How Do Gills Work? How Does It Make Breathing In Water Possible?

Just like humans breathe with the lungs, gills are the respiratory organs of many aquatic creatures.

To stay alive, underwater organisms also need to breathe in oxygen and excrete carbon dioxide. This is where the functionality of gills step in.

Oxygen diffuses 10,000 times more easily in the air than in water. Air-sac structures like lungs are not enough for absorbing the oxygen from water. Fish need something more powerful to draw the diffused oxygen from water to sustain. The gills help such organisms to take in the dissolved oxygen from water and breathe out the excreted carbon dioxide.

The gills fish have interestingly have a large surface area, which allows for a large space for exchanging gases with the external environment easily. The exchanged gases are absorbed by the thin walls of the capillaries and lamellae containing body fluids and blood. The bloodstream or the fluid flowing through the capillaries carries all necessary gases into different parts of the body. Similarly, carbon dioxide is excreted out by the surface of the thin walls of the capillaries.

If you liked what you read about gills in this article, check out how do amphibians breathe? And how do animals hibernate?

An Overview Of Gills

The gills help in the respiration process of most of the fishes underneath the water. When water passes through the mouth of a fish, it directly reaches the gills by crossing several tiny blood vessels within the gill slits.

The gills easily absorb the dissolved oxygen in the water, and it washes away the carbon dioxide and toxic ammonia produced by the fish's body. Gill or gill-like structures are not only present in fishes, but they are also found in many other animals that live underwater, like crustaceans, amphibians, aquatic insects, and mollusks.

In some animals, the gills are developed in such a way that it also helps them to breathe on land, provided they are moist at that moment. The gill of the hermit crab is an example of the modified gill.

The gills are protected by a flap of skin in rays, sharks, and other similar species. The structure of a vertebrate's gill is different from that of an invertebrate's gill. Amphibian and fish gills contain the gills of vertebrates, while invertebrates, like mollusks and crustaceans, contain plate-like gills.

Function Of Gills In Fish And Other Underwater Animals

There are a million microscopic organisms, along with some big, inactive ones living in fresh water, the sea, or the ocean, that can breathe through their entire body without gills. However, animals with complex structures need gills for breathing. Some animals have gills but can also absorb oxygen across the surface of their body.

The main function of the gills in underwater fishes is the exchange of essential gases. It is made up of fine filaments consisting of highly lamellae, tissue, branches, or tuft processes to increase the surface area of exchange. They are delicate, therefore, the diffusion of gas across the respiratory surface into the blood or body fluid becomes easier. The water outside the gill cover provides support to it.

Water consists of only a fraction of oxygen than that present in the air. Therefore fish need a large surface area, otherwise, it will become difficult to absorb the gas across it. The gaseous exchange takes place across the entire area of the vasculature gill, and the pressure is balanced by the one-way water current flowing by a pumping mechanism. The water pressure over the fish gills plays a very important role in keeping them safe. In some species like fish and mollusks, the water flow occurs in a direction opposite to the flow of blood. This mechanism, called countercurrent exchange, helps the organisms to breathe in 90% of the oxygen from water.

How do gills help in gas exchange?

A million fishes use the mechanism of gills to exchange oxygen and carbon dioxide dissolved inside the water. The water entering through the mouth passes to the back of the mouth, where an exchange takes place. The fine capillaries around the gill through which the blood flows are responsible for the exchange. They are covered by a flap of skin in sharks or ray-finned fish.

In animals that are vertebrates, the gills subtly transform into pharynx walls with many gill slits in the exterior part. This involves countercurrent exchange to keep the gaseous substances flowing. This results in a supported breathing mechanism in the animals. When an organism like fish draws the water in through the mouth, it forcefully passes out of the gills moving past the gill openings. This process helps in the exchange of oxygen in fish species.

In invertebrates, the gill has been modified in various forms, so the mechanism varies according to the structure. In some cases, they form a plate-like structure, while in others, the appendages of the animal are transformed into a gill. All of these modifications help them to draw in oxygen from water into their blood or body fluid.

How do gill filaments work in fish?

The filaments are an important part of the gills, and they have functions similar to the lungs in the vertebrates. Apart from absorbing oxygen, they are meant for maintaining the iron and pH levels in a fish as well as helping in removing nitrogenous waste in the form of ammonia.

These filaments are the biggest components of the gill, and they cover a large area. They are also called the primary lamellae, while the smaller branches are called the secondary lamellae. In the secondary lamellae, blood and water flow in opposite directions which naturally increases the concentration of oxygen in the water flowing beside it. Oxygen is absorbed in the body of the fish along the full length of the lamellae. The level of absorption by the filaments depends on the activity of the fish. A fish that moves fast can absorb oxygen faster, while a fish that is mostly sedentary will absorb lower units of oxygen.

Difference Between Gills And Lungs

Gills and lungs both work for respiration, but they are different from each other in structural forms. Gills are specialized for breathing in the water, while the lungs are a type of organ that helps in breathing air.

As we have known from the above discussions in this article, the gills help underwater organisms to breathe. They are found mainly in amphibians, fish, annelids, and some arthropods. They are enclosed by a very thin sheath under which the blood vessels of the organisms carry blood and other body fluids. When water passes through the mouth of the fish, it reaches the gill through the contraction of openings. On coming in contact with water, oxygen passes into the blood vessel by diffusion easily, and it is transported to the rest of the body parts of the fish. This is the mechanical process of gills.

The lungs work in a completely different way. It is an advanced organ that facilitates respiration in mammals, including human beings. The lungs come in a pair in human beings, and they are placed on either side of the heart. The lungs function by extracting the oxygen from the air and diffusing it into the bloodstream. Unlike the filamentous gill, lungs are made up of several tubes, and each of them is meant for carrying air. Some microscopic capillaries surrounding the air sacs are present, which enhances gaseous exchange in vertebrate animals.

Can we make artificial gills?

The system of the artificial gill is still hypothetical, and it has not been proven to date. It is a theoretical technology that is yet to be demonstrated. The core objective of this technology is to reduce the intake of surrounding oxygen by allowing humans to breathe in oxygen from water sources like fresh water and the sea.

Like how gills work in fish, the technology of artificial gills was created to help human beings to survive in a water body. However, the usability of the so-called discovery might not be successful since human beings survive on a huge amount of oxygen. According to statistics, a diver, when they swim, will require 0.4 gal (1.5 L) of oxygen per minute and 0.15 gal (0.6 L) of oxygen per minute while they rest.

According to this number, a moderate person will require 52 gal (196.8 L) of oxygen. The seawater of the tropical area contains much vegetation; therefore, the oxygen content is the highest in such water. The whole process seems a bit sketchy. Passing such a huge amount of water through the system will require a lot of energy, and the device will also become bulky.

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