Logan Steele
Gills are the respiratory organs of fish, which are located behind and to the side of the mouth cavity. They are covered by gill slits and are filled with blood vessels, giving them a bright red colour. Gills have a large surface area and thousands of tiny blood vessels, which allow oxygen to easily access the bloodstream. While gills are not muscles, the stalk of external gills, which are commonly observed on the aquatic larva of salamanders, lungfish, and bichirs, usually contains muscle tissue.
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What You'll Learn
Gills are not a muscle, they are a respiratory organ
Gills are not a muscle, but rather a respiratory organ. They are found in fish and some amphibians, such as salamanders, lungfish, and bichirs, and are used to extract oxygen from the water and expel carbon dioxide. Gills lie behind and to the side of the mouth cavity and consist of fleshy filaments supported by gill arches and filled with blood vessels, giving them a bright red colour.
The gill arches are made of bone or cartilage and provide a framework for the gills. The filaments are thin, leaf-like structures where the exchange of gases takes place. The blood capillaries in these filaments are close to the surface of the gills, allowing for the efficient exchange of oxygen and carbon dioxide. Water taken in through the mouth passes over these filaments, allowing the gills to absorb oxygen and release carbon dioxide.
While gills are not muscles, they are connected to the circulatory system, which includes the heart, arteries, capillaries, and veins. The oxygenated blood from the gills enters the efferent arteries of the gill arches and then flows into the dorsal aorta, which distributes oxygenated blood to the body's tissues and organs.
Over time, gills became less common as animals transitioned from water to land. Gills need to stay wet to function properly, which is not ideal for land animals. Instead, land animals, including humans, evolved to have a diaphragm, a muscle that regulates breathing, and lungs, which are more efficient at extracting oxygen from the air.
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Gills are made up of thousands of tiny blood vessels
Gills are the same in most, but not all, fishes. They are the means by which fish are able to extract oxygen from water. Fish take water into their mouths, bypassing the gills just behind their heads on either side. The oxygen is then dissolved in the water, and carbon dioxide is released into it, then expelled.
The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water. The capillaries are the smallest blood vessels, and they connect arteries and veins. They have thin walls, which allow them to be transfer stations for oxygen, nutrients, carbon dioxide, and waste. The blood vessels in the gills are protected by a membrane called an operculum, which allows fish to exchange carbon dioxide and oxygen with the water beneath the gills.
In addition to the well-established respiratory pathway through the gill lamellae, there is also evidence of a secondary non-respiratory circulation in the gills of some fish, such as Latimeria. This secondary circulation includes a central venous sinus (CVS), arterio-venous anastomoses (AVAs), and central filament arteries (CFAs).
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Gills are protected by a gill cover in most fish
Gills are organs that allow fish to breathe underwater. They have a large surface area and are made up of thousands of tiny blood vessels. As water rushes over the gills, oxygen is diffused into the bloodstream, and carbon dioxide diffuses out. Gills need to stay wet to function, which is why they are not ideal for animals that do not live underwater.
In most fishes, gills are set inside the pharynx and are covered by gill slits. These gills are protected by a hard but flexible gill cover, also known as an operculum, which forms the outer wall of the gill chamber. The operculum also plays a major role in the pumping mechanism that regulates the continuous flow of water over the gills.
The evolution of gill covers in fish dates back more than 430 million years. The formation of these covers was an important innovation that allowed vertebrates to become top predators. A study by USC Stem Cell scientists found that the Pou3f3 gene is responsible for the development of gill covers in zebrafish.
It is interesting to note that humans once had gill cover-like structures during our embryonic stage, inherited from our distant fish ancestors. However, over time, gills shrank and eventually disappeared in full-time land animals during the Carboniferous period, around 315 million years ago.
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Gills are located behind and to the side of the mouth cavity
Gills are organs that enable fish to breathe underwater. They are located right behind the head, behind and to the side of the mouth cavity. Gills are composed of comb-like filaments, the gill lamellae, which help increase their surface area for oxygen exchange. The gill filaments or lamellae are supported by the gill arches and are filled with blood vessels, which give gills a bright red colour. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.
When a fish breathes, it draws in a mouthful of water at regular intervals. Then it draws the sides of its throat together, forcing the water through the gill openings, so that it passes over the gills to the outside. The gills push the oxygen-poor water out through openings in the sides of the pharynx. The water taken in continuously through the mouth passes backward between the gill bars and over the gill filaments, where the exchange of gases takes place.
Most fish exchange gases like oxygen and carbon dioxide using gills on both sides of the pharynx (throat). Gills possess tissues resembling short threads, referred to as gill filaments or lamellae. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide. Other than respiration, these filaments have other functions, including the exchange of ions, water, acids, and ammonia.
The blood vascular system consists of the heart, the arteries, the capillaries, and the veins. The interchange of oxygen, carbon dioxide, nutrients, and other substances such as hormones and waste products takes place in the capillaries. The capillaries lead to the veins, which return the venous blood with its waste products to the heart, kidneys, and gills. There are two kinds of capillary beds: those in the gills and those in the rest of the body. The heart, a folded continuous muscular tube with three or four saclike enlargements, undergoes rhythmic contractions and receives venous blood in a sinus venosus. It passes the blood to an auricle and then into a thick muscular pump, the ventricle. From the ventricle, the blood goes to a bulbous structure at the base of a ventral aorta just below the gills.
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Gills are inefficient for land animals
Gills are external or internal respiratory organs found in aquatic animals that facilitate the exchange of gases. They are most commonly found in fish, but some amphibians, such as salamanders, lungfish, and bichirs, also have gills. Gills are inefficient for land animals because they require moisture to function effectively. If gills were to come into contact with air, they would quickly dry up, making it difficult for the animal to breathe.
The inefficiency of gills for land animals can be attributed to their specialized function in aquatic environments. Gills are designed to extract dissolved oxygen from water, which is then transported through the circulatory system to various parts of the body. This process involves the continuous flow of water over the gill filaments, where gas exchange occurs. The capillaries in the gill filaments play a crucial role in this process, allowing for the absorption of oxygen and the release of carbon dioxide.
In contrast, land animals rely on lungs for respiration, which are specifically adapted for extracting oxygen from the air. Lungs are highly efficient at taking oxygen from the air and transferring it into the bloodstream through gas exchange. This process is facilitated by the diaphragm, a muscle that regulates breathing in mammals. The presence of lungs allows land animals to obtain oxygen directly from the atmosphere, eliminating the need for a constant water supply for respiration.
Additionally, gills may not provide sufficient oxygen for larger land animals that require higher levels of oxygen intake. Seasonal changes can also impact the oxygen levels in water, further reducing the efficiency of gills in certain environments. On the other hand, lungs offer a more reliable source of oxygen, as they are capable of extracting large amounts of oxygen from the air, ensuring that the animal's oxygen demands are met.
While some terrestrial crustaceans, such as certain species of hermit crabs, land snails, and sand hoppers, have gills or gill-like structures, they are not solely dependent on these structures for respiration. These land animals have adapted to use alternative methods, such as carrying water with them or developing branchiostegal lungs that are better suited for oxygen absorption from air. Therefore, while gills may be present in a few rare cases of land animals, they are not the primary or efficient means of respiration in terrestrial environments.
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Frequently asked questions
No, gills are not muscles. Gills are respiratory organs that are set on a frill of stalks protruding from the sides of an animal's head. They have a large surface area and thousands of tiny blood vessels, giving oxygen easy access to the bloodstream.
Gills consist of fleshy filaments supported by gill arches and filled with blood vessels, which give gills a bright red colour. The blood capillaries in the gill filaments are close to the gill surface to take up oxygen from the water and to give up excess carbon dioxide to the water.
No, humans do not have gills. Gills need to stay wet in order to work, which isn't ideal for animals that do not live underwater. Humans breathe with lungs inside their bodies and have a diaphragm — the muscle that regulates breathing.
