ANSWER -
Parallel flow (concurrent exchange) limits the amount of oxygen that can be extracted from water because as the water and the respiratory medium (such as air) flow in the same direction, they quickly reach a point of equilibrium where the partial pressure of oxygen in the water is equal to that in the respiratory medium. This means that after a certain point, no further diffusion of oxygen occurs, and the oxygen extraction process becomes less efficient. In contrast, countercurrent exchange, where the water and respiratory medium flow in opposite directions, allows for a greater concentration gradient and more efficient oxygen extraction.