Why is one way flow an advantage to fish

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.

Oxygen would quickly pass from the water into the blood, until the oxygen levels of the blood and water rapidly became the same, and oxygen diffusion into the blood would stop. 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.

This impressive ability to acquire oxygen allows fish to thrive in water that has oxygen levels only a tenth of those at the top of Mount Everest, where most people cannot survive without supplemental oxygen.

It just shows that fish are truly masters at breathing. The Fish Report. The gill filaments have many protrusions called gill lamellae. One of the ways in which gas exchange is carried out efficiently is by the countercurrent flow principle. Sounds complicated but it just means that water and blood are flowing in different directions. The water that passes over the gill lamellae flows in the opposite direction to the blood within the gill lamellae.

This system maximises the amount of oxygen diffusing into the blood by having the most oxygenated blood meet the most oxygenated water, and the least oxygenated blood meet the least oxygenated water. This means that the concentration gradient is maintained the whole way through, allowing the maximum amount of oxygen to diffuse into the blood from the water.

This is important because there isn't much oxygen in the water, and fish need to absorb enough oxygen to survive. In addition to this, the lamellae have a rich blood supply so that a steep concentration gradient can be maintained between the blood in the lamellae and the water through. Hence, oxygen diffusing into the blood is rapidly removed by the circulating blood supply and more oxygen is able to difuse into the blood. Another way in which a steep concentration gradient is maintained is by ensuring water flows in one direction only.

The fish opens its mouth to let water in, then closes its mouth and forces the water through the gills and out through the operculum gill cover. This allows for more efficient gas exchange than if the water had to go in and out the same way.