-Blood with a relatively high oxygen concentration meets water with a lot oxygen in it
-Hence, there is still a diffusion gradient for oxygen into the blood
-Blood with a relatively low oxygen concentration meets water which has had most of its oxygen removed. (d) The countercurrent system allows the gills of a bony fish to achieve an 80% extraction of oxygen from water. That's three times the rate of extraction of human lungs from the air. (e) This helps to overcome the problem of low oxygen content in water.