Excretion: Removal of Metabolic Wastes | 排泄：代谢废物的去除

📚 Excretion: Removal of Metabolic Wastes | 排泄：代谢废物的去除

Excretion is the biological process by which organisms eliminate toxic metabolic waste products generated from cellular activities. Unlike egestion, which removes undigested food residues, excretion specifically deals with substances produced through metabolism, such as carbon dioxide, urea, and excess ions. Efficient excretion is vital for maintaining homeostasis, preventing the accumulation of harmful chemicals that could disrupt enzyme function, pH balance, and osmotic pressure within body fluids.

排泄是生物体清除细胞活动产生的有毒代谢废物的生物学过程。与排出未消化食物残渣的排遗不同，排泄专门处理代谢产生的物质，如二氧化碳、尿素和多余离子。有效的排泄对于维持内环境稳态至关重要，可防止有害化学物质累积，这些物质会扰乱体液中的酶功能、酸碱平衡和渗透压。

1. Defining Excretion and Its Importance | 排泄的定义及其重要性

Excretion should not be confused with egestion or secretion. Egestion is the removal of undigested material from the gut, while secretion refers to the release of useful substances from cells or glands. True excretion removes metabolic wastes that are often toxic if allowed to concentrate. In mammals, the primary excretory organs are the lungs (CO₂), kidneys (nitrogenous wastes and water), and skin (salts and water in sweat). Plants also excrete oxygen and water vapour, but lack a specialised excretory system, instead storing wastes in vacuoles or shedding leaves.

排泄不应与排遗或分泌混淆。排遗是从肠道排出未消化的物质，而分泌指细胞或腺体释放有用物质。真正的排泄是清除代谢废物，这些废物若累积起来往往有毒。哺乳动物的主要排泄器官是肺（排出CO₂）、肾（处理含氮废物和水）和皮肤（汗液中的盐分和水）。植物也会排泄氧气和水蒸气，但缺乏特化的排泄系统，而是将废物储存在液泡中或通过落叶丢弃。

The build-up of nitrogenous waste is particularly dangerous. Ammonia, produced during amino acid breakdown, is highly toxic and must be rapidly converted or diluted. Aquatic animals can excrete ammonia directly, but terrestrial animals convert it into less toxic urea or uric acid to conserve water. This illustrates how excretion is intimately linked with water balance and habitat adaptation.

含氮废物的积累尤为危险。氨基酸分解产生的氨毒性极强，必须迅速转化或稀释。水生动物可以直接排出氨，但陆生动物为节约水分会将其转化为毒性较低的尿素或尿酸。这说明排泄与水分平衡及栖息地适应密切相关。

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2. Metabolic Waste Products: CO₂, Urea, and Others | 代谢废物：二氧化碳、尿素及其他

The main metabolic wastes in humans are carbon dioxide from respiration, urea from protein metabolism, and excess mineral ions. Carbon dioxide dissolves in blood plasma to form carbonic acid, lowering pH and posing a threat to enzyme activity. The lungs maintain acid-base balance by ventilating CO₂, while the kidneys excrete hydrogen ions and reabsorb bicarbonate to maintain blood pH around 7.4.

人体主要的代谢废物是呼吸作用产生的二氧化碳、蛋白质代谢生成的尿素以及多余的矿物质离子。二氧化碳溶于血浆形成碳酸，降低pH值，对酶活性构成威胁。肺通过通气排出二氧化碳来维持酸碱平衡，而肾脏则排出氢离子并重吸收碳酸氢根，以维持血液pH值在7.4左右。

Urea is the primary nitrogenous waste in mammals. Its formation in the liver via the ornithine cycle combines two ammonia molecules with carbon dioxide, producing a relatively non-toxic compound that can be concentrated in urine. Other nitrogenous wastes include uric acid (from nucleic acid breakdown) and creatinine (from muscle metabolism). The liver plays a central role by deaminating excess amino acids, removing the amino group as ammonia, then converting it to urea.

尿素是哺乳动物主要的含氮废物。它在肝脏中通过鸟氨酸循环形成，将两分子氨与一分子二氧化碳结合，生成毒性相对较低的化合物，可浓缩于尿液中。其他含氮废物包括尿酸（来自核酸分解）和肌酐（来自肌肉代谢）。肝脏通过将过量氨基酸脱氨基，以氨的形式移除氨基后转化为尿素，发挥核心作用。

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3. The Human Urinary System: An Overview | 人体泌尿系统概述

The human excretory system for nitrogenous waste is the urinary system, comprising the kidneys, ureters, bladder, and urethra. The kidneys, situated at the back of the abdominal cavity, filter blood and produce urine. Urine flows via the two ureters into the muscular bladder for temporary storage, and is eliminated through the urethra during urination. The renal artery delivers blood to each kidney, while the renal vein carries away filtered blood.

人体处理含氮废物的排泄系统是泌尿系统，包括肾脏、输尿管、膀胱和尿道。肾脏位于腹腔后部，过滤血液并生成尿液。尿液经两条输尿管流入肌肉性膀胱暂存，排尿时经尿道排出。肾动脉将血液送入每个肾脏，肾静脉则运走过滤后的血液。

Each kidney contains about one million microscopic filtering units called nephrons. The nephron is where filtration, reabsorption, and secretion occur. The surrounding blood vessels play a critical role in maintaining the high pressure needed for ultrafiltration and supplying energy for active transport processes. Understanding the gross anatomy of the kidney – cortex, medulla, pelvis – is fundamental to grasping the urine formation process.

每个肾脏含有约一百万个称为肾单位的微观过滤单元。肾单位是过滤、重吸收和分泌发生的场所。周围的血管在维持超滤所需高压力以及为主动运输提供能量方面起着关键作用。理解肾脏的大体解剖——皮质、髓质和肾盂——是掌握尿液形成过程的基础。

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4. Kidney Structure: Cortex, Medulla, and Nephron | 肾脏结构：皮质、髓质与肾单位

A longitudinal section of the kidney reveals an outer cortex, an inner medulla, and a central pelvis that drains into the ureter. The cortex appears granular and contains the Bowman’s capsules, proximal and distal convoluted tubules. The medulla displays striped pyramids composed largely of loops of Henle and collecting ducts. The renal pelvis is a funnel-shaped cavity where urine collects before entering the ureter.

肾脏的纵剖面显示出外层的皮质、内层的髓质以及通向输尿管的中央肾盂。皮质呈颗粒状，包含鲍曼囊、近曲小管和远曲小管。髓质呈现出条纹状肾锥体，主要由亨利氏袢和集合管组成。肾盂是一个漏斗形腔室，尿液在进入输尿管前在此汇集。

The nephron spans both regions and is closely associated with a network of capillaries. Blood enters through the afferent arteriole into the glomerulus – a knot of capillaries inside the Bowman’s capsule. After filtration, blood exits via the efferent arteriole, which branches into peritubular capillaries entwining the nephron tubule. These capillaries later rejoin to form the renal vein. This arrangement ensures intimate contact between blood and tubular fluid, enabling efficient exchange.

肾单位跨越这两个区域，并与毛细血管网紧密相连。血液通过入球小动脉进入鲍曼囊内的毛细血管团——肾小球。过滤后，血液经出球小动脉离开，出球小动脉分支形成缠绕肾小管周围的管周毛细血管。这些毛细血管后来重新汇合形成肾静脉。这种结构保证了血液与管腔液的密切接触，从而实现高效的物质交换。

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

Each nephron is a long tubule with distinct regions specialised for different roles. It begins with the Bowman’s capsule, which encloses the glomerulus for ultrafiltration. Leading from the capsule is the proximal convoluted tubule (PCT), a highly coiled section with microvilli and numerous mitochondria for active reabsorption. The tubule then descends into the medulla as the loop of Henle, consisting of a thin descending limb and a thick ascending limb.

每个肾单位都是一条长小管，具有特化了不同功能的不同区域。它起始于包围肾小球的鲍曼囊，用于超滤。从囊中引出的是近曲小管（PCT），这是一个高度盘绕的部分，拥有微绒毛和大量线粒体，用于主动重吸收。随后小管作为亨利氏袢下降进入髓质，由细段降支和粗段升支组成。

The ascending limb returns to the cortex and transitions into the distal convoluted tubule (DCT). The DCT is shorter and less coiled than the PCT, but plays a key role in ion balance and pH regulation. Several nephrons join a collecting duct, which travels back through the medulla to empty urine into the renal pelvis. The collecting duct is now known to be more than a passive drain; it is under hormonal control for final water reabsorption.

升支返回皮质，移行为远曲小管（DCT）。DCT比PCT短且盘绕程度较低，但在离子平衡和pH调节中起关键作用。多个肾单位汇入一条集合管，集合管穿过髓质将尿液导入肾盂。如今已知集合管不仅是被动排放管道，还受激素控制进行最终的水分重吸收。

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6. Ultrafiltration in the Bowman’s Capsule | 鲍曼囊中的超滤作用

Ultrafiltration occurs when blood passes through the glomerular capillaries under high hydrostatic pressure. The afferent arteriole is wider than the efferent arteriole, creating a pressure build-up inside the glomerulus. This pressure forces water, ions, glucose, amino acids, and urea out of the blood and into the Bowman’s capsule, while blood cells and large proteins are retained due to their size. The filtration barrier consists of capillary endothelial pores, a basement membrane, and podocyte slits.

当血液在较高静水压下流经肾小球毛细血管时，发生超滤。入球小动脉比出球小动脉宽，在肾小球内产生压力积聚。这一压力迫使水、离子、葡萄糖、氨基酸和尿素离开血液进入鲍曼囊，而血细胞和大分子蛋白质则因其体积而被截留。滤过屏障由毛细血管内皮孔、基底膜和足细胞裂孔组成。

The resulting filtrate has a composition similar to blood plasma minus proteins. Around 125 cm³ of filtrate is produced per minute in the human kidneys, but over 99% of this volume is later reabsorbed. Filtration is passive and depends solely on pressure; any drop in blood pressure can severely impair kidney function. The glomerular filtration rate (GFR) is a key indicator of kidney health.

生成的滤液成分与血浆相似但不含蛋白质。人体肾脏每分钟产生约125 cm³滤液，但其中超过99%的体积随后被重吸收。过滤是被动的，仅依赖于压力；任何血压下降都可能严重损害肾功能。肾小球滤过率（GFR）是肾脏健康的关键指标。

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7. Selective Reabsorption in the Proximal Convoluted Tubule | 近曲小管中的选择性重吸收

The proximal convoluted tubule (PCT) is the main site for reclaiming useful solutes from the filtrate. All glucose, most amino acids, vitamins, and about 70% of filtered sodium ions are reabsorbed here, along with a substantial volume of water. The cells lining the PCT are packed with mitochondria, providing ATP for active transport. Microvilli on the luminal surface increase the surface area for absorption.

近曲小管（PCT）是从滤液中回收有用溶质的主要场所。所有葡萄糖、大部分氨基酸、维生素以及约70%的滤过钠离子都在此被重吸收，同时伴有大量水分。排布于PCT的细胞富含线粒体，为主动运输提供ATP。管腔内表面的微绒毛增大了吸收的表面积。

Sodium ions are actively pumped out of the PCT cells into the tissue fluid via sodium-potassium pumps on the basal membrane. This creates a low sodium concentration inside the cells, allowing Na⁺ to diffuse in from the filtrate through co-transporter proteins that simultaneously bring in glucose or amino acids. The movement of solutes into the tissue fluid lowers its water potential, causing water to follow by osmosis. Urea remains largely in the filtrate, though some passive reabsorption occurs.

钠离子通过基侧膜上的钠钾泵被主动泵出PCT细胞，进入组织液。这使得细胞内钠离子浓度降低，允许Na⁺通过协同转运蛋白从滤液中扩散进入细胞，同时带入葡萄糖或氨基酸。溶质向组织液的移动降低了其水势，导致水分通过渗透跟随进入。尿素则大部分留在滤液中，但也会发生一些被动重吸收。

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

The loop of Henle creates a concentration gradient in the medulla that allows the kidney to produce urine more concentrated than plasma. The descending limb is permeable to water but not to ions; water moves out by osmosis into the salty medullary interstitium, concentrating the tubular fluid as it descends. The thin ascending limb is impermeable to water but allows passive diffusion of Na⁺ and Cl⁻ into the interstitium.

亨利氏袢在髓质中建立了一个浓度梯度，使肾脏能够产生比血浆更浓的尿液。降支对水通透但对离子不通透；水通过渗透作用进入含盐的组织间液，使得小管液在下降过程中被浓缩。细段升支对水不通透，但允许Na⁺和Cl⁻被动扩散进入组织间液。

The thick ascending limb actively transports sodium and chloride ions out of the filtrate using a Na⁺-K⁺-2Cl⁻ co-transporter. This process, combined with the countercurrent flow of fluid in the two limbs, multiplies the gradient: the deeper into the medulla, the higher the solute concentration reaches, up to four times that of blood plasma. This countercurrent multiplication is essential for water conservation and is more developed in animals living in arid environments.

粗段升支利用Na⁺-K⁺-2Cl⁻协同转运蛋白主动将钠和氯离子运出滤液。这一过程结合两肢液体逆向流动，倍增了浓度梯度：越深入髓质，溶质浓度越高，可达血浆浓度的四倍。这种逆流倍增效应对水分保存至关重要，生活在干旱环境中的动物此结构更为发达。

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9. Distal Convoluted Tubule and Collecting Duct: Hormonal Control | 远曲小管和集合管：激素调控

The distal convoluted tubule (DCT) fine-tunes the reabsorption of sodium and calcium ions under the influence of hormones. Aldosterone, released from the adrenal glands when blood pressure drops, stimulates the DCT to reabsorb more Na⁺, which leads to water reabsorption and raises blood volume. Parathyroid hormone increases calcium reabsorption in the DCT. Simultaneously, potassium and hydrogen ions can be actively secreted into the filtrate to regulate electrolyte balance and pH.

远曲小管（DCT）在激素影响下精细调节钠离子和钙离子的重吸收。醛固酮在血压下降时从肾上腺释放，刺激DCT更多地重吸收Na⁺，从而引发水分重吸收，提高血容量。甲状旁腺激素增加DCT中钙的重吸收。同时，钾离子和氢离子可被主动分泌到滤液中，以调节电解质平衡和pH值。

The collecting duct receives urine from many nephrons and passes through the medullary gradient. Its permeability to water is controlled by antidiuretic hormone (ADH). When ADH is present, aquaporin channels are inserted into the cell membranes, allowing water to exit by osmosis into the hypertonic medulla, producing concentrated urine. In the absence of ADH, the collecting duct remains impermeable to water, and dilute urine is excreted. The collecting duct also participates in urea recycling, contributing to the medullary osmotic gradient.

集合管承接来自多个肾单位的尿液，穿过髓质梯度。它对水的通透性受抗利尿激素（ADH）控制。当ADH存在时，水通道蛋白被嵌入细胞膜，允许水通过渗透进入高渗的髓质，产生浓缩尿液。无ADH时，集合管保持不通透水，排泄稀薄尿液。集合管还参与尿素循环，帮助维持髓质的渗透梯度。

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10. Osmoregulation: ADH and Water Balance | 渗透调节：抗利尿激素与水平衡

Osmoregulation is the homeostatic control of the water potential of body fluids. The hypothalamus contains osmoreceptors that monitor the solute concentration of the blood. When the plasma becomes too concentrated, osmoreceptors shrink, triggering the posterior pituitary gland to release ADH into the bloodstream. ADH binds to receptors on collecting duct cells, initiating a signaling cascade that inserts aquaporin-2 proteins into the apical membrane.

渗透调节是体液中水势的稳态控制。下丘脑含有渗透压感受器，监测血液的溶质浓度。当血浆变得过浓时，渗透压感受器收缩，促使垂体后叶向血液释放ADH。ADH与集合管细胞上的受体结合，启动信号级联反应，将水通道蛋白-2插入顶端膜。

With more aquaporins, water reabsorption increases, urine volume falls, and the blood becomes more dilute. Once plasma water potential returns to normal, osmoreceptors stop stimulating ADH release. This negative feedback loop ensures precise control. Conversely, drinking large volumes of water suppresses ADH secretion, resulting in copious dilute urine. Disorders such as diabetes insipidus occur when ADH production or response is impaired, leading to excessive urination and dehydration.

有了更多水通道蛋白，水分重吸收增加，尿量减少，血液变稀。一旦血浆水势恢复正常，渗透压感受器停止刺激ADH释放。这一负反馈回路保证了精确调控。相反，大量饮水会抑制ADH分泌，导致大量稀薄尿液。当ADH生成或反应受损时，会出现如尿崩症等疾病，导致排尿过多和脱水。

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11. Formation of Urea: The Ornithine Cycle | 尿素的形成：鸟氨酸循环

Urea is produced exclusively in the liver through a cyclic pathway called the ornithine cycle (or urea cycle). The process begins with the deamination of excess amino acids in liver cells. The amino group (—NH₂) is removed, producing ammonia (NH₃) and a keto acid. Ammonia is highly toxic, so it is immediately combined with carbon dioxide to enter the ornithine cycle. The overall equation for urea synthesis is: 2NH₃ + CO₂ → (NH₂)₂CO + H₂O.

尿素仅在肝脏中通过一种称为鸟氨酸循环（或尿素循环）的环式途径生成。该过程始于肝细胞中过量氨基酸的脱氨基作用。氨基（—NH₂）被移除，产生氨（NH₃）和一个酮酸。氨毒性极高，因此它立即与二氧化碳结合进入鸟氨酸循环。尿素合成的总方程式为：2NH₃ + CO₂ → (NH₂)₂CO + H₂O。

The cycle starts with ornithine combining with ammonia and CO₂ to form citrulline, then aspartate contributes another amino group, eventually regenerating ornithine and releasing urea. This energy-dependent process uses ATP. The urea then diffuses into the bloodstream and is transported to the kidneys for excretion. The liver also converts ammonia from other sources, such as intestinal bacteria, maintaining a safe blood ammonia level.

循环起始于鸟氨酸与氨及CO₂结合形成瓜氨酸，然后天冬氨酸提供另一个氨基，最终重新生成鸟氨酸并释放尿素。这一耗能过程需要ATP。尿素随后扩散入血，被运送到肾脏排泄。肝脏还会转化来自肠道细菌等其他来源的氨，维持安全的血氨浓度。

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12. Kidney Failure and Dialysis | 肾衰竭与透析

Kidney failure occurs when the nephrons lose their ability to filter blood effectively. Causes include diabetes, chronic high blood pressure, and infections. As glomerular filtration rate declines, toxic wastes like urea and excess ions accumulate, causing symptoms such as fatigue, swelling, and electrolyte imbalances. In end-stage renal disease, the kidneys can no longer maintain homeostasis, requiring artificial filtration or transplantation.

当肾单位丧失有效过滤血液的能力时，就会发生肾衰竭。病因包括糖尿病、慢性高血压和感染。随着肾小球滤过率下降，尿素等毒性废物和多余离子积累，引起疲劳、水肿和电解质紊乱等症状。终末期肾病时，肾脏无法维持内稳态，需要人工过滤或移植。

Dialysis artificially reproduces kidney filtration. Haemodialysis pumps blood through a machine where it flows past a semipermeable membrane bathed in a carefully formulated dialysis fluid. Wastes diffuse down their concentration gradients from blood into the fluid, while needed substances are retained. Peritoneal dialysis uses the patient’s own peritoneal membrane. Both methods must be performed regularly to prevent waste build-up. Kidney transplantation offers a long-term solution but carries risks of rejection.

透析通过人工方式重现肾脏滤过。血液透析将血液泵经一台机器，在其中流经浸在精心配制的透析液中的半透膜。废物顺浓度梯度从血液扩散到透析液中，而所需物质则被保留。腹膜透析利用患者自身的腹膜。两种方法都必须定期进行以防止废物聚积。肾移植提供了长期解决方案，但存在排斥风险。

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