Gas Exchange

Principles of Gas Exchange

Why do organisms need efficient gas exchange?

• They require oxygen for aerobic respiration to produce ATP (required for nearly all life processes)
• They need to remove carbon dioxide produced by respiration — a build-up of CO₂ would lower the pH of body fluids and disrupt enzyme activity

Three general principles apply across every gas exchange system. Keep these in mind whenever a question asks about adaptations that increase the rate of gas exchange:

Large surface area

• A greater surface means that more diffusion is able to occur. This leads to a faster rate of exchange.
• This can be achieved by highly branched or folded structures (e.g. tracheoles in insects, bronchioles and alveoli in humans, lamellae in fish)

Short diffusion pathway (thin surface)

• The thinner the exchange surface, the faster gases can diffuse across it
• Most exchange surfaces are only one cell thick (e.g. alveolar epithelium, tracheole walls, gill lamellae)

Steep concentration gradient maintained

• The greater the difference in concentration across the surface, the faster the rate of diffusion
• This is maintained by a good blood supply (continuously carrying away absorbed gases) and ventilation (continuously bringing in fresh air/water)
• Examples: fish counter-current system, ventilation of alveoli

Single-celled Organisms

A single-celled organism such as an amoeba has a high surface area to volume ratio, so it doesn’t need a specialised gas exchange system. Gases simply diffuse in and out across the cell membrane.

Insects

Insects don’t use blood to transport oxygen. Instead, they have a network of air-filled tubes — the tracheal system — that delivers air directly to their tissues.

• Spiracles — small pores on the surface of the body where air enters and exits
• Tracheae — microscopic air-filled tubes (the main airways) leading inward from the spiracles
• Tracheoles — the tracheae branch into smaller tracheoles, which have thin, permeable walls and reach directly into the respiring tissues

Oxygen diffuses down its concentration gradient from the air into the tracheoles, across the thin tracheole walls, and into the respiring cells.

Carbon dioxide moves the other way to oxygen. Down its concentration gradient, out of the cells, along the tracheoles and tracheae, and out through the spiracles.

In active insects, body movements (e.g. abdominal pumping) help move air in and out.

Fish

Fish live in water, which has a much lower oxygen concentration than air. Efficient gas exchange is essential; this is achieved by the gills.

Structure of the Gills

• Water enters through the mouth, flows over the gills, and exits through the operculum (a flap that covers the gills)
• Each gill is made of many thin gill filaments stacked on top of each other
• Each filament is covered with rows of gill lamellae — thin folds that further increase the surface area
• Inside each lamella is a dense network of capillaries carrying blood
• (Note: Lamellae = plural, lamella = singular)

Gas Exchange and the Counter-current System

As water flows over a lamella:

• Oxygen diffuses from the water → across the lamella → into the blood
• Carbon dioxide diffuses from the blood → out of the lamella → into the water

The key adaptation in fish is the counter-current principle:

• Blood flows through the capillaries in the lamellae in one direction
• Water flows over the lamellae in the opposite direction

Because they flow in opposite directions, the water always has a higher oxygen concentration than the blood next to it. This maintains a steep concentration gradient at every point, so oxygen continues to diffuse from water into blood all the way along.

Dicotyledonous Plants

Plants need gas exchange for two processes:

• Photosynthesis (during the day) — take in carbon dioxide, release oxygen
• Respiration (continuously) — take in oxygen, release carbon dioxide

Where gas exchange happens

• Inside the leaf, gases diffuse between the mesophyll cells and the air spaces in the spongy mesophyll (large surface area)
• Gases enter and leave the leaf through pores called stomata, typically found on the underside of the leaf
  • (Note: stomata = plural, stoma = singular)
• Each stoma is opened or closed by a pair of guard cells:
  • When water enters the guard cells (by osmosis), they become turgid and the stoma opens — allowing gas exchange (but water vapour also escapes)
  • When water leaves the guard cells, they become flaccid and the stoma closes — reducing water loss (but also limiting gas exchange)

This is why stomata are typically open during the day (when photosynthesis is happening) and closed at night (to reduce water loss when not needed).

Water Loss vs Gas Exchange

Terrestrial insects and xerophytic plants live in dry environments and must balance two opposing needs:

• They need gas exchange surfaces that are large, thin and moist, but these features also lose water rapidly by evaporation
• They need to limit water loss, but doing so also reduces gas exchange

Terrestrial Insects

Key adaptations for reducing water loss:

• Spiracles can close (using small valves) when oxygen demand is low — reduces water vapour escaping
• Waxy, waterproof cuticle on the exoskeleton — reduces evaporation across the body surface
• Hairs around the spiracles trap a layer of moist air — reduces the water potential gradient between the inside and outside, slowing evaporation

Xerophytic Plants

Xerophytes are plants adapted to dry, hot, or windy habitats (e.g. cacti, marram grass) where water loss must be minimised.

Key adaptations for reducing water loss:

• Sunken stomata (in pits) — trap moist air around the stoma, reducing the water potential gradient between the leaf and the outside, so less water diffuses out
• Hairs on the epidermis — trap a layer of moist air at the leaf surface, also reducing the water potential gradient between leaf and outside
• Rolled / curled leaves — enclose the stomata so they are sheltered from the wind (wind would normally remove moist air from around the stomata and steepen the water potential gradient, increasing water loss)
• Reduced number of stomata — fewer stomata for evaporation of water
• Thick waxy cuticle — waterproof barrier across the rest of the leaf surface
• Small leaves or spines (e.g. cacti) — reduce the surface area available for water loss

All of these adaptations reduce water loss but also reduce the rate of gas exchange, so xerophytes typically photosynthesise more slowly than well-watered plants.

Human Gas Exchange System

The human gas exchange system delivers oxygen from the atmosphere to the blood (and removes carbon dioxide). Air follows this pathway:

Trachea → Bronchi → Bronchioles → Alveoli

• Trachea — the main windpipe, supported by C-shaped rings of cartilage that keep it open
• Bronchi — the trachea splits into two bronchi, one leading to each lung
• Bronchioles — each bronchus branches repeatedly into smaller and smaller bronchioles
• Alveoli — tiny air sacs at the end of each bronchiole, where gas exchange takes place

The whole system is moved by the ribcage, intercostal muscles, and diaphragm during breathing (covered in the next section).

Alveolar Epithelium

Each alveolus is lined with a single layer of thin, flat cells — the alveolar epithelium. This is the actual exchange surface, sitting right next to a network of capillaries:

• Oxygen diffuses from the alveolus → across the alveolar epithelium → across the capillary endothelium → into the blood
• Carbon dioxide diffuses the other way (blood → alveolus)

Ventilation

Ventilation is the movement of air in and out of the lungs. It works by changing the volume of the thoracic cavity, which changes the pressure inside the lungs. Air then moves down the pressure gradient — from high to low pressure.

Inspiration (breathing in) — active

1. Diaphragm contracts and moves downwards
2. External intercostal muscles contract, pulling the ribcage upwards and outwards
3. Volume of the thoracic cavity increases
4. So pressure inside the lungs decreases below atmospheric pressure
5. Air moves down the pressure gradient — from outside (high) → into the lungs (low)

Inspiration is an active process because it requires muscle contraction (uses ATP).

Expiration (breathing out) — usually passive

1. Diaphragm relaxes and domes upwards (returns to its resting shape)
2. External intercostal muscles relax, allowing the ribcage to fall downwards and inwards (helped by gravity and the elastic recoil of the lungs)
3. Volume of the thoracic cavity decreases
4. So pressure inside the lungs increases above atmospheric pressure
5. Air moves down the pressure gradient — out of the lungs → to the atmosphere

Antagonistic Intercostal Muscles

The external and internal intercostal muscles work as an antagonistic pair (they contract in opposite directions):

• Inspiration: External intercostals contract during inspiration, pulling ribs up and out. The internal intercostal muscles relax.

• Forced Expiration: Internal intercostals contract during forced expiration (e.g. exercise), pulling ribs down and in to expel more air. The external intercostal muscles relax.

At rest, normal expiration is passive. The internal intercostals only contract during forced/active breathing out (e.g. during exercise).

See Antagonistic Pairs (skeletal muscles) for more detail on how antagonistic muscle pairs work in general — e.g. the biceps/triceps at the elbow.

Lung Disease & Risk Factors

A typical exam question in this section will give you data (a graph or table) about smoking or pollution and lung function, and ask you to:

1. Describe what the data shows
2. Explain the mechanism — how damage to the lungs reduces gas exchange
3. Sometimes evaluate whether the data is enough to prove cause (vs just a correlation)

Smoking and lung function example question Focus

Scientists measured the lung function of three groups of men from age 25 to age 75:

• non-smokers
• continuous smokers
• ex-smokers (men who smoked until age 50, then stopped)

Lung function was measured using FEV₁ (forced expiratory volume in 1 second — the volume of air a person can breathe out in 1 second), expressed as a percentage of the average value at age 25.

(a) Describe the effect of smoking on FEV₁ shown in Figure 1. (2 marks)

(b) Smoking damages alveoli, reducing the number of alveoli and thickening the alveolar walls. Explain how this reduces the rate of gas exchange in the lungs. (3 marks)

(5 marks)

Hint

For (a): comment on the trend in non-smokers AND how the smokers’ line compares. What happens when smokers quit at age 50?

For (b): think about the three principles of an efficient gas exchange surface — what happens to surface area, diffusion pathway, and rate of diffusion?

Mark Scheme

(a) — 2 marks

1. FEV₁ declines with age (visible in both non-smokers and smokers from age 25, and in ex-smokers from age 50 onwards) (1 mark)
2. Smokers’ FEV₁ declines faster than non-smokers (so smokers have lower FEV₁ at any given age) OR when smokers stop at age 50, their decline slows down (follows the non-smokers’ rate) (1 mark)

(b) — 3 marks

1. Fewer alveoli means reduced (total) surface area for gas exchange (1 mark)
2. Thickened alveolar walls mean a longer / increased diffusion pathway (1 mark)
3. This (reduced surface area and longer diffusion pathway) leads to a slower / reduced rate of (gaseous) diffusion of oxygen into the blood (and CO₂ out) (1 mark)

Comments from mark scheme

• For (a): describe the trend — don’t just read off individual values without context
• For (b): say the rate of diffusion is reduced (not just “less gas exchange”). Accept “slower diffusion”
• Examiners flag students who confuse “less” gas exchange with “slower” gas exchange — at A-level the rate is what matters
• This graph alone shows correlation between smoking and lung decline — not direct proof of cause. Other variables (e.g. air pollution, occupation) could also contribute

Exam Question Practice