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 and remove carbon dioxide. 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 large surface area for gas exchange. The diagram shows how the many 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 to ensure blood flow through the gills. To see how the gas exchange happens, we need to zoom right in on a single lamella because the lamellae are the gas exchange surface. They are very thin to provide a short diffusion pathway.
Each lamella has its own blood supply flowing across 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 exchange system, and maintains a high concentration gradient of oxygen across the whole length of the lamella (see graph below). 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 get their oxygen from air. The air enters the insect gas exchange system through pores called spiracles then travels into tiny tubes called tracheae. The trachea branch off into even tinier tubes called tracheoles which is where the gas exchange occurs. The highly branched tracheoles provide a large surface area, and they have very thin walls to provide a short diffusion pathway. Tracheoles contain fluid which oxygen can dissolve in, and the walls permeable to oxygen. All these things make them well adapted for gas exchange by diffusion. Oxygen diffuses directly into the cells, and carbon dioxide diffuses out of the cells.
Of course, the air needs to move in and out of the system of tubes in order to maintain concentration gradients. To do this, the volume and pressure inside the insect need to change. Rhythmic abdominal movements cause these changes in pressure, and air is forced in and out the tracheae 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.
Summary
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 very thin to provide a short diffusion pathway, and are ventilated to maintain high concentration gradients of oxygen and carbon dioxide.
- Ventilation happens due to changing volume and pressure.
As always, learning how to relate structure to function will set you off on the right foot for A-level biology exam questions.
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