Figure 2.16 Concurrent versus countercurrent circulation. (a) Concurrent flow in aquatic animals would cause oxygen to diffuse from the water and into the capillaries of the gills. Once 50 percent of the oxygen was transferred, the two flows would come into equilibrium. (b) When animals use countercurrent circulation, more oxygen can be transferred into the gill capillaries because even if the capillaries contain a high amount of oxygen, the oxygen content of the water is higher, allowing oxygen diffusion to continue.