Fish and insects don’t have lungs like humans and other mammals do. Instead, they have clever alternative gas exchange systems in order to deliver oxygen to their cells. Let’s look at each organism individually.
Gas Exchange in Fish
I’m sure it’s fairly obvious that fish don’t breath air because they live underwater. Instead, they must get their oxygen from water. To do this, they use their specialised exchange surface: the gills.
Gills, like lungs, have a huge surface area for gas exchange. The diagram shows how the gill filaments and lamellae (also called gill plates) create the large surface area. There are blood vessels running through a structure called the gill arch which deliver and remove blood. To see how the gas exchange happens, we need to zoom right in on a single lamella.
Each lamella has its own blood supply which flows across the lamella through capillaries in one direction. The water flows across the surface of the lamella in the opposite direction to the blood flow. This is called a counter-current system, and maintains a high concentration gradient of oxygen between the blood and the water. Remember, the higher the concentration gradient, the faster the rate of diffusion.
Water must pass over the gills regularly in order for gas exchange to take place, just the same as humans need to breathe in and out. Fish must carry out ventilation, but of course it is a different process to ventilating lungs and there is some extra terminology needed. Before we look at the process, know that the space inside a fish’s mouth is called the buccal cavity, and the bony flap covering and protecting the gills is called the operculum. Here is how ventilation happens in bony fish (i.e fish that have a skeleton made of bone):
- The fish opens its mouth so the buccal cavity volume increases and the buccal cavity pressure decreases – water is drawn in.
- The fish closes its mouth so the buccal cavity volume decreases and the buccal cavity pressure increases – the operculum is forced open and the water is forced out across the gills.
This explains why fish open and close their mouths all the time!
Gas Exchange in Insects
Insects do not have tiny lungs, but they do use air to get their oxygen. The air enters the insect through pores called spiracles then travels into tiny tubes called tracheae. The trachea branch off into even tinier tubes called tracheoles. Tracheoles contain fluid which oxygen can dissolve in, and the walls are really thin and permeable. This makes them well adapted for gas exchange by diffusion – oxygen diffuses into the cells, and carbon dioxide diffuses out of the cells.
Of course, the air needs to move in and out of the insect to maintain those concentration gradients. To do this, the volume and pressure inside the insect need to change. Rhythmic abdominal movements cause these changes, and air is forced in and out through the spiracles. In larger winged insects there is an area above the abdomen called the thorax – wing movements change the volume of the thorax to help air move in and out.
Hopefully you have seen that gas exchange in fish and insects is quite different to gas exchange in humans, but the principles are the same:
- Gas exchange surfaces are adapted to maintain a high rate of diffusion: they have a large surface area, are thin, and are ventilated to maintain high concentration gradients of oxygen and carbon dioxide.
- Ventilation happens due to changing volume and pressure (in the buccal cavity in fish).
As always, learning how to relate structure to function will set you off on the right foot for A Level biology exam questions.