Unit 8 Gas exchange in animals and smoking related risks
Key unit competence: Locate structures of the gas exchange system affected by smoking.
Key ideas
- Gas exchange surfaces share features: large surface area, thin (short diffusion distance), moist (gases dissolve), permeable, a good blood supply and ventilation to keep steep concentration gradients. Fick's law: rate of diffusion ∝ (surface area × concentration difference) ÷ thickness. As organisms grow, the surface area to volume ratio falls, so large animals need specialised organs.
- Insects use a tracheal system: air enters through spiracles (with valves and hairs to reduce water loss), passes along tracheae (supported by rings of chitin) and fine tracheoles that reach every cell. Tracheole ends contain fluid; during activity, lactic acid lowers the water potential of muscle cells, the fluid is withdrawn and air reaches closer to the cells. Large insects ventilate by abdominal pumping.
- Bony fish have four pairs of gills under an operculum. Each gill arch carries gill filaments bearing gill lamellae (secondary lamellae) with many capillaries. Ventilation: mouth opens, floor of buccal cavity lowers, water enters; mouth closes, floor rises, water is forced over gills and out through the operculum. Counter-current flow: blood and water flow in opposite directions, so a concentration gradient is maintained along the whole lamella and up to about 80% of O₂ is extracted (parallel flow gives about 50%).
- Cartilaginous fish (sharks) have no operculum; gill slits open separately; many use ram ventilation (swimming with mouth open) and flow is mostly parallel, so extraction is less efficient. Amphibians exchange gases through moist skin (cutaneous), the buccal cavity lining and simple sac-like lungs (ventilated by buccal pumping); tadpoles use gills.
- Human gas exchange system: nasal cavity → pharynx → larynx → trachea → bronchi → bronchioles → alveoli. Trachea and bronchi: C-shaped cartilage rings (keep airway open), ciliated epithelium and goblet cells (mucus traps dust and microbes; cilia sweep it up), smooth muscle and elastic fibres. Bronchioles: no cartilage, smooth muscle controls diameter. Alveoli: squamous epithelium one cell thick, elastic fibres (recoil), surfactant (lowers surface tension), dense capillaries; about 300 million alveoli give ~70 m².
- Ventilation: inspiration: external intercostal muscles contract (ribs up and out), diaphragm contracts and flattens, thoracic volume increases, pressure falls below atmospheric, air enters. Expiration (at rest, passive): muscles relax, elastic recoil of lungs, volume decreases, pressure rises, air leaves; forced expiration uses internal intercostals and abdominal muscles.
- Lung volumes (spirometer): tidal volume (~0.5 dm³ at rest), inspiratory and expiratory reserve volumes, vital capacity (TV + IRV + ERV, ~4.5–5 dm³), residual volume (~1.5 dm³, cannot be expelled), total lung capacity (VC + RV). Dead space (~0.15 dm³) is air in airways that does not reach alveoli. Pulmonary ventilation = tidal volume × breathing rate. A spirometer trace falls over time as O₂ is used and CO₂ is absorbed by soda lime; the slope gives O₂ consumption.
- Control of breathing: the ventilation centre in the medulla oblongata sends impulses along intercostal and phrenic nerves. Chemoreceptors in the medulla, carotid and aortic bodies detect a rise in CO₂ (fall in pH); the centre increases the rate and depth of breathing. Stretch receptors in the bronchioles stop inspiration (Hering–Breuer reflex).
- Smoking: tar paralyses cilia and causes mucus build-up (chronic bronchitis), contains carcinogens (lung cancer); emphysema: elastic fibres destroyed by phagocyte enzymes, alveoli break down, surface area falls, breathlessness (both are COPD); nicotine raises heart rate and blood pressure, makes platelets sticky and narrows arterioles; carbon monoxide binds haemoglobin forming carboxyhaemoglobin, reducing O₂ transport and damaging the endothelium, promoting atherosclerosis and coronary heart disease. Lung cancer signs: persistent cough, coughing blood, chest pain, weight loss.
- Plants: gases diffuse through stomata (leaves) and lenticels (stems) into air spaces of spongy mesophyll. Stomatal opening: in light, guard cells actively take in K⁺ (proton pumps), malate forms, water potential falls, water enters by osmosis, guard cells become turgid and, because the inner wall is thicker, they bow apart. In darkness or drought (abscisic acid) K⁺ leaves and stomata close.
Common misconceptions
- "We breathe in because the lungs suck in air." No: muscles increase thoracic volume, lowering pressure; air is pushed in by atmospheric pressure.
- "Oxygen is the main stimulus for breathing." No: in healthy people the main stimulus is a rise in carbon dioxide (fall in pH).
- "Vital capacity is all the air the lungs can hold." No: that is total lung capacity; vital capacity excludes the residual volume.
- "Counter-current flow means water and blood move the same way." No: they flow in opposite directions.
Worked examples
Example 1. A student breathes 15 times per minute with a tidal volume of 0.5 dm³ and a dead space of 0.15 dm³. Calculate pulmonary ventilation and alveolar ventilation.
Solution
Pulmonary ventilation = 0.5 × 15 = 7.5 dm³ min⁻¹. Air reaching alveoli per breath = 0.5 − 0.15 = 0.35 dm³, so alveolar ventilation = 0.35 × 15 = 5.25 dm³ min⁻¹.
Example 2. Explain why counter-current flow in bony fish is more efficient than parallel flow.
Solution
In counter-current flow, blood always meets water with a higher O₂ concentration than itself, so a diffusion gradient is maintained along the whole length of the lamella. Blood can leave with up to about 80% saturation. In parallel flow the two reach equilibrium halfway, so only about 50% is extracted.
Practice questions
State five features of efficient gas exchange surfaces.
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Large surface area; thin; moist; permeable; good blood supply; ventilation to keep a steep gradient.
Common mistake: Saying surfaces must be thick to be strong.
Describe the tracheal system of an insect.
Show answer
Spiracles with valves open into tracheae supported by chitin rings; tracheae branch into tracheoles that reach the cells; tracheole ends contain fluid; large insects pump the abdomen to ventilate.
Common mistake: Saying insects carry oxygen in blood.
Explain the ventilation mechanism in bony fish.
Show answer
Mouth opens and buccal floor lowers, water enters; operculum closes; mouth closes and floor rises, increasing pressure; water forced over gills and out through the opened operculum; one-way flow.
Common mistake: Saying water flows in and out of the mouth.
Describe the mechanism of inspiration in humans.
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External intercostal muscles contract, raising ribs up and out; diaphragm contracts and flattens; thoracic volume increases; pressure falls below atmospheric; air flows into lungs.
Common mistake: Saying the lungs expand first and pull the ribs.
Define tidal volume, vital capacity, residual volume and dead space.
Show answer
Tidal volume: air in or out per normal breath. Vital capacity: maximum air breathed out after maximum breath in. Residual volume: air left after maximum expiration. Dead space: air in airways not reaching alveoli.
Common mistake: Mixing vital capacity with total lung capacity.
Explain how breathing rate increases during exercise.
Show answer
Respiration produces more CO₂; blood pH falls; chemoreceptors in medulla, carotid and aortic bodies detect it; impulses to the ventilation centre; more impulses to intercostal muscles and diaphragm; faster, deeper breathing.
Common mistake: Saying low oxygen is the main trigger.
Explain how smoking causes chronic bronchitis and emphysema.
Show answer
Tar paralyses cilia and stimulates goblet cells; mucus builds up, infections, chronic bronchitis. Phagocytes entering lungs release elastase that digests elastic fibres; alveoli burst and fuse; surface area falls; emphysema.
Common mistake: Saying nicotine causes emphysema.
Describe the short-term effects of nicotine and carbon monoxide on the cardiovascular system.
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Nicotine stimulates adrenaline release: heart rate and blood pressure rise, arterioles narrow and platelets become sticky, so clots form more easily. Carbon monoxide combines with haemoglobin to form carboxyhaemoglobin, so less oxygen is carried; the heart works harder and the lining (endothelium) of arteries is damaged.
Common mistake: Saying carbon monoxide is the substance that causes addiction (it is nicotine).
Explain how smoking contributes to atherosclerosis and coronary heart disease.
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Carbon monoxide and other chemicals damage the endothelium of arteries; fatty deposits (cholesterol) build up under the damaged lining, forming plaques (atheroma); the lumen narrows and sticky platelets make clots more likely; if a coronary artery is blocked, part of the heart muscle gets no oxygen: angina or a heart attack.
Common mistake: Saying smoking only affects the lungs.
State three symptoms of lung cancer and three symptoms of COPD.
Show answer
Lung cancer: persistent cough, coughing up blood, chest pain, weight loss, tiredness. COPD (chronic bronchitis and emphysema): long-lasting cough with mucus (phlegm), breathlessness on exertion, wheezing, frequent chest infections.
Common mistake: Mixing up COPD with asthma, which is usually caused by allergies and can be reversed.
Students boil tobacco leaves in water, cool the liquid and spray it on a plant covered with aphids. Describe a suitable control and state what the result shows.
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Control: spray an identical plant with aphids using only boiled and cooled water (same volume, same conditions). The aphids die only on the plant sprayed with the tobacco extract, which shows that tobacco contains a poisonous substance (nicotine) that dissolves in water.
Common mistake: Forgetting the control, so the death of the aphids cannot be linked to the tobacco.
Explain the opening of stomata.
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In light, proton pumps in guard cells pump H⁺ out; K⁺ enters; malate forms; water potential falls; water enters by osmosis; guard cells become turgid and bow apart due to thicker inner walls.
Common mistake: Saying guard cells lose water to open.
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. A frog at rest exchanges most gases through its: a. medulla oblongata 2. Cartilaginous fish such as sharks differ from bony fish because they: b. moist skin 3. The substance in smoke that binds haemoglobin is: c. carbon monoxide 4. The breathing centre is located in the: d. have no operculum Show answer
1 → b (moist skin) · 2 → d (have no operculum) · 3 → c (carbon monoxide) · 4 → a (medulla oblongata)
- Cutaneous respiration.
- Their gill slits open separately.
- It forms carboxyhaemoglobin.
- It controls the intercostal muscles.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
spiracles · carbon dioxide · contracts and flattens · lenticels- Air enters the insect tracheal system through .
- During inspiration the diaphragm .
- The main chemical stimulus for breathing is a rise in blood .
Show answer
1: spiracles · 2: contracts and flattens · 3: carbon dioxide
- They can be closed by valves.
- Thoracic volume increases.
- It lowers blood pH.
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. C-shaped rings of cartilage are found in the: a. residual volume 2. Mucus in the airways is made by: b. emphysema 3. Air remaining in the lungs after maximum expiration is the: c. goblet cells 4. Breakdown of alveolar walls causing breathlessness is: d. trachea Show answer
1 → d (trachea) · 2 → c (goblet cells) · 3 → a (residual volume) · 4 → b (emphysema)
- They stop the airway collapsing.
- It traps dust and microbes.
- It cannot be breathed out.
- Surface area falls.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
lowers surface tension · operculum · cilia of the airways · spiracle- In bony fish, the gill cover is the .
- Tar in cigarette smoke damages the .
- Surfactant in the alveoli .
Show answer
1: operculum · 2: cilia of the airways · 3: lowers surface tension
- It protects the gills.
- Mucus then builds up.
- It stops alveoli collapsing.