Before this goes any further, a few clarifications:
1. Gas exchange is central to life. Oxygen is needed in respiration* which generates usable energy without which life wouldn’t exist. Removing the resulting carbon dioxide is crucial too.
2. Water can be a gas too, in the form of water vapour. This may in certain organisms escape with the air, so water preservation versus gas exchange is always an important thing to bear in mind. This is especially important when talking about insects and plants.
Insects have a tracheal system made up of many tracheae which branch into smaller tracheoles. All tracheae connect to the exoskeleton of the insect, so that air diffuses in and out through the spiracles.
The technical terms highlighted above are important in describing what really is just a bunch of holes and tubes. Here’s a video that describes what happens. Don’t worry about the overly detailed labels. Just enjoy the smooth ride of a video!
In order to balance the opposing needs for conserving water and obtaining oxygen, insects are able to close their spiracles, as well as contract their abdomens. The former prevents water loss, while the latter enhances ventilation so that more oxygen gets inside their body.
Fish extract dissolved oxygen molecules from the surrounding water. The oxygen content of water is much lower compared to air, so fish have special adaptations which enable them to make the most of the available oxygen. These adaptations are gills.
1. Gill filaments have lamellae which increase the surface area available for diffusion, while keeping the diffusion pathway short.
2. The water flow through the fish’s mouth as well as the blood in gill capillaries follow the countercurrent principle. As seen in the above diagram, water and blood flow against each other, rather than along each other. This ensures that oxygen diffusion can take place along the whole length of the flow, not just for half of it – before the concentration of oxygen in the blood and in the water becomes equal.
This is easily exemplified (and an acceptable form of explanation in an exam) by a number table. The upper row is the oxygen concentration in the blood, while the lower is the one in the water. Along the flow, oxygen enters the bloodstream from the water, so that the concentration in blood increases, while the concentration in water decreases.
0 1 2 3 4 5 6 7 8 9 – deoxygenated blood becomes more and more oxygen-rich
^ ^ ^ ^ ^ ^ ^ ^ ^ ^ – oxygen from the water enters the bloodstream (from higher concentration to lower)
1 2 3 4 5 6 7 8 9 10
If water flowed in the same direction as blood, this is what it would look like:
0 1 2 3 4 5 5 5 5 5 – deoxygenated blood becomes slightly oxygenated, stalling halfway through
^ ^ ^ ^ ^ – when blood and water oxygen concentrations equal (5 and 5), diffusion stops
10 9 8 7 6 5 5 5 5 5
Here’s a video which explains nicely how fish carry out gas exchange: