• Fish have thick skin that enable them to be waterproof however the result of this is gases cannot readily diffuse through the skin.
  • Fish also have a small surface area : volume ratio
  • Body surface therefore cannot supply adequate amount of respiratory gases

Structure of the Gills


  • The gills are located within the body of the fish behind the head
  • They are made of gill filaments. These gill filaments are stacked up in a pile
  • At a right angle to the filaments are the gill lamellae which increase the surface area of the gills
  • Water is taken through the mouth and is forced over the gills and out through an opening on each side.


The gills of a fish are efficient at gas exchange as they have a large surface area with each lamellae. They also have a short diffusion pathway which further increases the gas exchange efficiency. The countercurrent flow maintains a high concentration gradient throughout, with the circulatory system moving the blood saturated with oxygen away with the ventilated water being replaced with new water, all to ensure gas exchange is effective and efficient.

Countercurrent Flow

Countercurrent flow means the blood flows in the opposite direction to the flow of water

The concentration of oxygen in the water compared to the concentration of oxygen in the blood is always higher, therefore a concentration gradient is maintained right across the surface. As seen above, when there is 100% oxygen saturation in the water when it enters, the blood is almost as saturated at 90% so there is a concentration gradient. At the next stage where the oxygen has donated 10% to the blood, the water now has 90% but the blood has 80% so again there is a concentration gradient. This continues along the entire surface. Equilibrium is never reached and diffusion of oxygen from the water into the blood is constantly occurring

Concurrent Flow (parallel System)

Concurrent flow means the blood flows in the same direction to the flow of water

When the blood flows in the same direction as the blood the concentration gradient diminishes to a point where the percentage of oxygen is equal in the blood and the water. Equilibrium has been reached, and therefore there is no concentration gradient so there is no net movement of oxygen.