5.2 Excretion | 5.2 排泄

📚 5.2 Excretion | 5.2 排泄

Excretion is the removal of metabolic waste products generated by chemical reactions in living cells. It is a vital homeostatic process that prevents the accumulation of toxic substances, regulates water potential and maintains pH balance. In mammals, the kidneys are the primary excretory organs, but the lungs, liver and skin also play essential roles. A clear understanding of deamination, ultrafiltration, selective reabsorption and osmoregulation forms the core of the A‑level syllabus for this topic. This revision guide breaks down every key concept you must master for the exam.

排泄是指将活细胞代谢反应产生的废物排出体外的过程。这是一个重要的稳态调节方式，可防止有毒物质积累，调控水势并维持pH平衡。在哺乳动物中，肾脏是主要的排泄器官，肺、肝和皮肤也发挥关键作用。透彻理解脱氨、超滤、选择性重吸收和渗透调节是该部分A‑level考纲的核心。本复习指南逐一剖析你必须掌握的各个关键概念。

1. Excretion vs. Egestion | 排泄与排遗的区分

Excretion specifically refers to the elimination of metabolic wastes—substances produced inside cells as by‑products of metabolism, such as carbon dioxide, urea and bile pigments. In contrast, egestion (defecation) removes undigested food residue that has never entered the body cells. Egestion is part of the digestive system, not the excretory system. Exams frequently test this distinction, so be precise: faeces are egested, not excreted.

排泄特指代谢废物的清除——这些物质是在细胞内产生的，属于新陈代谢的副产品，例如二氧化碳、尿素和胆色素。而排遗（排便）则是排出从未进入身体细胞的未消化食物残渣。排遗属于消化系统，而非排泄系统。考试中经常测试这一区别，务必精确：粪便是排遗产物，不是排泄产物。

2. Major Excretory Organs and Waste Products | 主要排泄器官与对应的废物

The body employs several organs to excrete different wastes. The table below links each organ to its principal excretory products and origin.

身体利用多个器官来排出不同的废物。下表将每个器官与其主要排泄产物及来源联系起来。

• Lungs – excrete carbon dioxide (CO₂) and water vapour produced by aerobic respiration.
• Kidneys – excrete urea, excess water, excess mineral salts (Na⁺, K⁺, Cl⁻) and creatinine from protein metabolism and muscle activity.
• Skin – excrete water, sodium chloride and small amounts of urea in sweat through sweat glands.
• Liver – produces urea from excess amino acids via deamination; bile pigments (bilirubin) are excreted into faeces via the gall bladder.

• 肺 — 排泄有氧呼吸产生的二氧化碳(CO₂)和水蒸气。
• 肾脏 — 排泄尿素、多余的水分、多余的矿物盐(Na⁺、K⁺、Cl⁻)以及来自蛋白质代谢和肌肉活动的肌酐。
• 皮肤 — 通过汗腺以汗液形式排泄水分、氯化钠和少量尿素。
• 肝脏 — 通过脱氨作用将过量氨基酸转化为尿素；胆色素（胆红素）经胆囊排入粪便。

3. The Liver and Deamination | 肝脏与脱氨基作用

Amino acids in excess of the body’s need cannot be stored. The liver breaks them down through deamination: the amino group (–NH₂) is removed, forming ammonia (NH₃) and a keto acid. Ammonia is highly toxic and must be converted immediately. The liver uses the ornithine cycle (urea cycle) to combine ammonia with carbon dioxide, producing urea, which is much less toxic. Urea is then released into the blood and transported to the kidneys for elimination. The remaining keto acid can be respired or converted to fat.

超过身体需求的氨基酸无法储存。肝脏通过脱氨基作用将其分解：去除氨基(–NH₂)，形成氨(NH₃)和酮酸。氨毒性极强，必须立即转化。肝脏利用鸟氨酸循环（尿素循环）将氨与二氧化碳结合，生成毒性低得多的尿素。尿素随后释放入血并被运输到肾脏排出。剩余的酮酸可用于呼吸作用或转化为脂肪。

4. Gross Anatomy of the Kidney | 肾脏的大体解剖结构

A longitudinal section of a kidney reveals three distinct regions. The outer cortex appears granular and contains the renal corpuscles and convoluted tubules. The inner medulla is striated and contains the loops of Henle and collecting ducts. The renal pelvis is a funnel‑shaped cavity that collects urine from the collecting ducts and channels it into the ureter. The renal artery brings unfiltered blood, and the renal vein carries filtered blood away. The ureter conveys urine to the bladder.

肾脏的纵切面显示出三个不同的区域。外层的皮质呈颗粒状，含有肾小体和曲部小管。内部的髓质有条纹，包含亨勒袢和集合管。肾盂是一个漏斗状的空腔，收集来自集合管的尿液并将其导入输尿管。肾动脉将未过滤的血液输入，肾静脉则运走过滤后的血液。输尿管将尿液输送至膀胱。

5. The Nephron – The Functional Unit | 肾单位——功能单元

Each kidney contains about one million nephrons, the microscopic tubules that produce urine. A nephron consists of the renal corpuscle (Bowman’s capsule and glomerulus), the proximal convoluted tubule (PCT), the loop of Henle (descending and ascending limbs), the distal convoluted tubule (DCT) and the collecting duct. The glomerulus is a knot of capillaries inside Bowman’s capsule. The afferent arteriole delivers blood to the glomerulus, and the efferent arteriole carries it away. The surrounding peritubular capillaries reabsorb useful substances.

每个肾脏含约一百万个肾单位，它们是生成尿液的结构和功能单位。一个肾单位由肾小体（鲍曼氏囊和肾小球）、近曲小管(PCT)、亨勒袢（降支和升支）、远曲小管(DCT)和集合管组成。肾小球是鲍曼氏囊内的一团毛细血管。入球小动脉将血液输入肾小球，出球小动脉则将血液运出。周围的管周毛细血管负责重吸收有用物质。

6. Ultrafiltration in the Bowman’s Capsule | 鲍曼氏囊中的超滤

Ultrafiltration occurs in the renal corpuscle. The afferent arteriole has a wider diameter than the efferent arteriole, generating a high hydrostatic pressure in the glomerular capillaries (~55 mmHg). This pressure forces water, glucose, amino acids, urea and inorganic ions (Na⁺, K⁺, Cl⁻, HCO₃⁻) out of the capillary and through the three‑layer filtration barrier: the capillary endothelium (with fenestrations), the basement membrane (a mesh of collagen fibres) and the podocyte foot processes (filtration slits). Blood cells and large plasma proteins (e.g. albumin) are retained because they are too large. The resulting filtrate entering Bowman’s capsule is protein‑free and isotonic with plasma.

超滤发生在肾小体中。入球小动脉的管径比出球小动脉粗，使得肾小球毛细血管内产生较高的静水压（约55 mmHg）。这一压力迫使水分、葡萄糖、氨基酸、尿素和无机离子(Na⁺、K⁺、Cl⁻、HCO₃⁻)穿过滤过屏障排出。该屏障由三层构成：有孔毛细血管内皮、基膜（胶原纤维网状结构）和足细胞足突（滤过裂隙）。血细胞和大分子血浆蛋白（如白蛋白）因体积过大而被截留。由此进入鲍曼氏囊的滤液不含蛋白质，且与血浆等渗。

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

About 65–70% of the glomerular filtrate is reabsorbed in the PCT. The epithelial cells lining the PCT are adapted with microvilli (providing a large surface area), numerous mitochondria (for ATP supply) and co‑transporter proteins. Sodium ions are actively pumped out of the cells into the blood by Na⁺/K⁺‑ATPase, creating a low Na⁺ concentration inside the cell. This allows Na⁺ to diffuse down its gradient from the filtrate into the cell through co‑transport proteins, bringing glucose and amino acids with it. Water follows by osmosis. All glucose and amino acids, most water and a large portion of mineral ions are reabsorbed here. Urea and other wastes remain in the filtrate.

约65–70%的肾小球滤液在近曲小管中被重吸收。PCT的上皮细胞具有微绒毛（增大表面积）、丰富的线粒体（供给ATP）和协同转运蛋白。钠离子通过Na⁺/K⁺‑ATP酶被主动泵出细胞进入血液，使胞内Na⁺浓度降低。这使得Na⁺沿浓度梯度从滤液扩散入细胞，并通过协同转运蛋白携带葡萄糖和氨基酸进入。水通过渗透作用跟随。所有葡萄糖和氨基酸、大部分水以及大量无机离子在此被重吸收。尿素及其他废物则留在滤液中。

8. The Loop of Henle and Countercurrent Multiplication | 亨勒袢与逆流倍增机制

The loop of Henle creates a concentration gradient in the medulla, allowing the production of concentrated urine. The descending limb is permeable to water but impermeable to Na⁺ and Cl⁻. As filtrate descends deeper into the hypertonic medulla, water leaves by osmosis, concentrating the filtrate. The thin ascending limb is impermeable to water but allows passive diffusion of Na⁺ and Cl⁻ into the medulla. The thick ascending limb actively transports Na⁺ and Cl⁻ out via a symport, further raising medullary osmolarity. This countercurrent multiplier effect establishes an osmolarity gradient from ~300 mOsm in the cortex to ~1200 mOsm in the inner medulla.

亨勒袢在髓质中形成浓度梯度，使浓缩尿的产生成为可能。降支对水通透，但对Na⁺和Cl⁻不通透。随着滤液深入高渗髓质，水因渗透作用离开，滤液逐渐浓缩。薄壁升支对水不通透，但允许Na⁺和Cl⁻被动扩散入髓质。厚壁升支通过协同运输蛋白主动将Na⁺和Cl⁻泵出，进一步提高髓质渗透压。这种逆流倍增效应建立起从皮层约300 mOsm至内髓约1200 mOsm的渗透压梯度。

9. ADH and Osmoregulation in the Collecting Duct | 抗利尿激素与集合管的水调节

As the filtrate passes through the DCT and collecting duct, its final concentration is adjusted according to the body’s hydration status. The collecting duct runs through the hypertonic medulla; if its walls are permeable to water, water exits by osmosis, producing a small volume of concentrated urine. Antidiuretic hormone (ADH) controls this permeability. When blood water potential drops (during dehydration), osmoreceptors in the hypothalamus stimulate the posterior pituitary to release ADH. ADH binds to receptors on collecting duct cells, triggering vesicles containing aquaporin‑2 channels to fuse with the luminal membrane, making it water‑permeable. As the body rehydrates, ADH secretion falls, aquaporins are removed and dilute urine is produced.

当滤液流经远曲小管和集合管时，其最终浓度会根据体内的水合状态进行调整。集合管穿过高渗的髓质；若管壁对水通透，水便因渗透作用流出，产生少量浓缩尿。抗利尿激素(ADH)控制此通透性。当血液水势下降（脱水时），下丘脑内的渗透压感受器刺激垂体后叶释放ADH。ADH与集合管细胞上的受体结合，触发含有水通道蛋白‑2的囊泡与管腔膜融合，使管壁对水通透。当身体水分充足后，ADH分泌减少，水通道蛋白被移除，产生稀释尿液。

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

If the kidneys fail to filter blood effectively, toxic urea and excess ions accumulate, disturbing osmotic balance and pH. The two main dialysis treatments are haemodialysis and peritoneal dialysis. In haemodialysis, blood is pumped through a dialysis machine where it flows counter‑current to dialysis fluid across a partially permeable membrane. The dialysis fluid contains normal levels of glucose and electrolytes but no urea, so urea diffuses out of the blood. Excess ions are also removed down concentration gradients. Peritoneal dialysis uses the patient’s peritoneum as a natural membrane; dialysis fluid is infused into the abdominal cavity and wastes move into it. Both methods replace the kidney’s excretory function but cannot replicate its endocrine roles.

若肾脏不能有效过滤血液，有毒的尿素和多余离子就会积累，扰乱渗透平衡和pH。两种主要透析疗法是血液透析和腹膜透析。血液透析中，血液被泵入透析机，与透析液在半透膜两侧逆向流动。透析液含有正常浓度的葡萄糖和电解质但不含尿素，因此尿素经扩散从血液进入透析液。多余离子也沿浓度梯度被移除。腹膜透析利用患者自身的腹膜作为天然膜；将透析液注入腹腔，废物便移入其中。两种方法均可替代肾脏的排泄功能，但无法复制其内分泌功能。

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