In these organs in most fish, water flows the opposite direction to the blood, increasing oxygen transfer
In the gills, the blood picks up oxygen from the surrounding water and 
leaves the ... By having the blood flow in the opposite direction, the gradient is 
always such that the water has more available oxygen than the blood, and 
oxygen... [DOC] [TLE] respiration facts, information, pictures | Encyclopedia.com articles ... [PAR] More complex is the method of diffusion into blood whereby oxygen passes ...