Insects

Insects have a hard, impermeable exoskeleton and so have had to come up with another solution. However, they don’t use a transport system: instead they have small holes called spiracles positioned along their sides.

Air enters the spiracles and flows down small tubes called tracheae. These divide into smaller ones, known as tracheoles, which are in direct contact with the cells. When air reaches a cell it dissolves in the cell’s fluid. When the insect’s muscles require energy it sucks up this fluid from the cells.

Valves can open and close the spiracles and are sometimes surrounded by hairs. These help prevent the insect losing too much water: the hairs ensure that a level of humidity is kept around the spiracle so that a lower water vapour concentration gradient is maintained.

——————————————————

Fish

Fish, on the other hand, use gills. They contain a number of gills situated between their mouth cavity or buccal cavity and the operculum, which is a chamber on either side of their mouths. Each gill is comprised of numerous filaments which in turn are covered with a multitude of lamellae, thereby increasing the surface area.

As water flows over the gills the oxygen is taken up by the blood passing through the lamellae. The water is flowing in the opposite direction to the blood which is known as a countercurrent flow. This means a fish is able to take up oxygen very rapidly, three times more quickly than a human can.

This is because the water flowing next to the blood is water that has released less oxygen. Put another way, if the blood and water flowed in the same direction then the first blood to come into contact with the water would take up the oxygen quickly as the concentration gradient would be high. However, as the water moved along the lamellae the concentration would decrease because the blood flowing with the water would take up more and more oxygen until there was no concentration gradient left for the oxygen to move across. The blood, however, would only be able to be 50% saturated.

With countercurrent flow the blood can reach at least 80% saturation: the blood is able to move through the lamellae and continue to absorb oxygen as it does because a concentration gradient it maintained.