WJEC Biology for AS: Study and Rev Guide

Counter-current flow : blood and water flow in opposite directions at the gill lamellae, maintaining the concentration gradient and, therefore, oxygen diffusion into the blood, along their entire length. Key Term Parallel flow 2. Counter-current flow is seen in bony fish, e.g. salmon, where blood and water flow in opposite directions. This is a much more efficient system because diffusion is maintained along the entire length of the gill filament because there is always a higher concentration of oxygen in the water than in the blood it meets, which results in higher oxygen absorption as an equilibrium is not reached. Bony fish have a more advanced ventilation mechanism than cartilaginous fish. 80% di usion numbers are % saturation with oxygen O 2 Water ow Blood ow 100% 60% 50% 50% 50% 20% 0% 40% 50% 50% 50% Counter-current flow across gill lamellae Oxygen concentration across the gill lamella of a bony fish 80 AS Biology: Study and Revision Guide