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Gas Exchange in Animals and Plants

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Senior 5 · Biology

Gas Exchange in Animals and Plants

5 lessons

Lesson 1 of 5

Gas-exchange surfaces in insects, fish and amphibians

25 min

Why does a fish suffocate in air?

Air contains about 21% oxygen by volume, roughly thirty times more oxygen per litre than water. Yet a tilapia lifted out of a fish pond in Rwamagana gasps and dies within minutes. Out of water, the delicate gill plates collapse and stick together, and their huge surface shrinks to almost nothing. The gills work only in the medium they were built for. Every animal and plant faces the same task: getting oxygen in and carbon dioxide out fast enough. This lesson asks what makes a good gas-exchange surface, and then compares how insects, fish and amphibians solve the problem in different ways.

Definition

Gas exchange: the movement of oxygen and carbon dioxide, by diffusion, between an organism and its environment across a gas-exchange surface.

Ventilation: the movement of air or water over a gas-exchange surface, by breathing movements, which keeps the concentration gradients steep.

Surface area to volume ratio (SA:V): an organism's surface area divided by its volume. It falls as an organism gets larger.

Counter-current flow: flow of two fluids in opposite directions on either side of an exchange surface, as water and blood flow in a fish gill.

What makes a good exchange surface

Gases cross exchange surfaces by diffusion. Fick's law sums up what controls the rate:

\text{rate of diffusion} \propto \frac{\text{surface area} \times \text{difference in concentration}}{\text{thickness of the surface}}

So an efficient gas-exchange surface has:

a large surface area, often folded or branched;
a thin barrier, often one cell thick, for a short diffusion path;
a steep concentration gradient, kept up by ventilation on one side and blood flow on the other;
a moist surface, since gases must dissolve before they diffuse through cells; and it must be permeable to oxygen and carbon dioxide.

A single-celled organism, or a thin flatworm, can use its whole body surface, because its SA:V is large and no cell is far from the outside. Larger, active animals have a small SA:V and a high demand for oxygen, so they need a special surface and, usually, a blood system to carry the gases.

Worked example 1: surface area to volume ratio

Model two animals as cubes of side 1 mm and 5 mm. Calculate the SA:V of each, and explain what it means for gas exchange.

1 mm cube: surface area $= 6 \times 1^2 = 6\,\mathrm{mm^2}$ ; volume $= 1^3 = 1\,\mathrm{mm^3}$ ; SA:V $= \frac{6}{1} = 6 : 1$ .
5 mm cube: surface area $= 6 \times 5^2 = 150\,\mathrm{mm^2}$ ; volume $= 5^3 = 125\,\mathrm{mm^3}$ ; SA:V $= \frac{150}{125} = 1.2 : 1$ .
For a cube of side $L$ , SA:V $= \frac{6L^2}{L^3} = \frac{6}{L}$ : making the side 5 times longer makes the ratio 5 times smaller.

Meaning: each cubic millimetre of the larger animal has only a fifth as much surface to supply it, and its central cells are further from the surface. Its body surface alone cannot supply its oxygen needs.

Insects: air piped straight to the cells

An insect's body is covered by an exoskeleton with a waxy, waterproof cuticle, so very little gas exchange happens through its surface. Instead, air travels in tubes to every tissue (Figure 1).

Spiracles: pores along the sides of the thorax and abdomen. Valves open and close them; keeping them closed when possible cuts water loss.
Tracheae: tubes reinforced by spiral rings of chitin, which keep them open.
Tracheoles: very fine branches, less than 1 µm wide, that reach between the cells and end on the muscle fibres. Oxygen diffuses down the tubes to the cells; carbon dioxide diffuses out.

The tips of the tracheoles contain fluid. When a muscle is very active, lactate builds up in it, lowering its water potential, so water is drawn out of the tracheoles into the muscle; more of the tracheole surface then holds air, and oxygen diffuses in faster. Large, active insects such as locusts also ventilate: muscles squeeze the abdomen and pump air in and out of the tracheae. Because diffusion along air tubes is only efficient over short distances, the tracheal system is one reason why insects stay small.

Fish: a one-way flow and a clever trick

A bony fish has four pairs of gills under a bony flap, the operculum. Each gill is a curved gill arch bearing many thin gill filaments, and each filament carries rows of tiny plates, the gill lamellae, packed with capillaries. Water is a hard medium to breathe: it is dense and holds little oxygen, so fish need a very efficient system.

Ventilation: the fish lowers the floor of its mouth (buccal cavity), so the volume rises and the pressure falls, and water flows in. It then closes its mouth and raises the floor, forcing water over the gills and out under the operculum. Water flows in one direction only.
Counter-current flow: blood in the lamellae flows in the opposite direction to the water (Figure 2A). Blood always meets water with more oxygen than itself, so oxygen diffuses into the blood along the whole length of the lamella.

Worked example 2: how much better is counter-current flow?

Use Figure 2. (a) What percentage of the oxygen in the water does the blood gain in each system? (b) By what factor is counter-current flow better?

(a) Counter-current: the water's saturation falls from 100% to 20%, so $100 - 20 = 80$ percentage points of oxygen are taken into the blood, which rises from 10% to 90%: 80% of the oxygen is extracted.
Parallel: the water falls from 100% to 55%, so 45% is extracted; the blood rises from 10% to 55%.
(b) $\frac{80}{45} = 1.8$ : counter-current flow extracts about 1.8 times as much oxygen.

Why: in parallel flow the two fluids reach the same saturation, and diffusion stops. In counter-current flow a difference in concentration is kept along the whole lamella, so diffusion never stops.

Amphibians: three surfaces in one animal

A frog, such as those calling from Rwanda's marshes in the rainy season, exchanges gases in three places:

The skin: thin, moist with mucus, and well supplied with capillaries. It works in water and on land, and when a frog is resting it can supply much of its oxygen and lose much of its carbon dioxide this way.
The lining of the mouth (buccal cavity), which is also moist and full of capillaries.
Simple, sac-like lungs, with little internal folding. A frog fills them by a buccal pump: with its mouth shut and nostrils open, it lowers the floor of its mouth to draw air in; then it closes its nostrils and raises the floor, pushing air into the lungs under pressure.

The skin must stay moist to work, which is one reason why amphibians live in damp places. Tadpoles breathe with gills, first external and then internal, as fish do.

Feature	Insect	Bony fish	Frog	Human
Exchange surface	Tracheole walls	Gill lamellae	Skin, mouth lining, lungs	Alveoli
Medium	Air	Water	Air and water	Air
Ventilation	Abdominal pumping (active insects)	One-way flow by mouth and operculum	Buccal pump (positive pressure)	Ribs and diaphragm (negative pressure)
Gases carried by blood?	No: tubes reach the cells	Yes	Yes	Yes

Warning

Common mistakes

"Insects breathe through their mouths." Air enters through the spiracles, and the blood of an insect does not carry oxygen to the cells.
"Counter-current flow means water and blood flow in the same direction." It means opposite directions; same direction is parallel flow.
Explaining counter-current flow. Do not just say "it is more efficient". Say that a concentration gradient is maintained along the whole length of the lamella.
"Larger animals have a larger SA:V." They have a larger surface area, but a smaller surface area to volume ratio.

TED-Ed: "Why fish are better at breathing than you are", by Dan Kwartler. Watch for the structure of the gill filaments and lamellae and for the counter-current flow of blood and water, then compare it with Figure 2.

Tip

Memory hook: for any exchange surface, check the four marks: Big area, Thin barrier, Steep gradient, Moist: "Big Tilapia Swim More". Then say which feature of the animal provides each one.

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Practice: gas-exchange surfaces in animals

List four features of an efficient gas-exchange surface.
Calculate the SA:V of a cube of side 3 cm, and compare it with a cube of side 1 cm.
Using Fick's law, by what factor does the rate of diffusion change if the surface area doubles and the barrier becomes three times thicker?
Explain why insects can close their spiracles, and what the benefit is.
Explain how the fluid in tracheole tips helps an active flight muscle get more oxygen.
Describe how a bony fish ventilates its gills.
In a model of parallel flow, water enters at 100% saturation and blood at 20%. If they leave with equal saturation and equal flow rates, what will it be? Why can counter-current flow do better?
Suggest why a frog that has been kept in dry air for some hours becomes short of oxygen, even though its lungs work.

Show answersHide answers

Large surface area; thin barrier (short diffusion path); steep concentration gradient maintained by ventilation and blood flow; moist; permeable.
Side 3 cm: $\frac{6 \times 9}{27} = \frac{54}{27} = 2 : 1$ . Side 1 cm: 6 : 1. The larger cube has a third of the ratio.
$\frac{2}{3}$ : the rate becomes about 0.67 times as fast (it falls by a third).
Muscles work valves in the spiracles. Closing them when the insect is resting reduces water loss by evaporation from the tracheal system.
Lactate from anaerobic respiration lowers the water potential of the muscle, so water moves out of the tracheoles by osmosis; air fills more of each tracheole, so oxygen reaches the muscle faster, because diffusion in air is much faster than in water.
The floor of the mouth is lowered, increasing the volume and lowering the pressure, so water enters; the mouth closes and the floor rises, pushing water over the gills and out under the operculum.
60%, halfway between 100% and 20%. In counter-current flow the blood always meets water with more oxygen, so a gradient is kept along the whole lamella and the blood can rise above 60%.
Much of its gas exchange happens through the skin, which must be moist for gases to dissolve and diffuse; in dry air the skin dries out and this route stops working.

Key points

Rate of diffusion $\propto \frac{\text{area} \times \text{concentration difference}}{\text{thickness}}$ ; good surfaces are large, thin, moist and ventilated.
SA:V falls as size rises, so large animals need special surfaces and transport systems.
Insects: spiracles, tracheae and tracheoles carry air directly to the cells.
Fish: one-way flow over lamellae and counter-current exchange keep a gradient along the whole gill.
Frogs: skin, mouth lining and simple lungs filled by a buccal pump.

Sources and credits

Written by Ganzaa. Facts were checked against 39.1 Systems of Gas Exchange, from Biology 2e by Mary Ann Clark, Matthew Douglas and Jung Choi, OpenStax, licensed under CC BY 4.0. No text was copied.
Photo: Fish gills, Krishna satya 333, CC BY-SA 4.0, via Wikimedia Commons.
Video: Why fish are better at breathing than you are, Dan Kwartler, TED-Ed, embedded from YouTube.

Figures 1 and 2, the table, the worked examples and the practice questions are by Ganzaa. The saturation values in Figure 2 are illustrative.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Why does a fish suffocate in air?

2What makes a good exchange surface

3Insects: air piped straight to the cells

4Fish: a one-way flow and a clever trick

5Amphibians: three surfaces in one animal

Why does a fish suffocate in air?

What makes a good exchange surface

Insects: air piped straight to the cells

Fish: a one-way flow and a clever trick

Amphibians: three surfaces in one animal