How fish breathe

Like the rest of the group of animals, fish need oxygen in their tissues to survive, thus meeting their fundamental energy needs to stay alive.

Fish obtain oxygen from the external environment thanks to vascularized gills, lungs , and even through the skin (in fish without scales), or through temporary respiratory structures (such as the yolk sac tissue of fish embryos). On the other hand, the blood of the fish’s circulatory system also plays an important role in gas exchange, allowing the transport of carbon dioxide resulting from cellular oxidative processes to the gills and other respiratory structures for its correct elimination from the body.

In the following sections of the article we will see in detail the role played by both the gills and the lungs in the respiratory system of fish, as well as the characteristics of the complex breathing process.

There are different structural types of gills in fish , depending on the group to which they belong, and can be placed in the fish classification as: agnate (fish without jaws: lampreys and hagfish), chondrichthyans (cartilaginous fish: sharks and rays) or else osteichthians (bony fish: such as salmon and tuna).

All of them share the presence of gill openings as external respiratory organs , generally constituted by a single opening on each side of the head, opposite the pectoral fin of the fish. These gills are made up of very fine epidermal filaments, adapting the structure of numerous folds rich in blood vessels.

The importance of the gills in the correct functioning of the respiratory process is vital for the survival of all fish . In the section dedicated to explaining the respiratory system of fish we will see in more detail how the gills work.

Did you know that fish are not the only ones with gills? In this other AgroCorrn post we talk about 40 animals that breathe through gills .

Fish have developed different mechanisms throughout their evolution that allow them to obtain oxygen from the air when the amounts of oxygen in the aquatic environment are scarce or not sufficient to meet their vital energy needs. The lungs (and sometimes the gas bladder) are the main internal structures adapted to air respiration, present in many tropical freshwater and some saltwater fish during their temporary air respiration habit.

Thus, the lungs of lungfish have the same respiratory function as that of the lungs of other groups of vertebrate animals that take oxygen from the air. In this way, the lungs present internal septa and roughness that divide the air spaces into smaller compartments, ending in alveolar sachets covered by numerous blood vessels.

The smooth muscle present in the lungs of fish that breathe outside of the water makes it possible for the respiratory mechanism to be responsible for providing and distributing air within the lung. In addition, the fish receive blood supply in their lungs from their last efferent branchial artery, subsequently returning the already oxygenated blood from the lungs to the heart.

The basic process that takes place in the respiratory system of fish begins with the use of oxygen from the water through the lamellar filaments of the gills. This uptake of oxygen is favored in turn by the direction of the body’s own blood flow and by the circulation of water from the environment in which the fish lives. In this way, the countercurrent of blood and breathable water maintains a smooth diffusion gradient in the gills for oxygen to penetrate and carbon dioxide to release into the fish.

As inspiration begins, the protective covers of the gills close tightly and the fish opens its mouth, entering the water with the oxygen dissolved in it. Thanks to the contraction of different muscles, the water begins to flow through the gill lamellae , the oral cavity now functioning as a pumping system, thus preventing the water from flowing out of the mouth.

Later, both the gill cover and the gill fins open and the water is expelled from the body, also preventing the return of water through the action of an oral valve.