When water flows through the gills, oxygen in the water diffuses quickly into the bloodstream.
- Oxygen can diffuse from the water into the blood because of the counter-current system.
The Counter-Current System
- The circulatory system of a fish is a counter-current system.
- Blood flows through the lamellae in the opposite direction of the flow of water through the gills.
- The system ensures there is always a steep concentration gradient between the water and the blood.