The arrangement of capillaries in a fish gill allows for __, the exchange of a substance or heat between two fluids flowing in opposite directions.
- In a fish gill, this process maximizes gas exchange efficiecy. Because blood flows in the direction opposite to that of water passing over the gills, at each point in ts travel blood is less saturated with O2 than the water it meets. As blood enters a gill capillary, it encounters water that is completing its passage through the gill. Depleted of much of its dissolved O2, this water nevertheless has a higher __ than the incoming blood, and O2 transfer takes place. As the blood continues its passage, its __ steadily increaeses, but so does that of the water it encounters, since each successive position in the blood's travel corresponds to an earlier position in the water's passage over the gills. Thus a partial pressure gradient favoring the diffusion of O2 from water to blood exists along the entire length of the capillary.
- countercurrent exchange
- PO2 x2