To understand countercurrent flow, it is easiest to start by looking at concurrent flow where water and blood flow over and through the lamellae in the same direction.
When the blood first comes close to the water, the water is fully saturated with oxygen and the blood has very little.
There is therefore a very large concentration gradient and oxygen diffuses out of the water and into the blood.
As you move along the lamella, the water is slightly less saturated and blood slightly more but the water still has more oxygen in it so it diffuses from water to blood.
This continues until the water and the blood have reached equal saturation.
After this the blood can pick up no more oxygen from the water because there is no more concentration gradient. The maximum saturation of the water is 100% so the maximum saturation of the blood is 50%.