📚 345. Digestion and Absorption in Humans | 人体消化与吸收
The human digestive system is a long, muscular tube that runs from the mouth to the anus. Its main function is to break down large, insoluble food molecules into small, soluble substances that can be absorbed into the bloodstream and used by cells for energy, growth and repair. This process involves both mechanical digestion (physical breakdown) and chemical digestion (enzymatic hydrolysis). In IGCSE Biology, understanding how the alimentary canal and associated organs work together is essential, as it forms the basis of nutrition and metabolism.
人体的消化系统是一条从口腔延伸到肛门的肌肉长管。其主要功能是将大分子不溶性食物分解为可溶的小分子物质，以便被吸收进入血液，供细胞用于能量、生长和修复。这一过程同时涉及机械消化（物理性粉碎）和化学消化（酶催化水解）。在IGCSE生物学中，理解消化管及相关器官如何协同工作是基础，因为它构成了营养与代谢的核心。
1. Overview of the Alimentary Canal | 消化管概述
The alimentary canal (gut) is a continuous tube that includes the mouth, oesophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine (caecum, colon, rectum) and anus. Accessory organs — the salivary glands, liver, gall bladder and pancreas — lie outside the tube but secrete enzymes and other substances into it. The wall of the canal is muscular, allowing peristaltic movement that pushes food along.
消化管（肠道）是一条连续的管道，包括口腔、食管、胃、小肠（十二指肠、空肠、回肠）、大肠（盲肠、结肠、直肠）和肛门。附属器官——唾液腺、肝脏、胆囊和胰腺——位于管道之外，但会向其中分泌酶和其他物质。消化管的管壁由肌肉构成，能够通过蠕动推动食物前进。
Digestion can be subdivided into mechanical digestion, which physically breaks food into smaller pieces without changing its chemical nature, and chemical digestion, where enzymes hydrolyse large insoluble polymers into small soluble monomers. Mechanical digestion begins in the mouth with chewing and continues in the stomach with churning; chemical digestion occurs mainly in the mouth, stomach and small intestine.
消化可分为机械消化和化学消化。机械消化将食物物理性地粉碎而不改变其化学性质，而化学消化则通过酶将大分子不溶性聚合物水解为可溶的小分子单体。机械消化始于口腔的咀嚼，并在胃中通过搅动持续进行；化学消化主要发生在口腔、胃和小肠。
2. Mouth and Salivary Glands | 口腔与唾液腺
Inside the mouth, teeth mechanically break down food, increasing the surface area for enzyme action. Three pairs of salivary glands secrete saliva, a watery fluid that contains the enzyme salivary amylase. Saliva also contains mucus to lubricate food, making it easier to swallow, and has a pH close to 7.0, which is optimal for amylase.
在口腔中，牙齿对食物进行机械粉碎，增大了酶作用的表面积。三对唾液腺分泌唾液，其中含有唾液淀粉酶。唾液中还含有黏液，起润滑作用以便于吞咽，其pH接近7.0，是淀粉酶的最适pH。
Salivary amylase catalyses the hydrolysis of starch (a polysaccharide) into the disaccharide maltose. This is the first step in carbohydrate digestion. The reaction can be summarised as:
唾液淀粉酶催化淀粉（一种多糖）水解为二糖麦芽糖。这是碳水化合物消化的第一步。该反应可总结为：
Starch + H₂O → Maltose
淀粉 + 水 → 麦芽糖
However, food remains in the mouth only for a short time, so only a small fraction of starch is digested there. The bolus is then pushed to the back of the mouth by the tongue and swallowed.
然而，食物在口腔中停留时间很短，因此只有少量淀粉在此被消化。随后，食团被舌头推到口腔后部，并被吞咽。
3. Oesophagus and Peristalsis | 食管与蠕动
The oesophagus is a muscular tube that connects the pharynx to the stomach. During swallowing, the epiglottis closes over the trachea to prevent food from entering the airway. Once the bolus enters the oesophagus, waves of muscle contraction — called peristalsis — push it down towards the stomach.
食管是连接咽部和胃的肌肉质管道。吞咽时，会厌软骨盖住气管，防止食物进入气道。食团进入食管后，波状肌肉收缩——即蠕动——将其推向胃部。
Peristalsis involves the circular muscles behind the bolus contracting while the circular muscles ahead relax. Longitudinal muscles also contract rhythmically, shortening the tube. This process occurs throughout the entire alimentary canal and does not rely on gravity; it can even move food upwards if you are upside down.
蠕动涉及食团后方的环肌收缩、前方的环肌舒张。纵肌也有节律地收缩，使管道缩短。这一过程贯穿整个消化管，不依赖于重力；即使倒立，食物仍可被推送上去。
4. Stomach: Site of Protein Digestion | 胃：蛋白质消化的场所
The stomach is a muscular, J-shaped organ that churns food and mixes it with gastric juice, forming a semi-liquid mixture called chyme. Gastric juice is secreted by gastric glands in the stomach lining and contains hydrochloric acid (HCl), pepsinogen and mucus.
胃是一个J形的肌肉器官，通过搅动食物并将其与胃液混合，形成半液态的食糜。胃液由胃壁中的胃腺分泌，含有盐酸、胃蛋白酶原和黏液。
Hydrochloric acid kills most bacteria that enter with food and provides an acidic environment, pH 1.5–2.0, which is the optimum pH for the enzyme pepsin. Pepsin, initially released as inactive pepsinogen, is activated by HCl and then begins to hydrolyse proteins into shorter polypeptide chains. This is an example of chemical digestion.
盐酸可杀死随食物进入的大多数细菌，并提供pH 1.5–2.0的酸性环境，这是胃蛋白酶的最适pH。胃蛋白酶最初以无活性的胃蛋白酶原形式分泌，被盐酸激活后开始将蛋白质水解为较短的肽链。这属于化学消化。
Mucus lines the stomach wall, protecting it from being digested by pepsin and from corrosion by acid. Mechanical churning also continues to break down food particles. After 2–4 hours, chyme is gradually released into the duodenum through the pyloric sphincter.
黏液覆盖在胃壁上，保护其不被胃蛋白酶消化和胃酸腐蚀。机械搅动持续粉碎食物颗粒。2–4小时后，食糜通过幽门括约肌逐渐排入十二指肠。
5. Duodenum: The Enzyme Hub | 十二指肠：酶的中枢
The duodenum is the first, C-shaped section of the small intestine. Here, chyme is mixed with three crucial secretions: pancreatic juice from the pancreas, bile from the liver and gall bladder, and intestinal juice from the intestinal glands. This mixture neutralises stomach acid and completes the digestion of carbohydrates, proteins and fats.
十二指肠是小肠的第一段，呈C形。在此，食糜与三种关键分泌液混合：来自胰腺的胰液、来自肝脏和胆囊的胆汁，以及来自肠腺的肠液。这一混合液中和了胃酸，并完成碳水化合物、蛋白质和脂肪的消化。
Pancreatic juice contains several enzymes: pancreatic amylase (continues starch digestion), trypsin (a protease that digests polypeptides into shorter peptides) and lipase (breaks fats into fatty acids and glycerol). Bicarbonate ions in pancreatic juice raise the pH to about 8, which is optimum for these enzymes.
胰液含有多种酶：胰淀粉酶（继续消化淀粉）、胰蛋白酶（一种蛋白酶，将多肽分解为更短的肽）和脂肪酶（将脂肪分解为脂肪酸和甘油）。胰液中的碳酸氢根离子将pH提高到约8，这对这些酶最为有利。
Bile, produced by the liver and stored in the gall bladder, does not contain digestive enzymes. Instead, bile salts emulsify fat: they break large fat globules into tiny droplets, greatly increasing the surface area for lipase to attack. Chyme entering the duodenum also triggers the release of intestinal juice, which contains maltase, sucrase and peptidases to complete digestion of disaccharides and small peptides.
胆汁由肝脏生成并储存于胆囊，不含消化酶。然而，胆盐能乳化脂肪：将大的脂肪球打碎成微小液滴，大大增加了脂肪酶作用的表面积。进入十二指肠的食糜还会触发肠液的释放，其中含有麦芽糖酶、蔗糖酶和肽酶，以完成二糖和小肽的消化。
6. Enzymatic Breakdown of Food Molecules | 食物分子的酶促分解
The table below summarises the main digestive enzymes, their substrates, products and sources. All these reactions are examples of hydrolysis — water molecules split the chemical bonds of the substrate.
下表总结了主要的消化酶、其底物、产物和来源。所有这些反应都是水解反应——水分子断裂底物的化学键。
| Enzyme | Substrate | Products | Source |
|---|---|---|---|
| Salivary amylase | Starch | Maltose | Salivary glands |
| Pancreatic amylase | Starch | Maltose | Pancreas |
| Maltase | Maltose | Glucose | Small intestine |
| Pepsin | Protein | Polypeptides | Stomach |
| Trypsin | Polypeptides | Shorter peptides | Pancreas |
| Peptidases | Short peptides | Amino acids | Small intestine |
| Lipase | Fats (lipids) | Fatty acids + glycerol | Pancreas |
It is important to note that enzymes are specific; each acts only on its particular substrate. Also, end-products such as glucose, amino acids, fatty acids and glycerol are now small enough to be absorbed through the wall of the ileum.
重要的是，酶具有专一性；每种酶只能作用于其特定的底物。此外，终产物如葡萄糖、氨基酸、脂肪酸和甘油，此时已足够小，能够通过回肠壁被吸收。
7. Bile and Emulsification | 胆汁与乳化作用
Bile is a yellowish-green fluid produced continuously by the liver and stored in the gall bladder. When chyme enters the duodenum, the gall bladder releases bile via the bile duct. Bile contains bile salts, bile pigments (from haemoglobin breakdown), cholesterol and water, but no digestive enzymes.
胆汁是由肝脏持续生成的黄绿色液体，储存于胆囊中。当食糜进入十二指肠时，胆囊通过胆管释放胆汁。胆汁含有胆盐、胆色素（来自血红蛋白的分解）、胆固醇和水，但不含消化酶。
The key role of bile in digestion is the emulsification of fats. Bile salts have a hydrophobic side and a hydrophilic side, allowing them to coat small fat droplets and prevent them from coalescing. This physical process splits large fat globules into millions of tiny droplets, dramatically increasing the surface area available for lipase to act, thus speeding up digestion.
胆汁在消化中的关键作用是乳化脂肪。胆盐具有疏水端和亲水端，能够包裹小的脂肪微滴并防止它们重新聚集。这一物理过程将大的脂肪球分散成数百万个微小液滴，极大地增加了脂肪酶作用的表面积，从而加快了消化速度。
Because bile does not chemically change fats, emulsification is an example of mechanical digestion, even though it occurs in the small intestine.
由于胆汁并未化学改变脂肪，乳化作用是机械消化的一个实例，哪怕它发生在小肠中。
8. Absorption in the Ileum | 回肠内的吸收
Most absorption of the products of digestion occurs in the ileum, the final and longest part of the small intestine. Glucose, amino acids, water-soluble vitamins and mineral ions are absorbed into the blood capillaries inside the villi. Fatty acids and glycerol are absorbed into the lacteals (lymphatic vessels) of the villi.
消化产物的大部分吸收发生在回肠，即小肠最长也是最后的一段。葡萄糖、氨基酸、水溶性维生素和矿物质离子通过绒毛内的毛细血管被吸收。脂肪酸和甘油则被吸收到绒毛的乳糜管（淋巴管）中。
Absorption happens by several processes. Simple diffusion moves small, lipid‑soluble molecules and some mineral ions down their concentration gradient. However, most nutrients such as glucose and amino acids are absorbed by active transport using carrier proteins and ATP, allowing uptake even against a concentration gradient. Water is absorbed by osmosis, following the absorption of solutes.
吸收通过多种过程进行。简单扩散使脂溶性小分子和一些矿物质离子顺浓度梯度移动。然而，大多数营养素如葡萄糖和氨基酸通过主动运输被吸收，需要载体蛋白和ATP，即使逆浓度梯度也能摄入。水则随着溶质的吸收，通过渗透作用被吸收。
9. Adaptations of the Small Intestine for Absorption | 小肠对吸收的适应
The ileum is highly adapted for efficient absorption. Its structure provides an enormous surface area and short diffusion distances, maximising the rate of uptake.
回肠在结构上高度适应高效吸收。其结构提供了巨大的表面积和短扩散距离，使吸收速率最大化。
- Length: The small intestine is about 6–7 metres long in an adult, providing ample time for digestion and absorption. / 长度：成人小肠长约6–7米，为消化和吸收提供了充足时间。
- Villi: The inner lining is folded into millions of finger-like projections called villi (singular: villus). Each villus is ~1 mm long and contains a dense network of blood capillaries and a central lacteal. / 绒毛：内壁折叠成数百万个指状的突起，称为绒毛。每个绒毛长约1毫米，含有密集的毛细血管网和一条中央乳糜管。
- Microvilli: The epithelial cells covering each villus have their cell membrane further folded into microscopic projections, the microvilli, forming a 'brush border'. This increases the surface area enormously. / 微绒毛：覆盖绒毛的上皮细胞，其细胞膜进一步折叠为微小的微绒毛，形成“刷状缘”，极大地增加了表面积。
- Thin epithelium: The wall of each villus is only one cell thick, minimising the diffusion distance for nutrients to reach the capillary or lacteal. / 薄上皮：每个绒毛的肠壁仅有一个细胞的厚度，最大限度地缩短了营养素进入毛细血管或乳糜管的扩散距离。
- Rich blood supply: Each villus is supplied by an arteriole and drained by a venule, maintaining a steep concentration gradient by continuously carrying away absorbed products. / 丰富的血液供应：每个绒毛由一根微动脉供血、一根微静脉引流，通过持续运走吸收产物来维持陡峭的浓度梯度。
10. Large Intestine and Egestion | 大肠与排遗
Material that remains undigested and unabsorbed enters the large intestine. This includes dietary fibre (mainly cellulose), dead gut cells, bacteria and water. The large intestine consists of the caecum, appendix, colon, rectum and anus.
未被消化和吸收的物质进入大肠，包括膳食纤维（主要是纤维素）、脱落的肠细胞、细菌和水。大肠由盲肠、阑尾、结肠、直肠和肛门组成。
The main functions of the large intestine are to absorb water and mineral salts from the remaining material, and to provide storage for faeces. As water is absorbed, the contents become more solid. A large population of symbiotic bacteria in the colon ferments some fibre and synthesises vitamin K and certain B vitamins, which are then absorbed.
大肠的主要功能是从剩余物质中吸收水分和矿物质盐，并储存粪便。随着水分被吸收，内容物变得更为固体。结肠中有大量共生细菌，它们发酵部分纤维，并合成维生素K和某些B族维生素，随后被人体吸收。
Egestion is the removal of undigested, solid waste (faeces) through the anus. This is not excretion because the material has never been inside body cells. Fibre increases the bulk of faeces and aids regular bowel movements, reducing the risk of constipation.
排遗是指通过肛门将未消化的固体废物（粪便）排出。这不属于排泄，因为该物质从未进入过体细胞。膳食纤维增加粪便体积，帮助规律排便，降低便秘风险。
11. Summary of Key Reactions and pH Optima | 关键反应与最适pH总结
Below is a concise overview of the major digestive events, linking each stage to its optimal pH and end-product.
以下是对主要消化事件的简要概述，将每个阶段与其最适pH和终产物联系起来。
Mouth: Starch → Maltose (pH 7, salivary amylase)
口腔：淀粉 → 麦芽糖（pH 7，唾液淀粉酶）
Stomach: Protein → Polypeptides (pH 1.5–2, pepsin)
胃：蛋白质 → 多肽（pH 1.5–2，胃蛋白酶）
es → shorter peptides (pH 8, trypsin); Fats → fatty acids + glycerol (pH 8, lipase + bile)
十二指肠：淀粉 → 麦芽糖（pH 8，
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