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. It has an upper and lower part. The upper part has the nose, nasal cavities, sinuses, pharynx, and part of the larynx. The lower part 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. The main job of the respiratory system is to swap gases between the air and the blood. Oxygen moves from the air 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. 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. This helps keep food from entering their lungs when they swallow. Because of this, 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 is different from that of mammals. Birds have lungs that do not change size when they breathe. Instead, they have many air sacs in their bodies that work like bellows. These air sacs pull fresh air in and push used air out.

Birds do not have diaphragms or pleural cavities like mammals. Their lungs are smaller, but the air sacs take up more space in their bodies. When birds breathe in, muscles connected to their ribs move to make their body cavity bigger. This lets fresh air fill the air sacs and lungs. When they breathe out, the muscles move back, pushing the used air out. This special system helps birds get a steady flow of fresh air with every breath.

Reptiles

Main article: Reptile § Respiratory system

Reptiles, such as snakes and lizards, have simpler lungs than mammals. Their lungs are not as complex, but they still use tiny air sacs called alveoli to trade gases. Unlike mammals, reptiles do not have a diaphragm to help them breathe. Instead, they change the size of their body cavity using muscles between their ribs. Turtles are different; they use special muscles in their flanks to breathe in and out.

Amphibians

Main article: Amphibian § Respiratory system

Amphibians, like frogs and salamanders, breathe using both their lungs and their skin. On land, they take in air through their nostrils and push it into their lungs. Their skin stays moist and helps them breathe, especially in water. Young amphibians, such as tadpoles, have gills to breathe underwater, but they often start using lungs as they grow.

Fish

Main article: Fish physiology § Respiration

Fish need oxygen to live, but water has less oxygen than air. Fish use special organs called gills to get oxygen from water. Gills have thin walls and a large surface area, which helps them take in oxygen as water flows over them.

Some fish, like sharks, must keep moving to push water through their gills. Others can pump water through their gills while resting. A few special fish, like lungfish and labyrinth fish, can breathe air directly from above the water using extra organs they develop. 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. They release oxygen as a by-product. This process uses sunlight to turn carbon dioxide and water into sugar and oxygen.

Plants take in carbon dioxide through tiny holes on their leaves called stomata. They need carbon dioxide to make their food, but they do not need much oxygen. 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.