All living organisms respire. Cells need and use the energy that is formed through this
process to assist with life processes in order for organisms to survive and reproduce. Oxygen
and carbon dioxide are the main gases involved in aerobic respiration. Gas exchange is a
physical process by which oxygen is extracted from the air and into the bloodstream, while
carbon dioxide is simultaneously released. Gas exchange is the diffusion of these gases into
and out of cells, and this is essential for respiration to occur.
For diffusion and therefore gas exchange to occur quickly there must be a large surface area
to volume ratio for the gas exchange to take place, a partial pressure gradient for the gases to
diffuse down, a thin surface that gases can diffuse rapidly across and a moist surface that
gases can dissolve and diffuse into and out of. These conditions are called gas exchange
surfaces.
Reptiles, mammals and fish are vertebrates which all require oxygen (O2) to survive, and all
carry out gas exchange. They have different ways of carrying this out, related to their habitat
in order to occupy a specific ecological niche.
Fish live in water. They carry out gas exchange in a different way to mammals. They use gills
and the flow of water over their gills to take O2 and to remove CO2. A fish breathes as it
swims by opening its mouth and allowing water to flow over the gills. When the fish opens
its mouth the opercula close, meaning that water can flow over the gills and O2 can be
extracted and CO2 can be removed. Then the fish closes the mouth and the opercula open,
allowing the water to flow out of the fish.
The gills of a fish have filaments, which comb through the water. On the filaments are
lamellas. The lamella are shaped like a ladder, so that the O2 poor blood travels up one side
before crossing over to the other side and travelling back down as O2 rich blood. As the blood
travels across, through the capillaries, it takes in O2 by diffusion.