Respiratory system

All living things need air.

Plants and animals use a system to get it.

In humans, air travels through tubes. These tubes look like a tree.

The tubes lead to tiny air sacs.

These sacs are very small. They help the air reach your blood. This lets your body use the air.

Fish breathe in a different way. They use gills to get air from water.

Even plants have a way to breathe. They use tiny parts to take in air. Breathing helps all life stay alive.

All living things need to swap gases to stay alive. Animals and plants use a respiratory system to do this.

In humans, air travels through many tubes. This looks like a tree. Scientists call this the respiratory tree.

The air goes down the trachea, which is a large tube. Then it moves into smaller tubes called bronchi. These branch into even smaller tubes called bronchioles. At the very end are tiny air sacs. These are called alveoli. These sacs are right next to blood vessels. This allows oxygen to move into the blood.

Breathing is a set of steps using muscles. The diaphragm is a sheet of muscle in your chest. When it contracts, it moves down. This makes more room in your chest. This change in space pulls air into your lungs. When the muscle relaxes, air moves out.

A single breath is called a tidal volume. In a resting adult, this is about 500 ml. Some air always stays in your tubes. This is called dead space. It is about 150 ml of air. Fish use gills to get air from water instead.

Gills help them take gases from the water.

The respiratory system is a vital biological system. It helps animals and plants swap gases to stay alive.

In land animals, this gas exchange happens inside the lungs. The lungs contain millions of tiny air sacs. In mammals and reptiles, these sacs are called alveoli. Birds have similar sacs known as atria. These microscopic sacs have a rich blood supply. This brings the air into close contact with the blood.

Air travels through a series of hollow tubes. This system of airways is often called the respiratory tree.

First, air enters the trachea, which is the largest tube. The trachea branches into two main bronchi in the chest. These bronchi enter the lungs and branch into narrower tubes. In mammals, these smaller tubes are called bronchioles. In birds, these tubes are called parabronchi. These tubes eventually lead to the tiny alveoli or atria.

Breathing is the way air is pumped into these sacs. This happens using the muscles of respiration. In mammals, the diaphragm does most of the work. The diaphragm is a sheet of muscle that separates the chest from the belly. When it contracts, it flattens and moves downward. This increases the space inside the chest cavity. This change in space makes the air pressure fall. As a result, air flows into the lungs.

Humans have about 23 branching generations in their respiratory tree. An adult human breathes about 12 to 16 times every minute. A single resting breath is called the tidal volume. This is about 500 ml for a resting adult. Some air stays in the airways after you breathe out. This is called dead space and is about 150 ml. Even after a hard breath out, some air remains. This is the residual volume, which is about 1.0 to 1.5 liters.

Different animals use different ways to breathe. Most fish use gills to get gases from the water. Gills consist of thin filaments and lammellae. These parts create a large surface area for the water to touch. Insects have very simple respiratory systems. In amphibians, the skin also helps with gas exchange. Even plants have respiratory systems. They use tiny features called stomata to move gases.

The respiratory system is a vital biological system used for gas exchange. This process allows animals and plants to move gases in and out of their bodies. In land animals, the respiratory surface is internalized as linings of the lungs.

This internal placement helps protect the delicate surfaces where gas exchange occurs. In mammals and reptiles, these surfaces are found in millions of tiny air sacs called alveoli. Birds use similar microscopic sacs known as atria. These sacs are highly vascularized, meaning they have a very rich blood supply. This brings the air into close contact with the blood to facilitate exchange.

In mammals, air travels through a complex network of airways called the respiratory tree.

The largest airway is the trachea, which is about 1.8 cm in diameter. The trachea branches in the middle of the chest into two main bronchi. These bronchi are 1 to 1.4 cm in diameter and enter the lungs at the hilum. They then branch into secondary bronchi, also called lobar bronchi. These further divide into tertiary or segmental bronchi, which are 1 to 6 mm in diameter. The smallest tubes are the bronchioles, which lack any cartilaginous support. In birds, these branching tubes are instead called parabronchi.

Humans possess approximately 23 branching generations within this respiratory tree. The earlier generations, from 0 to 16, act primarily as air conduits. These include the trachea, bronchi, and larger bronchioles. The later generations, from 17 to 23, consist of respiratory bronchioles, alveolar ducts, and the alveoli. This is where the actual gas exchange takes place. The number of branches varies between species; for example, a mouse has only about 13 branching generations. Because the alveoli are dead-end terminals, any air that enters them must exit via the same route.

Breathing is the process of pumping air into these microscopic sacs. In mammals, inhalation at rest is driven primarily by the diaphragm. The diaphragm is an upwardly domed sheet of muscle that separates the thoracic cavity from the abdominal cavity. When the diaphragm contracts, it flattens and moves downward. This increases the volume of the thoracic cavity. Simultaneously, the intercostal muscles pull the ribs upward to enlarge the chest.

These actions cause the intrathoracic pressure to fall. Because the lungs are elastic, they expand to fill the increased space, drawing air in.

During exhalation, the process becomes largely passive. The diaphragm and intercostal muscles relax, allowing the chest and abdomen to return to a resting position. This position is determined by anatomical elasticity. Resting exhalation typically lasts about twice as long as inhalation. This is because the diaphragm relaxes more gently than it actively contracts.

Some air always remains in the airways after exhalation, a volume known as dead space. In an adult human, this dead space is about 150 ml. This air is breathed back into the alveoli before fresh environmental air reaches them.

We can measure various lung volumes using a tool called a spirometer. A resting adult human has a tidal volume of about 500 ml per breath. Even after a maximally forced exhalation, a person cannot expel all the air. This remaining amount is the residual volume, which is about 1.0 to 1.5 liters. The functional residual capacity, which includes this residual volume, is about 2.5 to 3.0 liters. The total lung capacity for an adult human is approximately 6 liters.

An average healthy human maintains a respiratory rate of 12 to 16 breaths per minute.

Different environments require different respiratory structures. Most fish and many aquatic animals use gills for gas exchange. Gills consist of thin filaments and lammellae that provide a large surface area for highly vascularized tissue.

Water flows over these gills through active or passive means. Other organisms use different methods; for instance, insects have very simple anatomical features. Amphibians often use their skin to assist in gas exchange. Even plants participate in gas exchange using anatomical features called stomata.