Question

how does counter-current exchanger of gasses work?

Answer 1

Ans: Fish need oxygen to survive,but rather than breathing air, fish extract their oxygen from the water around them.Fish have evolved a clever way to overcome their oxygen-deprived environment through their highly specialized gills. The gill is where fish absorb oxygen from the surrounding water into their blood.

Fish gills use a design called countercurrent oxygen exchange to maximize the amount of oxygen that their blood can pick up. They achieve this by maximizing the amount of time their blood is exposed to water that has a higher oxygen level, even as the blood takes on more oxygen. Countercurrent oxygen exchange means the blood flows through the gills in the opposite direction as the water flowing over the gills. This flow pattern ensures that as the blood progresses through the gills and gains oxygen from the water, it encounters increasingly fresh water with a higher oxygen concentration that is able to continuously offload oxygen into the blood. The low-oxygen blood, which is just entering the gill, meets low-oxygen water. Since there is more oxygen in the water, the oxygen can flow from water to blood. Likewise, the high-oxygen blood, which has nearly passed the entire length of the gill, meets fresh, high-oxygen water, and oxygen continues to flow from water to blood.

If fish instead had blood flowing in the same direction as water through their gills (called ‘concurrent flow’), the low-oxygen blood entering the gill would first meet the high-oxygen water also entering the gill. The maximum amount of oxygen that the blood could pick up would be only half of the total amount of oxygen in the water. In contrast, countercurrent oxygen exchange allows the blood to pick up 90 percent of the oxygen in the water.