Unit 8 Structure and functions of the human gas exchange system
Key unit competence: Describe the structure and functions of the human gas exchange system.
Key ideas
- Cells need oxygen for respiration (glucose + oxygen → carbon dioxide + water + energy) and must remove carbon dioxide. Gas exchange is the movement of oxygen and carbon dioxide in opposite directions across a surface, by diffusion (from high to low concentration). Breathing (ventilation) is the movement of air into and out of the lungs. They are not the same.
- Pathway of air: nostrils → nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. The lungs lie in the thorax, protected by the ribs, with intercostal muscles between the ribs and the diaphragm below.
- The nasal cavity has hairs and mucus that filter dust and germs; its blood vessels warm the air and the mucus moistens it. The epiglottis closes the trachea when we swallow. The larynx (voice box) produces sound.
- The trachea (windpipe) is kept open by C-shaped rings of cartilage. It is lined with ciliated cells and goblet cells (mucus): mucus traps dust and germs and cilia sweep it up to the throat. The trachea divides into two bronchi (one to each lung), which branch into many bronchioles.
- Bronchioles end in clusters of tiny air sacs, the alveoli: the gas exchange surface. Adaptations: very large surface area (millions of alveoli), walls one cell thick (short diffusion distance), moist lining (gases dissolve), dense network of capillaries and constant ventilation (keeps a steep concentration gradient).
- In the alveoli oxygen diffuses into the blood (joins haemoglobin) and carbon dioxide diffuses from the blood into the alveolus to be breathed out.
- Inhalation: external intercostal muscles contract, ribs move up and out; the diaphragm contracts and flattens; the volume of the thorax increases, pressure decreases, air flows in. Exhalation: intercostal muscles relax (internal intercostals contract in forced breathing), ribs move down and in; the diaphragm relaxes and domes up; volume decreases, pressure increases, air is pushed out.
- The breathing rate increases with exercise, is higher in young children, rises with fever and illness and at high altitude (less oxygen per breath).
- Diseases: asthma (narrow airways, wheezing), bronchitis (inflamed bronchi), pneumonia (fluid in alveoli), tuberculosis (bacterial, cough for more than 3 weeks), common cold, influenza, COVID-19 (viral), lung cancer and emphysema (mainly from smoking). Keep lungs healthy: do not smoke, avoid smoke and dust, cover coughs, ventilate rooms, exercise, eat well, seek early treatment.
Common misconceptions
- "We breathe in oxygen and breathe out carbon dioxide only." No: we breathe in and out air; exhaled air has less oxygen (about 16%) and more carbon dioxide (about 4%) than inhaled air (21% oxygen, 0.04% CO₂).
- "The lungs suck air in by themselves." No: the lungs have no muscles; the diaphragm and intercostal muscles change the volume of the thorax.
- "Breathing and respiration are the same." No: breathing moves air; respiration is the chemical release of energy in cells.
- "During inhalation the diaphragm moves up." No: it contracts and flattens (moves down).
Worked examples
Example 1. Explain how air enters the lungs during inhalation.
Solution
The external intercostal muscles contract, moving the ribs up and out; the diaphragm contracts and flattens. The volume of the thorax increases, so the pressure inside falls below atmospheric pressure, and air rushes in through the trachea to the alveoli.
Example 2. Give four features of alveoli that make gas exchange efficient.
Solution
A very large total surface area; walls only one cell thick (short diffusion distance); a moist surface where gases dissolve; a rich capillary network that keeps carrying oxygen away and bringing carbon dioxide.
Practice questions
List, in order, the structures air passes through from the nostrils to the alveoli.
Show answer
Nostrils, nasal cavity, pharynx, larynx, trachea, bronchus, bronchiole, alveolus.
Common mistake: Putting bronchioles before bronchi.
Distinguish between breathing and gas exchange.
Show answer
Breathing is the movement of air into and out of the lungs by muscles. Gas exchange is the diffusion of oxygen into the blood and carbon dioxide out of the blood across the alveolar surface.
Common mistake: Using "respiration" for breathing.
State the function of: (a) cartilage rings; (b) cilia; (c) epiglottis.
Show answer
(a) Keep the trachea and bronchi open. (b) Sweep mucus with dust and germs up to the throat. (c) Covers the trachea during swallowing so food does not enter.
Common mistake: Saying cartilage helps air flow by moving.
Describe the changes in the ribs, diaphragm, volume and pressure during exhalation.
Show answer
Intercostal muscles relax, ribs move down and in; diaphragm relaxes and becomes dome-shaped; volume of the thorax decreases; pressure increases above atmospheric, so air is pushed out.
Common mistake: Saying volume increases during exhalation.
Explain four adaptations of the alveoli for gas exchange.
Show answer
Large surface area; thin walls one cell thick; moist lining; good blood supply (many capillaries); constantly ventilated.
Common mistake: Listing features without explaining how they help.
Name three respiratory diseases and give one way to prevent each.
Show answer
Tuberculosis: vaccination (BCG), early treatment, ventilated rooms. Lung cancer: do not smoke. Influenza/COVID-19: cover coughs, wash hands, vaccination.
Common mistake: Naming diseases of other systems.
Give four factors that change the rate of breathing.
Show answer
Physical activity, age, illness (fever), altitude; also emotions such as fear.
Common mistake: Listing "breathing" as a factor.
Explain why smokers often cough in the morning.
Show answer
Smoke damages and paralyses cilia, so mucus with dirt collects in the airways overnight and must be coughed out.
Common mistake: Saying smoke makes extra oxygen.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. Hairs and mucus in the nose: a. Carbon dioxide 2. Which is the correct order of air flow? b. filter dust and germs from the air 3. During inhalation the diaphragm: c. contracts and flattens 4. Which gas diffuses from the blood into the alveoli? d. Trachea, bronchus, bronchiole, alveolus Show answer
1 → b (filter dust and germs from the air) · 2 → d (Trachea, bronchus, bronchiole, alveolus) · 3 → c (contracts and flattens) · 4 → a (Carbon dioxide)
- They clean the incoming air.
- Air passes through narrower and narrower tubes.
- Flattening increases the volume of the thorax.
- CO₂ is breathed out.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
asthma · bronchi · tuberculosis · rings of cartilage- The trachea is kept open by .
- Narrowing of the airways causing wheezing is typical of .
- The trachea divides into two .
Show answer
1: rings of cartilage · 2: asthma · 3: bronchi
- Cartilage stops the trachea collapsing.
- Asthma attacks can be triggered by dust, smoke or allergies.
- One bronchus goes to each lung.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. The site of gas exchange in the lungs is the: a. more carbon dioxide and less oxygen 2. Compared with inhaled air, exhaled air contains: b. intercostal muscles 3. The muscles between the ribs are the: c. epiglottis 4. The flap that closes the trachea during swallowing is the: d. alveolus Show answer
1 → d (alveolus) · 2 → a (more carbon dioxide and less oxygen) · 3 → b (intercostal muscles) · 4 → c (epiglottis)
- Alveoli are tiny thin-walled air sacs.
- Exhaled air still has about 16% oxygen.
- Inter = between, costa = rib.
- It stops food entering the airway.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
lung cancer · cilia · rickets · one cell thick- The walls of the alveoli are .
- The cells that sweep mucus out of the trachea have .
- Long-term smoking is a major cause of .
Show answer
1: one cell thick · 2: cilia · 3: lung cancer
- A thin wall gives a short diffusion distance.
- Cilia beat towards the throat.
- Tar in smoke contains cancer-causing chemicals.