Gill lamellae.

Today in Biology we were all let loose with some scalpels and a pair of scissors with the attempt of finding the gill lamellae in fish heads. The experience taught us more about the satisfaction you can get from mauling the eye out of dead fish, than what the actual purpose of the gill lamellae is.

Above: The delightful Grace holding the product of 40 minutes dissection.

The gill lamellae is an important respiratory structure in the gills of a fish, it uses the idea of a countercurrent flow, which is where a constant low diffusion gradient is necessary to transport oxygen around the respiring tissues. This system is efficient due to the use of at least 75 per cent of oxygen from the water. These structures are founded upon gill filaments which are stacked upon one another. The lamellae are important as they increase the surface area of the gills which in turn increases the rate of diffusion withing the fish itself.
The countercurrent flow system previously mentioned is a more effective method of diffusion of oxygen than the system us mammals possess; parallel flow. This is because the blood and water which flow over the lamellae do so in opposite directions. This means that blood that is well loaded with oxygen meets water, which also is well loaded with oxygen. Therefore little, but sufficient, oxygen will diffuse over the small diffusion gradient into the blood. The system also ensures that when the water contains little oxygen, diffusion will still occur. This is because there will always be a diffusion gradient favouring the diffusion of oxygen from water into the blood all the way across the gill lamellae hence then 75 per cent oxygen use.
Contrasted with the countercurrent flow, the diffusion system we posses only uses up a maximum of 50 per cent of oxygen available for diffusion due to the gradient being cancelled out. This is because of the flow of water and blood being 'parallel' to each other, creating a high diffusion gradient between the water's high saturation of oxygen, and the blood's low saturation of oxygen.
Fish contain these structures as they have a small surface area to volume ratio meaning they would simply not be able to survive without developing a specialised internal gas exchange surface.


Sources: AQA AS Biology textbook and Mrs Palmer, biology teacher.
Author: Katie-Jo Mawson