Human Nutrition and Gas Exchange
FoundationFood is broken down step by step by enzymes in the mouth, stomach and small intestine and absorbed through villi; the air sacs offer a vast, moist surface for exchange, while the diaphragm and intercostal muscles create pressure differences that draw air in and push it out.
- Mechanical digestion grinds and mixes food; chemical digestion uses enzymes to break large molecules into small absorbable ones — the two work hand in hand.
- The small intestine is where digestion peaks: bile emulsifies fat while pancreatic and intestinal juices break carbohydrates, proteins and lipids down completely into glucose, amino acids, glycerol and fatty acids.
- The small intestine multiplies its absorbing surface with length, folds, villi and microvilli, keeps the diffusion path short with a single-cell epithelium, and removes nutrients quickly via capillaries and lacteals.
文體小知識：科學知識說明文——追蹤食物在消化道的逐站旅程和空氣在呼吸系統的進出路線，講解消化酶、絨毛吸收與氣囊交換的結構適應，是人體系統單元的開篇。
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1Humans are heterotrophs: we cannot make our own food and must eat ready-made organic matter. A balanced diet needs carbohydrates, lipids, proteins, vitamins, minerals, dietary fibre and water in proportions that vary with age, activity level and physiological state. Large food molecules cannot cross cell membranes directly; they must first be digested into small, soluble molecules before the body can absorb them.
2Digestion is both mechanical and chemical: chewing and peristalsis break food into smaller pieces and mix it, while digestive enzymes split large molecules into small ones. In the mouth, salivary amylase breaks starch into maltose; hydrochloric acid in the stomach kills bacteria and provides the acidic environment in which pepsin breaks proteins into peptides; the small intestine is where most digestion happens.
3The small intestine is the hub of digestion and absorption. Bile made by the liver emulsifies fat into tiny droplets, enlarging the surface for enzymes; pancreatic and intestinal juices carry enzymes that break carbohydrates, proteins and lipids down completely into glucose, amino acids, glycerol and fatty acids. Folds, villi and microvilli multiply the absorbing surface enormously, and nutrients enter the blood through capillaries and lacteals.
4The breathing system consists of the nasal cavity, trachea, bronchi, bronchioles and air sacs. The air sacs are the site of gas exchange: they are extremely numerous, giving a vast total surface area; their walls are a single layer of flattened cells lying against a dense network of capillaries; and their inner surface is moist, so oxygen and carbon dioxide diffuse across quickly to serve every cell of the body.
5Ventilation is driven by pressure differences. During inhalation the diaphragm contracts and flattens while the external intercostal muscles lift the ribs up and out; the chest cavity enlarges, the pressure inside the lungs falls below atmospheric pressure and air flows in — an active process. In quiet exhalation these muscles relax, the chest recoils and air is pushed out — a passive process. The breathing rhythm is controlled by the breathing centre in the medulla oblongata.
1Summary 重點摘要
Food is broken down step by step by enzymes in the mouth, stomach and small intestine and absorbed through villi; the air sacs offer a vast, moist surface for exchange, while the diaphragm and intercostal muscles create pressure differences that draw air in and push it out.
2Vocabulary cards Vocabulary
The physical breakdown of food by chewing and peristalsis, which grinds and mixes it without changing its chemical nature.
The process by which digestive enzymes break carbohydrates, proteins and lipids into small, absorbable molecules.
The enzyme in saliva that breaks starch down into maltose in the mouth.
The breaking of large fat masses into tiny droplets by bile, enlarging the surface for lipase; bile itself contains no enzyme.
A finger-like projection of the small intestine wall containing capillaries and a lacteal, greatly enlarging the absorbing surface.
Tiny projections on the epithelial cells of villi that enlarge the absorbing surface even further.
A tiny pouch at the end of the airways where gas exchange takes place; thin-walled, moist and wrapped in capillaries.
The muscular sheet dividing chest from abdomen; when it contracts and flattens it drives inhalation.
The movement of air into and out of the lungs, driven by pressure differences created as the chest volume changes.
The removal of undigested, unabsorbed food remains (faeces) from the body through the anus.
3Step by step Steps
- Step 1: Human dietary needsHumans are heterotrophs: unlike plants we cannot make our own food and must eat ready-made organic matter. A balanced diet should include carbohydrates, lipids, proteins, vitamins, minerals, dietary fibre and water: carbohydrates and lipids mainly supply energy, proteins build and repair tissue, vitamins and minerals regulate metabolism, and fibre keeps the gut moving. Needs vary with age, activity level and physiological state. For example, growing teenagers need extra protein and calcium to grow and strengthen bones, while pregnant women need extra iron to make blood for themselves and the foetus.
- Step 2: The overall structure of the digestive systemThe alimentary canal is a long tube from mouth to anus, running mouth, oesophagus, stomach, small intestine, large intestine and anus, with three groups of accessory glands — salivary glands, liver and pancreas — pouring digestive juices into it. Food is pushed along by peristalsis: circular and longitudinal muscles alternate their contractions to grind, mix and advance the food, and this mechanical digestion greatly increases the contact area between food and enzymes. For example, a mouthful of rice is pushed down the oesophagus, then churned by rhythmic stomach muscles into semi-liquid chyme — all without conscious control.
- Step 3: Chemical digestion, enzyme by enzymeChemical digestion relies on enzymes, each acting on a specific substrate. Salivary amylase in the mouth breaks starch into maltose; hydrochloric acid in the stomach kills bacteria and supplies the acidic conditions for pepsin to break proteins into peptides; the small intestine is the main battlefield, where pancreatic and intestinal enzymes finish the job. For example, starch becomes maltose in the mouth and is then split into glucose by maltase in the small intestine before absorption, while bile emulsifies fat into droplets so lipase can quickly convert it into glycerol and fatty acids.
- Step 4: How the small intestine is built for absorptionThe small intestine is the main site of absorption, and its adaptations are layered: it is long; its inner wall bears circular folds; the folds carry dense villi; and each villus's epithelial cells carry microvilli — each level multiplies the absorbing surface. The villus wall is a single layer of cells, shortening the diffusion distance; inside each villus run a capillary network and a lacteal, so glucose and amino acids enter the blood while lipid products mainly travel by lacteal, and rapid removal keeps the concentration gradient steep. The large intestine absorbs the remaining water and salts, moulding the residue into faeces egested through the anus. For example, picture the small intestine as a long tunnel lined with finger-shaped villi, themselves bristling with microvilli; the total absorbing surface far outstrips a smooth tube, so nutrients are snapped up as soon as they are broken down.
- Step 5: Airways and air sacs of the breathing systemAir, warmed, moistened and filtered in the nasal cavity, passes in turn through the trachea, bronchi and bronchioles before reaching the air sacs. The air sacs are the site of gas exchange, and their adaptations make it highly efficient: they are numbered in hundreds of millions, giving an enormous total surface; each wall is a single layer of flattened cells, so the diffusion path is very short; a dense capillary network hugs the outside, sweeping oxygen away the moment it enters; and the inner surface is moist, because gases must dissolve in the film of water before diffusing. For example, where sac wall and capillary wall meet, the barrier is barely two cells thick, and oxygen crosses into the blood within seconds.
- Step 6: The mechanics of inhaling and exhalingVentilation is driven by pressure differences: air always flows from high to low pressure. During inhalation the diaphragm contracts and flattens and the external intercostal muscles pull the ribs up and out; the chest cavity enlarges, the lungs expand with it, the pressure inside falls below atmospheric pressure and air flows in — an active process. In quiet exhalation these muscles relax, the chest recoils elastically, the pressure inside rises above atmospheric and air is pushed out — a passive process. For example, during strenuous exercise breathing deepens and quickens as the internal intercostals and abdominal muscles join in, making exhalation active and greatly increasing ventilation to meet the demand for oxygen.
- Step 7: Gas exchange and the transport routeGas exchange proceeds by diffusion, its direction set by concentration gradients. In the air sacs, oxygen is more concentrated than in the blood, so it crosses the sac wall into the capillaries and binds to haemoglobin in red blood cells for transport; carbon dioxide brought by the blood diffuses the other way and leaves with each breath. In body tissues the situation reverses: cells constantly respire, consuming oxygen and releasing carbon dioxide, so oxygen leaves the blood and diffuses into the cells. For example, when you run, your leg muscles burn oxygen so fast that the tissue concentration plummets and haemoglobin releases extra oxygen — automatic adjustment that supplies exactly the right amount.
4Diagrams Diagrams
5Worked example Example
A student eats a pizza made from flour (mainly starch) and cheese (mainly protein and lipid). List, in order, where each of the three food classes is digested along the alimentary canal, the digestive juices and enzymes involved, and the final products absorbed; then state which section does most of the absorption and two structural adaptations related to absorption.
- Starch is partly broken into maltose by salivary amylase in the mouth; in the small intestine it is completely broken into glucose by enzymes in the pancreatic and intestinal juices.
- Protein is broken into peptides by pepsin in the stomach (in the acidic environment of hydrochloric acid); on reaching the small intestine it is broken into amino acids by proteases in the pancreatic and intestinal juices.
- Lipid is first emulsified into tiny droplets by bile in the small intestine, then broken into glycerol and fatty acids by lipases in the pancreatic and intestinal juices.
- Absorption happens mainly in the small intestine: villi and microvilli multiply the surface area in layers; the villus wall is a single layer of epithelial cells, shortening the diffusion path; and the capillaries and lacteal inside each villus carry nutrients away quickly, maintaining the concentration gradient.
- Answer: Starch: mouth (salivary amylase) to maltose, then small intestine (pancreatic and intestinal juices) to glucose. Protein: stomach (pepsin) to peptides, then small intestine to amino acids. Lipid: small intestine, emulsified by bile and split by lipases into glycerol and fatty acids. Glucose and amino acids are absorbed into the villus capillaries; lipid products mainly travel via the lacteal. Absorption occurs mainly in the small intestine, whose adaptations include folds, villi and microvilli enlarging the surface, and a single-cell-thick epithelium shortening the diffusion distance.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which activities in the alimentary canal count as mechanical digestion? Give two examples.
Q2What role does bile play in fat digestion?
Q3What are the final products of protein digestion, and where are they absorbed?
Q4What do the diaphragm and external intercostal muscles do during inhalation, and why does air flow into the lungs?
Q5What adaptations of the air sacs make gas exchange efficient?
8Recap Recap
- Mechanical digestion grinds and mixes food; chemical digestion uses enzymes to break large molecules into small absorbable ones — the two work hand in hand.
- The small intestine is where digestion peaks: bile emulsifies fat while pancreatic and intestinal juices break carbohydrates, proteins and lipids down completely into glucose, amino acids, glycerol and fatty acids.
- The small intestine multiplies its absorbing surface with length, folds, villi and microvilli, keeps the diffusion path short with a single-cell epithelium, and removes nutrients quickly via capillaries and lacteals.
- Four adaptations of the air sacs: huge numbers, single-cell walls, a moist lining and a dense capillary network — together they make oxygen and carbon dioxide exchange highly efficient.
- Inhalation is active (diaphragm flattens, ribs rise); quiet exhalation is passive; ventilation is driven by pressure differences and paced by the medulla oblongata.