na⁺ 和 Cl⁻ 等溶质不通透。随着滤液向浓度逐渐升高的髓质深部下行，水分通过渗透作用离开小管，使管腔液浓缩。相反，升支对水不通透，但通过厚段中的 Na⁺-K⁺-2Cl⁻ 共转运体主动将 Na⁺、K⁺ 和 Cl⁻ 泵出。
This active transport is termed the ‘countercurrent multiplier’ mechanism because the arrangement of descending and ascending limbs enables a small transport gradient to be amplified into a much larger osmotic gradient. The vasa recta, a set of capillaries running parallel to the loop, acts as a countercurrent exchanger to remove reabsorbed water and solutes without disturbing the medullary gradient.
这种主动转运被称为”逆流倍增”机制，因为降支和升支的排列方式使一个较小的转运梯度被放大为显著更大的渗透梯度。直小血管是与髓袢平行排列的毛细血管网，作为逆流交换器，在不扰乱髓质梯度的前提下移除重吸收的水分和溶质。
Interstitial osmotic gradient: cortex (~300 mOsm L⁻¹) → outer medulla (~600 mOsm L⁻¹) → inner medulla (~1200 mOsm L⁻¹)
组织间渗透梯度：皮质（约 300 mOsm L⁻¹）→ 外髓质（约 600 mOsm L⁻¹）→ 内髓质（约 1200 mOsm L⁻¹）
6. The Distal Convoluted Tubule: Finely Controlled Reabsorption | 远曲小管：精细调节的重吸收
The DCT receives fluid that has already had the majority of its solutes and water reabsorbed. Its main roles are the fine-tuning of ion balance and pH regulation. Unlike the PCT, the DCT is not permeable to water unless antidiuretic hormone (ADH) is present.
远曲小管接收已经历大部分溶质和水重吸收的液体。其主要功能是微调离子平衡和 pH 值。与近曲小管不同，远曲小管仅在抗利尿激素（ADH）存在时才对水具有通透性。
The DCT reabsorbs Na⁺ under the control of aldosterone. Aldosterone promotes the synthesis of Na⁺ channels and Na⁺/K⁺ ATPase pumps, thereby increasing Na⁺ reabsorption and K⁺ secretion. This is a key mechanism in the renin-angiotensin-aldosterone system (RAAS) for long-term blood pressure regulation.
远曲小管在醛固酮的调控下重吸收 Na⁺。醛固酮促进 Na⁺ 通道和 Na⁺/K⁺ ATPase 泵的合成，从而增加 Na⁺ 重吸收和 K⁺ 分泌。这是肾素-血管紧张素-醛固酮系统（RAAS）中长期血压调节的关键机制。
Additionally, the DCT actively secretes H⁺ ions and reabsorbs HCO₃⁻, contributing to acid-base balance. Parathyroid hormone (PTH) also acts on the DCT to increase Ca²⁺ reabsorption when blood calcium levels fall.
此外，远曲小管主动分泌 H⁺ 离子并重吸收 HCO₃⁻，参与酸碱平衡的维持。甲状旁腺激素（PTH）也作用于远曲小管，在血钙水平下降时增加 Ca²⁺ 的重吸收。
7. The Collecting Duct: Osmoregulation and ADH | 集合管：渗透调节与 ADH
The collecting duct runs from the cortex through the medulla to the renal pelvis. Its permeability to water is controlled by the hormone ADH. In the absence of ADH, the collecting duct is relatively impermeable to water, resulting in dilute urine. When ADH is present, aquaporin-2 channels are inserted into the apical membrane of collecting duct cells, allowing water to exit by osmosis into the hypertonic medullary interstitium.
集合管从皮质经髓质延伸至肾盂。其对水的通透性受激素 ADH 的调节。当 ADH 缺失时，集合管对水相对不通透，导致产生稀释尿液。当 ADH 存在时，水通道蛋白-2（AQP2）通道被插入集合管细胞的顶端膜，使水分通过渗透作用进入高渗的髓质组织间液。
ADH is synthesised in the hypothalamus and released from the posterior pituitary gland. Osmoreceptors in the hypothalamus detect an increase in blood plasma osmolarity and trigger ADH release as an action potential signalling cascade.
ADH 在下丘脑合成，并由垂体后叶释放。下丘脑中的渗透压感受器检测到血浆渗透压升高时，会触发 ADH 的释放，通过动作电位信号级联传递。
Urea also contributes to the medullary gradient: in the inner medulla, the collecting duct is permeable to urea, allowing it to diffuse into the interstitium. This recycled urea accounts for approximately 40-50% of the medullary osmotic gradient, enabling maximally concentrated urine of up to 1200 mOsm L⁻¹ in humans.
尿素也参与形成髓质梯度：在髓质内层，集合管对尿素具有通透性，使其扩散进入组织间液。这种被循环利用的尿素约占髓质渗透梯度的 40%-50%，使人类能够产生高达 1200 mOsm L⁻¹ 的最大浓缩尿液。
ADH present → more aquaporin-2 → more water reabsorbed → small volume, concentrated urine
ADH 存在 → 更多水通道蛋白-2 → 更多水被重吸收 → 尿量少、尿液浓缩
ADH absent → fewer aquaporin-2 → less water reabsorbed → large volume, dilute urine
ADH 缺失 → 水通道蛋白-2 减少 → 水重吸收减少 → 尿量大、尿液稀释
8. Comparison of Nephron Segments: Structure-Function Relationships | 肾单位各节段比较：结构与功能的关系
The table below summarises the key structural adaptations and corresponding functions of each nephron segment, which is a frequent examination focus.
下表总结了肾单位各节段的关键结构适应性及其对应功能，这是考试中的高频考点。
| Segment | Key structural feature | Main function |
| Bowman’s capsule / glomerulus | Fenestrated endothelium, basement membrane, podocytes | Ultrafiltration of blood |
| PCT | Microvilli, many mitochondria, co-transport proteins | Bulk reabsorption of glucose, amino acids, ions, water |
| Loop of Henle | U-shaped; descending permeable to water; ascending impermeable to water | Create medullary osmotic gradient |
| DCT | Fewer microvilli; hormone receptors | Fine-tuning ions, pH regulation |
| Collecting duct | ADH-regulated aquaporins; urea transporters | Variable water reabsorption; concentrates urine |
| 节段 | 关键结构特征 | 主要功能 |
| 鲍曼囊 / 肾小球 | 有窗孔的内皮、基底膜、足细胞 | 血液的超滤作用 |
| 近曲小管 | 微绒毛、大量线粒体、共转运蛋白 | 大量重吸收葡萄糖、氨基酸、离子、水 |
| 髓袢 | U 形；降支透水、升支不透水 | 建立髓质渗透梯度 |
| 远曲小管 | 微绒毛较少；含激素受体 | 精细调节离子、调节 pH |
| 集合管 | 受 ADH 调节的水通道蛋白；尿素转运体 | 可变性水重吸收；浓缩尿液 |
9. The Renin-Angiotensin-Aldosterone System (RAAS) | 肾素-血管紧张素-醛固酮系统（RAAS）
When blood pressure or blood sodium levels fall, juxtaglomerular cells in the afferent arteriole secrete renin. Renin cleaves angiotensinogen (produced by the liver) into angiotensin I, which is then converted to angiotensin II by angiotensin-converting enzyme (ACE) in the lungs.
当血压或血钠水平下降时，入球小动脉中的球旁细胞分泌肾素。肾素将肝脏产生的血管紧张素原裂解为血管紧张素 I，随后血管紧张素 I 在肺部被血管紧张素转换酶（ACE）转化为血管紧张素 II。
Angiotensin II exerts multiple effects: it stimulates aldosterone secretion from the adrenal cortex, promotes ADH release, causes vasoconstriction of arterioles, and directly stimulates Na⁺ reabsorption in the PCT. These coordinated actions restore blood pressure and blood volume.
血管紧张素 II 发挥多种效应：刺激肾上腺皮质分泌醛固酮、促进 ADH 释放、引起小动脉血管收缩，并直接促进近曲小管中的 Na⁺ 重吸收。这些协同作用共同恢复血压和血容量。
Low BP / low Na⁺ → renin ↑ → angiotensinogen → angiotensin I → ACE → angiotensin II → aldosterone ↑ → Na⁺ reabsorption ↑ → water follows → blood volume ↑ → BP restored
低血压 / 低钠 → 肾素 ↑ → 血管紧张素原 → 血管紧张素 I → ACE → 血管紧张素 II → 醛固酮 ↑ → Na⁺ 重吸收 ↑ → 水随之吸收 ↑ → 血容量 ↑ → 血压恢复
10. Common Exam Pitfalls and Key Definitions | 常见考试误区与关键定义
One common error is confusing the directions of reabsorption and secretion. Reabsorption moves substances from the tubular filtrate into the blood; secretion moves substances from the blood into the tubular fluid. A second common error is stating that the PCT reabsorbs all nutrients under hormonal control, when in fact PCT reabsorption is largely constitutive and not hormone-regulated.
一个常见错误是混淆重吸收和分泌的方向。重吸收是将物质从小管滤液中移入血液；分泌是将物质从血液移入小管液。第二个常见错误是认为近曲小管在激素调控下重吸收所有营养物质，而事实上近曲小管的重吸收主要是固有性的，不受激素调控。
Students frequently fail to distinguish between osmoregulation (regulation of water potential in the body) and excretion (removal of nitrogenous waste). The kidney performs both functions simultaneously but through distinct mechanisms. Also, remember that ADH does not directly act on the loop of Henle in humans; its primary targets are the collecting duct and, to a lesser extent, the DCT.
学生经常无法区分渗透调节（调节体内水势）与排泄（清除含氮废物）。肾脏同时执行这两项功能，但通过不同的机制实现。此外，请记住 ADH 在人体中并不直接作用于髓袢；其主要靶点是集合管，以及较小程度上的远曲小管。
Finally, a terminological note: the term ‘glomerular filtration rate’ (GFR) represents the volume of filtrate produced by both kidneys per minute, normally about 125 mL min⁻¹. A GFR that falls dramatically indicates impaired kidney function, which can be assessed clinically to identify renal disease.
最后需要说明一个术语：’肾小球滤过率’（GFR）代表双肾每分钟产生的滤液体积，正常约为 125 mL min⁻¹。GFR 显著下降提示肾功能受损，临床上可通过检测 GFR 来识别肾脏疾病。
Published by TutorHao | Biology Revision Series | aleveler.com
更多咨询请联系16621398022（同微信）
屏轩国际教育cambridge primary/secondary checkpoint, cat4, ukiset,ukcat,igcse,alevel,PAT,STEP,MAT, ibdp,ap,ssat,sat,sat2课程辅导，国外大学本科硕士研究生博士课程论文辅导Cancel reply