Respiratory system

The respiratory system is a special group of parts in living things that helps them get the air they need. All animals and plants have this system, even though it looks different in each one. It lets them trade gases, which means taking in fresh air and letting out used air.

In land animals like us, the respiratory system includes the lungs. Inside the lungs are tiny air sacs called alveoli where the trading of gases happens. Air moves through a tube called the trachea, then through smaller tubes called bronchioles before reaching the alveoli. Special muscles help push the air in and out, which we call breathing.

In water animals like fish, the respiratory system looks very different. Fish have gills that let them get oxygen from water. The water flows over the gills, and the oxygen moves into the fish’s body.

Even insects and plants have their own ways of getting gases. Insects have small holes and tubes, while plants use tiny openings called stomata to trade gases with the air around them.

Mammals

Main articles: Lung and Respiratory tract

In humans and other mammals, the respiratory system includes the respiratory tract, which has an upper and lower part. The upper tract has the nose, nasal cavities, sinuses, pharynx, and part of the larynx. The lower tract has the rest of the larynx, the trachea, bronchi, bronchioles, and alveoli, where gas exchange happens.

Fig. 1. Respiratory system

Breathing makes the lungs expand and contract. At rest, a typical adult takes in about 500 ml of air per breath. During heavy breathing, like when exercising, people breathe faster and deeper to get more oxygen.

The main job of the respiratory system is to swap gases between the air and the blood. Oxygen moves from the air in the alveoli into the blood, and carbon dioxide moves from the blood into the air to be breathed out. This helps keep the body’s oxygen and carbon dioxide levels balanced.

The breathing rate changes based on the body’s needs. For example, it speeds up during exercise because muscles need more oxygen. Special sensors in the brain and blood detect changes in carbon dioxide and oxygen levels and help control breathing.

Fig. 2. The lower respiratory tract, or "Respiratory Tree"TracheaMainstem bronchusLobar bronchusSegmental bronchusBronchioleAlveolar ductAlveolus

When moving to high altitudes where there’s less oxygen, people breathe deeper and faster. The body also makes more red blood cells to carry extra oxygen.

The respiratory system also helps protect the body. Coughing and sneezing can clear out irritants. The lining of the airways produces substances that fight germs and keep the lungs clean.

Breathing also plays a role in speaking and making sounds. Moving air through the larynx creates voice in humans, and in birds, it’s done through an organ called the syrinx.

Some problems with the respiratory system include blockages, infections, and damage to the lungs. Doctors called pulmonologists and respiratory therapists help treat these issues. In severe cases, machines called medical ventilators can help a person breathe.

Measurement	Equation	Description
Minute ventilation	tidal volume * respiratory rate	the total volume of air entering, or leaving, the nose or mouth per minute or normal respiration.
Alveolar ventilation	(tidal volume – dead space) * respiratory rate	the volume of air entering or leaving the alveoli per minute.
Dead space ventilation	dead space * respiratory rate	the volume of air that does not reach the alveoli during inhalation, but instead remains in the airways, per minute.

Exceptional mammals

Cetaceans

Horses

Horses breathe only through their noses, unlike many other mammals. They cannot breathe through their mouths, which helps keep food from entering their lungs when they swallow. This special way of breathing means horses rely completely on their nostrils for air.

Elephants

The elephant is unique among mammals because it does not have a space called the pleural space in its lungs. Instead, its lung tissues are connected by strong fibers. This helps elephants stay underwater for long periods when they breathe through their trunk, which acts like a snorkel. Their breathing mainly depends on a strong muscle called the diaphragm.

Birds

The respiratory system of birds works differently from that of mammals. Birds have rigid lungs that do not expand and contract during breathing. Instead, they have many air sacs spread throughout their bodies that act like bellows, pulling fresh air in and pushing used air out.

Birds do not have diaphragms or pleural cavities like mammals. Their lungs are smaller, but the air sacs make up a larger part of their body volume. When birds breathe in, muscles attached to their ribs move to increase the size of their body cavity, which lets fresh air fill the air sacs and lungs. When they breathe out, these muscles reverse, pushing the used air out. This special system allows birds to have a constant flow of fresh air through their lungs with each breath.

Reptiles

Main article: Reptile § Respiratory system

Reptiles, such as snakes and lizards, have simpler lungs compared to mammals. Even though their lungs are not as complex, they still use tiny air sacs called alveoli for trading gases. Unlike mammals, reptiles do not have a diaphragm to help them breathe. Instead, they change the size of their body cavity by using muscles between their ribs. Turtles are an exception, using special muscles in their flanks to breathe in and out.

Amphibians

Main article: Amphibian § Respiratory system

Amphibians, like frogs and salamanders, use both their lungs and their skin to breathe. When on land, they take in air through their nostrils and push it into their lungs using special muscles. Their skin, which stays moist and is covered in a slippery liquid, also helps them breathe, especially when they are in water. Young amphibians, such as tadpoles, have gills to help them breathe underwater, but many keep using their lungs as they grow older.

Fish

Main article: Fish physiology § Respiration

Fish need oxygen to live, but water holds much less oxygen than air. Fish solve this problem with special organs called gills. Gills have thin walls and a large surface area, which helps them take in oxygen from the water as it flows over them.

Some fish, like sharks, move constantly to push water through their gills, while others can pump water through their gills when they are resting. A few special fish, like lungfish and labyrinth fish, can also breathe air directly from above the water using extra organs they develop as they grow older. These fish can survive out of water for short periods if they stay moist.

Invertebrates

Arthropods

Insects

Most insects breathe through tiny holes in their bodies called spiracles. Air travels through a network of tubes called 'tracheae' and even smaller tubes called 'tracheoles' that reach every cell. These tubes help deliver oxygen to where it’s needed, especially when the insect is active.

Molluscs

Molluscs, such as snails and clams, usually have gills for breathing in water. They also have a heart that carries a special molecule called hemocyanin to transport oxygen. Some molluscs, like land snails, have a lung instead of gills.

Plants

Main article: Photosynthesis

Plants use carbon dioxide to make food through photosynthesis, and they release oxygen as a by-product. This process uses sunlight to turn carbon dioxide and water into sugar and oxygen. The opposite of this process is called respiration, where plants take in oxygen and release carbon dioxide to get energy for their cells.

Plants breathe in carbon dioxide through tiny holes on their leaves called stomata. They don’t need a lot of oxygen, but they do need carbon dioxide to make their food. It takes a lot of air for a plant to make even a small amount of sugar because the air we breathe has only a tiny bit of carbon dioxide in it.