Gill

Many animals live in the water. They use gills to breathe.

Gills take the air from the water. This air goes into their blood. This helps them stay alive.

Some fish use gills to catch food too. Gills can be very thin and soft. They need water to stay strong. Without water, they might fall apart. Gills are a very special way to breathe underwater.

Many animals living in water need a way to breathe. They use special parts called gills.

Gills take oxygen from the water and put it into the blood. This is how they get the air they need.

Gills have a very large surface area. They use many tiny folds called lamellae to do this.

These folds help the animal catch as much oxygen as possible. This is important because water holds much less oxygen than air.

Fish move water over their gills to breathe. Some fish swim fast to push water through. Others use a pumping way to move water. Bony fish have a hard cover called an operculum. This cover helps them control water pressure.

Some animals have different types of gills.

Newt larvae have feathery gills on the outside of their bodies.

Sea slugs also have gills on their bodies. Even some crabs have gills to help them live. Gills are a vital part of life in the water.

Many animals living in water need a special way to breathe. They use organs called gills to take in oxygen from the water. Scientists sometimes use the word branchia to talk about gills.

These organs help animals get oxygen and get rid of carbon dioxide. Without gills, most active water animals could not survive. Gills are very important for life in the sea or a lake.

Gills work by using a very large surface area. They are made of tiny tissue threads called filaments. These filaments have many small folds called lamellae.

These folds allow more water to touch the gill at once. This is a big deal because water has much less oxygen than air. A cubic meter of air has about 275 grams of oxygen. Fresh water has less than 1/25th of that amount.

To be efficient, many fish use a countercurrent exchange. This means blood and water flow in opposite directions. This clever way of working can recover 90% of the oxygen.

People have studied gills for a very long time. An ancient thinker named Galen noticed fish had many tiny openings. He thought these openings were just for gases. Another writer named Pliny the Elder believed fish breathed with gills. He also noted that the thinker Aristotle had a different idea. The name branchia comes from the Greek word for gills.

Different animals have many different types of gills.

Most bony fish have five pairs of gills. They use a hard bony cover called an operculum to help pump water. Sharks and rays often have five pairs of gill slits.

Some sharks, like the Broadnose sevengill shark, have more than five. Some amphibians, like newt larvae, have feathery gills on the outside.

Even sea slugs have gills on their bodies.

You can think of gills like a filter for breathing. Just as a screen catches things, gills catch oxygen from the water. Some animals like crabs even keep water in chambers to breathe on land.

This helps them stay moist so they can still get oxygen. If a fish is taken out of water, its gills can collapse. The water helps hold the delicate gill shapes open. This is why gills work so well in the sea.

Gills, also known by the academic name branchia, are specialized respiratory organs. They allow aquatic animals to perform gas exchange. This process extracts dissolved oxygen from water and excretes carbon dioxide. While many tiny or inactive animals can absorb oxygen through their entire body surface, more complex or active organisms require gills. These organs are essential because water presents a much harder breathing environment than air. For example, a cubic meter of air contains about 275 grams of oxygen at standard temperature and pressure. In contrast, fresh water holds less than 1/25th of that amount.

The mechanism of a gill relies on maximizing surface area for diffusion. Gills typically consist of thin filaments of tissue and tiny folds called lamellae. These structures create a massive surface area in contact with the environment. Inside these filaments and lamellae are blood vessels or coelomic fluid. Gases move through the thin gill walls into the blood or hemolymph. This blood then carries the oxygen to the rest of the body via the circulatory system. Because water is 777 times more dense than air and 100 times more viscous, breathing requires constant movement.

To move oxygen efficiently, many fish use a countercurrent exchange mechanism. In this system, water passes over the gills in the opposite direction to the flow of blood. This clever arrangement allows the animal to recover as much as 90% of the dissolved oxygen in the water. Water is moved across the gills in several ways. Some animals use a specialized pumping mechanism to create a one-way current. Others rely on the animal's motion through the water, the beating of cilia, or other appendages. Without the support of surrounding water, these delicate gills would collapse and lie on top of each other.

Different groups of animals have evolved distinct gill structures. Cartilaginous fish, such as sharks and rays, typically have five pairs of gill slits. These slits open directly to the outside of the body. They are separated by cartilaginous gill arches, which support the interbranchial septum. Some sharks, like the Broadnose sevengill shark, have more than five pairs. Many sharks use ram ventilation, which means they must swim forward to force water over their gills. Other species, like skates, may use a spiracle to suck water in.

Bony fish have a different setup involving a branchial chamber. This chamber is covered by a bony structure called an operculum. The operculum helps adjust water pressure to allow for proper ventilation. This means many bony fish do not have to swim constantly to breathe. Most bony fish also have five pairs of gills. Some species also use their gills to regulate salt and water levels. They use specialized cells called ionocytes to manage the balance of ions in their blood.

Amphibians and invertebrates show even more variety in gill design. Many amphibian larvae, such as newt larvae, develop feathery external gills. These grow from the outer surface of the gill arches. While most amphibians lose these during metamorphosis, some species like the olm or mudpuppy keep them into adulthood.

Invertebrates also display diverse forms. Horseshoe crabs possess book gills, which are external flaps with many thin membranes. Some crustaceans protect their gills inside a gill chamber. Even sea slugs have visible gills on their bodies.

Humans have studied these biological structures for centuries. The word branchia comes from the Greek word for gills. Ancient thinkers provided different observations on how they work. The physician Galen noted that fish had many tiny openings, or foramina, that allowed gases to pass through. Pliny the Elder believed that fish breathed specifically through their gills. He also noted that the philosopher Aristotle held a different opinion. Today, we understand how these organs connect to broader biological systems, including the evolution of other structures like the ear opening in higher vertebrates.