The structure of the kidney | 肾脏的结构

📚 The structure of the kidney | 肾脏的结构

In A-Level Biology, understanding the structure of the kidney is fundamental to grasping how the body maintains water balance, excretes waste, and regulates blood pressure. The kidney is a complex organ composed of specialised functional units called nephrons, each intimately associated with an intricate network of blood vessels. From the gross anatomy of the renal cortex and medulla to the microscopic arrangement of the glomerulus, Bowman’s capsule, and the renal tubule, every structural feature is adapted to carry out filtration, selective reabsorption, and secretion. This article examines the macroscopic and microscopic architecture of the mammalian kidney, linking structure to function at every level, with a focus on the Cambridge A-Level Biology syllabus.

在A-Level生物学中，理解肾脏的结构是掌握人体如何维持水平衡、排泄废物和调节血压的基础。肾脏是一个复杂的器官，由称为肾单位的高度特化的功能单位组成，每个肾单位都与精密的血管网紧密相连。从肾皮质和髓质的大体解剖，到肾小球、鲍曼囊和肾小管的显微排列，每一种结构特征都经过适应，以实现超滤、选择性重吸收和分泌的功能。本文将从宏观和微观两个层面详细探讨哺乳动物肾脏的结构，并在每个层面上将结构与功能联系起来，紧扣剑桥A-Level生物学的考纲。

1. The main functions of the kidney | 肾脏的主要功能

The kidneys perform several vital roles in homeostasis. Primarily, they excrete nitrogenous waste products such as urea, which is formed from the breakdown of excess amino acids in the liver. In addition, the kidneys regulate the water potential of the blood by controlling the volume of water lost in urine, a process that involves the loop of Henle and the action of antidiuretic hormone (ADH). They also play a key role in maintaining the balance of essential ions like sodium, potassium, hydrogen, and bicarbonate, thus helping to stabilise blood pH. Furthermore, the kidneys are involved in the production of hormones such as erythropoietin, which stimulates red blood cell formation, and renin, an enzyme that helps control blood pressure. All these functions depend critically on the precise anatomical arrangement of the renal tissues, from the gross arrangement of the cortex and medulla down to the cellular specialisations of the nephron.

肾脏在稳态中发挥着多项至关重要的功能。其主要功能是排泄含氮代谢废物，例如尿素，它是在肝脏中由过量氨基酸分解形成的。此外，肾脏通过控制尿液中流失的水量来调节血液的水势，这一过程涉及髓袢和抗利尿激素（ADH）的作用。肾脏还在维持钠、钾、氢和碳酸氢盐等必需离子的平衡方面起着关键作用，从而有助于稳定血液的pH值。不仅如此，肾脏还参与生成某些激素，例如刺激红细胞生成的促红细胞生成素，以及有助于控制血压的酶——肾素。所有这些功能都严格依赖于肾脏组织的精确解剖排列，从皮质和髓质的大体结构，一直到肾单位的细胞特化。

2. Gross anatomy of the kidney | 肾脏的大体解剖

Externally, each kidney is covered by a tough fibrous capsule that protects the delicate internal structures. On a longitudinal section, the kidney reveals three distinct regions: an outer cortex, a middle medulla, and an inner chamber called the renal pelvis. The cortex is the dark reddish-brown outer zone where the glomeruli, Bowman’s capsules, and the convoluted tubules are packed. The medulla consists of several conical masses called renal pyramids, which appear striated due to the parallel arrangement of collecting ducts and loops of Henle. The apex of each pyramid, termed the renal papilla, projects into a minor calyx. These minor calyces merge to form major calyces, which in turn converge to form the renal pelvis, a funnel-shaped cavity that drains urine into the ureter. This layered organisation ensures that urine flows unidirectionally from the site of formation towards the bladder.

从外部看，每个肾脏都被一层坚韧的纤维囊所覆盖，以保护内部脆弱的结构。在纵切面上，肾脏显示出三个截然不同的区域：外层的皮质、中部的髓质，以及一个称为肾盂的内腔。皮质是深红褐色的外层区域，其中密集分布着肾小球、鲍曼囊以及肾小管的弯曲部分。髓质由数个称为肾锥体的圆锥形组织块构成，这些锥体因集合管和髓袢的平行排列而呈现条纹状。每个锥体的顶端称为肾乳头，它突入一个小肾盏中。这些小肾盏汇合形成大肾盏，进而汇聚成肾盂——一个漏斗状的腔体，将尿液排入输尿管。这种分层结构确保了尿液能单向地从其生成部位流向膀胱。

3. The nephron: the functional unit | 肾单位：功能单位

The nephron is the microscopic structural and functional unit of the kidney, with approximately one million nephrons in each human kidney. Each nephron is a blind-ended tube that begins at the renal corpuscle and continues through a series of specialised segments: the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The distal convoluted tubule drains into a collecting duct, which passes down through the medulla and empties urine into the renal pelvis. The nephron is closely associated with a network of blood vessels that supply and drain the glomerulus and subsequently form peritubular capillaries. The precise structure of each segment determines its role in the processes of ultrafiltration, selective reabsorption, and tubular secretion, forming the basis for urine formation.

肾单位是肾脏微观结构和功能的基本单位，每个人的肾脏中约有100万个肾单位。每一个肾单位都是一条盲端的小管，起始于肾小体，并依次经过一系列特化的节段：近曲小管、髓袢和远曲小管。远曲小管汇入集合管，集合管穿过髓质下行，将尿液排入肾盂。肾单位与一个血管网络紧密相连，这些血管负责向肾小球供应和引流血液，并随后形成管周毛细血管。每一个节段的精确结构决定了它在超滤、选择性重吸收以及肾小管分泌过程中的作用，从而构成了尿液形成的基础。

4. The renal corpuscle: site of ultrafiltration | 肾小体：超滤的部位

The renal corpuscle is the initial filtering component of the nephron and consists of a tuft of capillaries called the glomerulus, surrounded by the cup-shaped Bowman’s capsule. Blood enters the glomerulus via a wide afferent arteriole and leaves through a narrower efferent arteriole, creating a high hydrostatic pressure inside the capillary tuft. The endothelial cells of the glomerular capillaries are fenestrated, with pores that allow most components of blood plasma to pass through but retain blood cells and platelets. On the outer side, the inner layer of Bowman’s capsule is composed of highly specialised epithelial cells called podocytes, which have foot-like processes (pedicels) that wrap around the capillaries. Between the podocytes and the capillary endothelium lies a shared basement membrane, which acts as the main molecular filter. Together, these three layers—fenestrated endothelium, basement membrane, and podocyte filtration slits—form the filtration barrier, permitting the passage of water, glucose, ions, and urea while restricting proteins and cells. The resulting filtrate, known as glomerular filtrate, collects in the Bowman’s space and flows into the proximal convoluted tubule.

肾小体是肾单位的初始滤过组件，由一簇称为肾小球的毛细血管和杯状的鲍曼囊构成。血液通过较宽的入球小动脉进入肾小球，再经由较窄的出球小动脉离开，从而在毛细血管簇内产生较高的静水压。肾小球毛细血管的内皮细胞具有窗孔，允许血浆中的大多数成分通过，但截留血细胞和血小板。在外侧，鲍曼囊的内层由高度特化的上皮细胞——足细胞构成，它们具有足状突起（足突），包裹在毛细血管周围。在足细胞和毛细血管内皮之间，有一层共享的基底膜，它是主要的分子滤网。这三层结构——有窗孔的内皮、基底膜和足细胞滤过裂隙——共同构成了滤过屏障，允许水、葡萄糖、离子和尿素通过，同时阻挡蛋白质和细胞。由此产生的滤液，称为肾小球滤液，进入鲍曼囊腔后流往近曲小管。

5. The proximal convoluted tubule: reabsorption powerhouse | 近曲小管：重吸收的主力

The proximal convoluted tubule (PCT) is the first segment of the renal tubule, leading directly from Bowman’s capsule. It is located entirely within the renal cortex and is lined with cuboidal epithelial cells that are richly endowed with adaptations for bulk reabsorption. The luminal surface of these cells possesses a dense brush border formed by thousands of microvilli, which dramatically increase the surface area for absorption. In addition, the basal membrane of the cells is deeply infolded, housing numerous mitochondria that provide the ATP required for active transport. The cells also contain many carrier proteins and channel proteins for cotransport and facilitated diffusion. Approximately 65–70% of the filtered water and sodium ions, 100% of glucose and amino acids, and a significant proportion of chloride and bicarbonate ions are reabsorbed in the PCT. This reabsorption is largely iso-osmotic, meaning water follows solutes by osmosis, so the fluid leaving the PCT has the same osmotic concentration as the plasma, though its volume is greatly reduced. The structural specialisations of the PCT directly enable its high reabsorptive capacity.

近曲小管是肾小管的第一段，直接延续于鲍曼囊。它完全位于肾皮质内，内衬以立方上皮细胞，这些细胞具有丰富的适应结构以进行大量重吸收。细胞的管腔面上具有由数千根微绒毛构成的致密刷状缘，极大地增加了吸收表面积。此外，细胞的基底膜深度内褶，内藏大量的线粒体，为主动运输提供所需的ATP。细胞还含有许多载体蛋白和通道蛋白，用于协同转运和易化扩散。大约65–70%的滤过水和钠离子、100%的葡萄糖和氨基酸，以及相当比例的氯离子和碳酸氢根离子在近曲小管中被重吸收。这一重吸收过程在很大程度上是等渗的，即水通过渗透作用跟随溶质，因此离开近曲小管的液体尽管体积大幅减少，但其渗透浓度与血浆相同。近曲小管的结构特化直接造就了其强大的重吸收能力。

6. The loop of Henle: creating the medullary gradient | 髓袢：建立髓质渗透梯度

The loop of Henle is a U-shaped extension of the tubule that dips into the medulla and plays a critical role in producing a high solute concentration in the tissue fluid of the medulla, known as the medullary osmotic gradient. It consists of a descending limb and an ascending limb. The descending limb is thin-walled and permeable to water but relatively impermeable to sodium and chloride ions. As tubular fluid descends deeper into the medulla, water moves out by osmosis into the hypertonic interstitial fluid, concentrating the filtrate. The ascending limb, by contrast, is thicker in its distal portion and is impermeable to water. Its epithelial cells actively transport sodium and chloride ions out of the tubular fluid into the medullary interstitium, using the energy-consuming sodium–potassium pump on their basolateral membranes. This countercurrent multiplier mechanism builds up an osmotic gradient that increases from about 300 mOsm in the cortex to as high as 1200 mOsm at the tip of the papilla. This gradient is subsequently essential for the reabsorption of water in the collecting duct under the influence of ADH. Additionally, the hairpin arrangement of the vasa recta, the blood vessels that supply the medulla, acts as a countercurrent exchanger to preserve the gradient by removing reabsorbed water and solutes without washing them away.

髓袢是肾小管的一个U形延伸段，深入髓质，在髓质组织液中产生高溶质浓度——即髓质渗透梯度——的过程中起关键作用。它由降支和升支组成。降支壁薄，对水具有通透性，但对钠离子和氯离子相对不通透。当小管液深入髓质时，水通过渗透作用进入高渗的组织液中，使滤液浓缩。与之相反，升支的远段较粗，且对水不通透。其上皮细胞利用基底侧膜上的耗能钠钾泵，将钠离子和氯离子从小管液中主动转运至髓质间质。这种逆流倍增机制建立起一个渗透梯度，从皮质处的约300 mOsm增加至乳头尖部的约1200 mOsm。这一梯度随后对于在ADH作用下集合管对水的重吸收至关重要。此外，供应髓质的直小血管呈发夹状排列，充当逆流交换器，通过带走重吸收的水和溶质而不冲刷掉它们，从而维持了这一梯度。

7. The distal convoluted tubule: fine-tuning ion balance | 远曲小管：精细调节离子平衡

After the ascending limb of the loop of Henle re-enters the cortex, the tubule becomes the distal convoluted tubule (DCT). The DCT is shorter and has a less prominent brush border compared with the PCT, but its cells still contain many mitochondria to fuel active transport. Crucially, the epithelium of the DCT is involved in the regulated reabsorption of sodium ions and the secretion of potassium and hydrogen ions, under the influence of hormones such as aldosterone. Additionally, the DCT plays a role in the reabsorption of calcium ions, a process stimulated by parathyroid hormone. Because the DCT is impermeable to water in the absence of ADH, this segment helps to produce dilute urine when water conservation is not required. The DCT of several nephrons joins a collecting duct, marking the point where hormonal control begins to dominate over fixed reabsorptive pathways.

在髓袢升支重新进入皮质后，肾小管变为远曲小管。与近曲小管相比，远曲小管较短，刷状缘也不那么发达，但其细胞内仍含有许多线粒体，为主动运输提供能量。尤其重要的是，远曲小管的上皮在醛固酮等激素的影响下，参与了对钠离子的调节性重吸收，以及钾离子和氢离子的分泌。此外，远曲小管在甲状旁腺激素刺激下，还参与钙离子的重吸收。由于缺乏ADH时远曲小管对水不通透，这一节段有助于在无需保存水分时产生稀释的尿液。多个肾单位的远曲小管汇入同一条集合管，这一点标志着激素调控开始占据主导地位，超越固定的重吸收路径。

8. The collecting duct: water and urea regulation | 集合管：水分与尿素的调节

The collecting duct is not strictly part of the nephron, as it receives tubular fluid from several nephrons, but it is an integral part of the excretory system. It begins in the cortex and descends through the medulla, increasing in diameter as tributaries merge. The collecting duct is lined by two main cell types: principal cells and intercalated cells. Principal cells are the targets of antidiuretic hormone (ADH), which is released from the posterior pituitary gland when blood water potential is low. ADH binds to receptors on the basolateral membrane of principal cells, triggering a signalling cascade that causes vesicles containing aquaporin-2 water channels to fuse with the luminal membrane, making the duct highly permeable to water. Water then flows out into the hypertonic medullary interstitium by osmosis, a process that concentrates the urine. Intercalated cells are involved in acid–base balance by secreting either hydrogen ions or bicarbonate ions, thus helping to regulate blood pH. The collecting duct also allows some urea to diffuse out into the medulla, contributing to the recycling of urea that helps maintain the medullary osmotic gradient.

集合管严格来说并不属于肾单位，因为它接收来自多个肾单位的管液，但它是排泄系统不可分割的一部分。它起始于皮质，穿过髓质向下延伸，随着支流的汇合，其管径逐渐增大。集合管内衬有两种主要细胞：主细胞和闰细胞。主细胞是抗利尿激素（ADH）的靶细胞，ADH在血液水势降低时由垂体后叶释放。ADH与主细胞基底侧膜上的受体结合，触发信号级联反应，导致含有水通道蛋白2水通道的囊泡与管腔膜融合，使集合管对水高度通透。随后，水通过渗透作用流入高渗的髓质间质，这一过程使尿液得以浓缩。闰细胞则通过分泌氢离子或碳酸氢根离子，参与酸碱平衡调节，从而帮助稳定血液pH值。集合管还允许一部分尿素扩散进入髓质，有助于尿素的再循环，以此为维持髓质渗透梯度做出贡献。

9. Blood supply of the kidney | 肾脏的血液供应

The kidneys receive a remarkably high blood flow—approximately 20–25% of the cardiac output at rest—delivered via the renal arteries that branch directly from the abdominal aorta. The renal artery divides into interlobar arteries that run between the renal pyramids, then arcuate arteries at the cortex–medulla boundary, and finally smaller interlobular arteries that radiate into the cortex. From the interlobular arteries arise the afferent arterioles, each of which supplies a single glomerulus. The blood that is not filtered passes into the narrower efferent arterioles. The efferent arteriole then gives rise to a second capillary network: in cortical nephrons this forms the peritubular capillaries that surround the PCT and DCT, while in juxtamedullary nephrons it forms the vasa recta that descend alongside the loops of Henle into the medulla. The peritubular capillaries and vasa recta provide an enormous surface area for reabsorbing water and solutes, and they eventually drain into the interlobular veins, arcuate veins, interlobar veins, and finally the renal vein, which returns blood to the inferior vena cava. This portal-like arrangement—an arteriole sandwiched between two capillary beds—is unique and is essential for allowing high-pressure filtration in the glomerulus while maintaining low-pressure reabsorption in the peritubular network.

肾脏的血流量异常丰富——静息时约占心输出量的20–25%——血液通过直接从腹主动脉分出的肾动脉供应。肾动脉分支为肾叶间动脉，穿行于肾锥体之间，继而在皮髓质交界处形成弓形动脉，最后再分为向皮质辐射的较小的小叶间动脉。从小叶间动脉上发出入球小动脉，每条入球小动脉供应单个肾小球。未被滤过的血液进入较细的出球小动脉。出球小动脉随后再发出第二个毛细血管网：在皮质肾单位中形成包围近曲小管和远曲小管的管周毛细血管，而在近髓肾单位中则形成直小血管，沿髓袢深入髓质。管周毛细血管和直小血管为重吸收水和溶质提供了巨大的表面积，并且最终汇入小叶间静脉、弓形静脉、叶间静脉，并最终进入肾静脉，将血液送回下腔静脉。这种类似门脉系统的排列——一条小动脉夹在两个毛细血管床之间——是独一无二的，并且对于在肾小球内实现高压滤过、同时在管周网络维持低压重吸收至关重要。

10. Cortical and juxtamedullary nephrons | 皮质肾单位与近髓肾单位

There are two distinct populations of nephrons that differ in their location and structure: cortical nephrons and juxtamedullary nephrons. Cortical nephrons make up about 85% of all nephrons; their renal corpuscles are located in the outer part of the cortex, and their loops of Henle are relatively short, only just dipping into the outer medulla. These nephrons are primarily involved in routine filtration and reabsorption under normal conditions. In contrast, juxtamedullary nephrons have their corpuscles situated near the cortex–medulla boundary, and they possess exceptionally long loops of Henle that plunge deep into the medulla. The vasa recta associated with these nephrons are also long and well-developed. Because of their long loops, juxtamedullary nephrons are the main drivers of the countercurrent multiplier system and are therefore essential for producing concentrated urine. The structural differences between these two nephron types are a key example of form suiting function at the organ level.

存在两类位置和结构各不相同的肾单位：皮质肾单位和近髓肾单位。皮质肾单位约占全部肾单位的85%；它们的肾小体位于皮质外层，髓袢相对较短，仅稍稍伸入外髓质。这些肾单位主要在正常情况下执行常规的滤过和重吸收功能。相反，近髓肾单位的肾小体靠近皮髓质交界处，并且拥有异常长的髓袢，可深入髓质深处。与这些肾单位相关联的直小血管也很长且发达。由于其长袢的存在，近髓肾单位是逆流倍增系统的主要推动力量，因此对于产生浓缩尿液至关重要。这两类肾单位的结构差异，是器官层面上形态适应功能的典型例证。

Feature 特征	Cortical nephron 皮质肾单位	Juxtamedullary nephron 近髓肾单位
Location of renal corpuscle 肾小体位置	Outer cortex 皮质外部	Near corticomedullary junction 靠近皮髓质交界
Length of loop of Henle 髓袢长度	Short, barely reaches outer medulla 短，仅达外髓质	Long, extends deep into inner medulla 长，深入内髓质
Vasa recta 直小血管	Reduced or absent 不发达或缺失	Long and well-developed 长且发达
Role 作用	Routine filtration and reabsorption 常规滤过与重吸收	Production of concentrated urine 产生浓缩尿液

11. Microscopic tissue organisation | 显微镜下的组织排列

Examining transverse sections across the cortex and medulla reveals how the kidney’s tubules and vessels are packed. In the cortex, circular profiles representing proximal convoluted tubules are easily recognised by their tall cuboidal cells with a well-developed brush border and a relatively small, irregular lumen. Distal convoluted tubules, by contrast, have a wider, clearer lumen and lack a prominent brush border, with fewer nuclei visible per cross-section. Glomeruli appear as spherical tangles of cells and capillaries within the Bowman’s capsule. The medulla is dominated by parallel arrays of thin limb segments and collecting ducts, with the vasa recta running alongside. Recognising these histological features is important for the A-Level practical endorsement and helps reinforce understanding of how each tubular section’s cellular specialisation supports its function. Furthermore, the number of mitochondria visible in different segments reflects the energy demand for active transport—abundant in PCT, thick ascending limb, and DCT, but sparser in the thin descending limb.

在显微镜下观察皮质和髓质的横切面，可以揭示肾脏小管和血管的排列方式。在皮质中，代表近曲小管的圆形切面很容易辨认，其上皮为高立方状细胞，具有发达的刷状缘，管腔较小且不规则。与之相比，远曲小管的管腔更大、更清晰，缺少明显的刷状缘，每个切面中的细胞核数目较少。肾小球呈球形，看起来像是鲍曼囊内细胞和毛细血管的缠绕体。髓质中则以平行排列的薄壁段小管和集合管为主导，直小血管穿插其中。识别这些组织学特征对于A-Level的实验技能认可评估很重要，同时也有助于巩固理解每段肾小管的细胞特化如何支持其功能。此外，不同节段中线粒体数量的多少反映了主动运输的能量需求——在近曲小管、髓袢升支粗段和远曲小管中丰富，而在降支细段中则较为稀少。

12. Integrating structure and function | 结构与功能的整合

Every structural feature of the kidney can be interpreted in functional terms. The simple squamous epithelium and fenestrated capillaries of the glomerulus permit rapid, pressure-driven filtration. The brush border and basal membrane infoldings of the PCT maximise reabsorption of bulk solutes and water. The countercurrent multiplier arrangement of the loop of Henle depends on the contrasting permeability properties of its descending and ascending limbs, which are the direct result of differences in their membrane composition and transport machinery. The presence of connecting segments between the DCT and the collecting duct provides anatomical sites for the action of aldosterone and ADH, linking the kidney’s structure to the body’s hormonal regulation of salt and water. Even the dual capillary bed arrangement—glomerular and peritubular—ensures that filtration and reabsorption can proceed in series without competing for pressure. Understanding the kidney’s structure therefore provides a powerful narrative of physiological logic that is central to the A-Level Biology specification for Cambridge. The interplay between anatomy, cellular biology, and biochemistry makes the kidney a model organ for studying the principle that form underlies function.

肾脏的每一个结构特征都可以从功能的角度加以解读。肾小球的单层扁平上皮和有窗孔毛细血管允许快速的、压力驱动的滤过作用。近曲小管的刷状缘和基底膜内褶最大化了对大量溶质和水分的重吸收。髓袢的逆流倍增排列依赖于其降支和升支截然不同的通透性，这直接源于它们在膜组成和转运机制上的差异。远曲小管和集合管之间的连接段为醛固酮和ADH的作用提供了解剖学场所，将肾脏的结构与机体对盐和水的激素调控联系起来。甚至肾小球和管周毛细血管这种双毛细血管床的排列，也确保了滤过和重吸收能够串联进行，而不会对压力产生竞争。因此，理解肾脏的结构，为掌握A-Level剑桥生物学大纲核心的生理学逻辑提供了强有力的叙述。解剖学、细胞生物学和生物化学之间的相互作用，使肾脏成为研究“形态决定功能”这一原理的典型器官。

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Published by TutorHao | Biology Revision Series | aleveler.com

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