Types of Respiratory System in Animals
Different animals use different respiratory organs, the body surface, gills, a tracheal system or lungs, but all share the same features: a large surface area, a thin moist lining, and a good supply of the gas being exchanged.
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What a respiratory surface needs
Every respiratory (gas exchange) surface, no matter which animal it belongs to, must do the same job: let oxygen in and carbon dioxide out fast enough to meet the animal's needs. To do this efficiently, a respiratory surface is thin so gases diffuse across it quickly, moist so gases can dissolve before crossing it, has a large surface area so many gas molecules can diffuse at the same time, and is well supplied with the gas being exchanged, either by blood vessels beneath it or by direct contact with air or water.
The organ an animal actually uses to meet these needs depends on its size, whether it lives in water or on land, and how active it is.
Respiratory organs in different animals
- Simple animals such as Amoeba and Hydra have no special respiratory organ, gases diffuse directly across the whole body surface, which is enough because they are small with a large surface-area-to-volume ratio.
- An earthworm exchanges gases across its moist, thin skin. Mucus keeps the skin moist, and a dense network of capillaries just under the skin carries the gases to and from the rest of the body.
- Insects use a tracheal system. Air enters through small openings called spiracles on the body surface and travels along branching tracheae and tracheoles directly to individual cells, so oxygen does not need to be carried by blood.
- Fish use gills. Water is drawn over thin gill filaments, and oxygen diffuses from the water into blood capillaries while carbon dioxide diffuses out, helped by a countercurrent flow of water and blood.
- Amphibians such as frogs exchange gases through moist skin and simple lungs, depending on their life stage and activity.
- Mammals, including humans, and reptiles use lungs containing millions of alveoli, which give an enormous internal surface area for gas exchange.
Comparing the main systems
| Animal group | Respiratory organ | Key adaptation |
|---|---|---|
| Insects | Tracheal system | Air reaches cells directly through tracheae, bypassing the blood |
| Fish | Gills | Countercurrent flow of water and blood keeps a steep diffusion gradient |
| Earthworms | Moist skin | A dense capillary network lies just beneath the thin, moist skin |
| Humans | Lungs (alveoli) | Millions of alveoli give a very large total surface area |
How it is examined
Exam questions on this standard usually ask you to identify a named organism's respiratory organ, label a diagram of a gill, tracheal system or alveolus, or explain how a structure's features (thin walls, moist lining, large surface area, good blood or air supply) suit it for gas exchange. Comparison questions are also common, asking you to state one similarity and one difference between two respiratory organs, such as gills and lungs.
Worked exam-style question
Question. Diagram Y shows a section through a fish gill. Structure P is a thin filament richly supplied with blood capillaries, and arrow R shows water flowing over P in the direction opposite to the blood flow inside it.
(a) Name structure P. (b) State the term used to describe the flow pattern shown by arrow R.
(c) Explain how this flow pattern increases the efficiency of gas exchange at the gill. (d) State one reason why the gill would not work efficiently if the fish were taken out of water.
Model answer. (a) P is a gill filament (lamella). (b) This is called countercurrent flow.
(c) Because water and blood flow in opposite directions, a diffusion gradient for oxygen is maintained along the whole length of the filament, so blood keeps picking up oxygen even where it is nearly saturated, instead of the gradient disappearing halfway along as it would if both flowed the same way. (d) Out of water, the gill filaments collapse and stick together, drastically reducing the surface area available for gas exchange, and the surface also dries out, so gases can no longer dissolve and diffuse across it.
Practice question
Try this. A locust's tracheal system carries air directly to its tissues without involving the blood at all. Suggest one advantage and one disadvantage of this arrangement compared with a mammal's lungs, which rely on the blood to transport oxygen.
Exam tip
Key terms
These Chapter 8 terms describe the structures and process examined in this topic:
- Gaseous exchange, the diffusion of oxygen and carbon dioxide between an organism and its environment across a respiratory surface.
- Alveolus, one of millions of tiny air sacs in a mammal's lung where gaseous exchange with the blood takes place.
- Trachea, the windpipe that carries air between the throat and the bronchi, kept open by rings of cartilage.
- Diaphragm, the sheet of muscle below the lungs whose contraction and relaxation help drive breathing.
Source:SRC-DSKP-EN
Frequently asked questions
What features do all respiratory surfaces have in common?
How is an insect's tracheal system different from a fish's gills?
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