Gills are a feature that increases the surface area of fish for gas exchange. Fish gills are made up of many layers of thin filaments, which are stacked one upon the other like our lungs. These lamellae are at an angle to the filaments, ensuring the maximum surface area for gas exchange. This allows water and blood to flow in opposite directions through fish gills, maximizing oxygen transfer.
A countercurrent exchange in a fish gill is a process that allows the blood to become fully loaded with oxygen when it meets the water. In a parallel flow, a small percentage of oxygen would enter the blood. Fish prefer the countercurrent flow because water has more oxygen than blood. This is because gas exchange takes place in the gas phase faster than in water.
The countercurrent exchange system allows maximum oxygen diffusion into the blood. By maintaining a concentration gradient, the countercurrent exchange system prevents the blood from containing water with the same oxygen concentration as the surrounding water. Once water and blood mix, oxygen diffusion into the blood stops. This is essential for the survival of the fish.
A fish gill has several different functions. The most important one is to carry oxygen from water to the blood. It does this by converting the water into oxygen that is needed for our cells. This is called diffusion. Using a countercurrent exchange in the fish gill maximizes oxygen transport. And it does it by increasing the flow of water over the fish’s gills. It also opens and closes the fish’s gill flaps, allowing fresh water to enter and exit the mouth.
The gills of a fish allow oxygen to diffuse from high pressure into the blood. To maintain a concentration gradient, the oxygen content of the water and blood must be equal. A fish gill can use up to 50% of the oxygen in the water. However, it is not entirely clear how fish achieve this, so scientists are still working on the exact mechanism.
A fish gill works by allowing the water to pass through a bulbous structure at the base of the ventral aorta. This then passes to a network of capillaries that absorb oxygen and waste gases. Oxygen-rich blood then enters the efferent gill arches and flows into the dorsal aorta, where it is distributed to various tissues.