When the blood flows through the blood vessels against the flow of water, it is called countercurrent system. Backflow occurs because the concentration of oxygen is higher than that of blood in retrograde flow. Oxygen present in water dissolves easily in blood due to a chronic low concentration. As a result, maximum oxygen (80%) is received by the blood. The lamellae present in the gills increase the oxygen absorption surface. As the tips of the gill rakers cross each other, water flow slows down and gas exchange takes longer.