At a Glance
The human respiratory system is made of the upper respiratory tract (nose, nasal cavity, sinuses, mouth, pharynx and larynx), the lower respiratory tract (trachea, bronchi, bronchioles and alveoli inside the two lungs) and the breathing muscles, mainly the diaphragm. At rest an adult breathes about 12 to 20 times a minute.
- Main job
- Take in oxygen, remove carbon dioxide
- Two divisions
- Upper and lower respiratory tract
- Gas exchange site
- Alveoli inside the lungs
- Main breathing muscle
- Diaphragm
- Resting breathing rate
- About 12 to 20 breaths per minute (adult)
On this page
What Is the Respiratory System?
The respiratory system is the body system that moves air in and out of the lungs and swaps oxygen for carbon dioxide in the blood. Every cell needs a steady supply of oxygen to release energy from food, and every cell produces carbon dioxide as waste. The respiratory system is how the body handles both, thousands of times a day.
It is built from three working parts. The airways (the respiratory tract) carry air. The lungs hold the gas exchange surface. The breathing muscles, led by the diaphragm and the intercostal muscles, pump air in and out. The diagram above shows how these parts sit together in the head, neck and chest.
Explained simply: your airways are a branching set of pipes, your lungs are two sponges full of tiny air bubbles, and your diaphragm is the pump underneath that keeps fresh air coming.
Main Sections
Upper Respiratory Tract
Lower Respiratory Tract
From the Glossary
What Are the Parts of the Respiratory System?
The main parts of the respiratory system, in the order air passes through them, are the nose and nasal cavity, the pharynx, the larynx, the trachea, the bronchi, the bronchioles and the alveoli. The paranasal sinuses open into the nasal cavity, the epiglottis guards the entrance to the larynx, and the oral cavity serves as a second way in when you breathe through your mouth.
Each part has its own guide on this site. The table lists them with the job each one does.
| Part | Tract | Main job |
|---|---|---|
| Nose and nasal cavity | Upper | Lets air in; warms, moistens and filters it; holds the smell receptors |
| Paranasal sinuses | Upper | Air-filled spaces that lighten the skull, make mucus and shape the voice |
| Oral cavity (mouth) | Upper | Second entrance for air, used when the nose is blocked or during exercise |
| Pharynx (throat) | Upper | Shared passage for air and food behind the nose and mouth |
| Larynx (voice box) | Upper | Holds the vocal cords and protects the airway |
| Epiglottis | Upper | Flap that covers the larynx when you swallow |
| Trachea (windpipe) | Lower | Carries air from the larynx to the bronchi |
| Bronchi | Lower | Carry air into each lung and branch inside it |
| Bronchioles | Lower | Small airways that lead to the alveoli |
| Alveoli | Lower | Tiny air sacs where oxygen and carbon dioxide are exchanged |
| Lungs | Lower | Hold the bronchial tree and alveoli; right lung 3 lobes, left lung 2 |
| Diaphragm | Breathing muscle | Main muscle of breathing; contracts to draw air in |
| Source: Standard human anatomy, our editors | ||
Upper vs Lower Respiratory Tract: What Is the Difference?
The upper respiratory tract runs from the nose down to the larynx, and the lower respiratory tract is everything from the trachea down. The upper respiratory tract warms, moistens and filters the air, gives us our sense of smell and lets us speak. The lower respiratory tract conducts the air deep into the chest and performs gas exchange.
Doctors use the same split when they talk about infections. A cold usually stays in the upper tract, while bronchitis and pneumonia involve the lower tract. You can read about both groups on our respiratory diseases page.
How Does Breathing Work, Step by Step?
Breathing works by changing the size of the chest so that air pressure inside the lungs falls below or rises above the pressure outside.
- The diaphragm contracts and flattens, and the external intercostal muscles lift the ribs up and out.
- The chest cavity gets larger, so pressure inside the lungs drops and air flows in through the nose or mouth.
- The air is warmed, moistened and filtered in the nose and nasal cavity, then passes the pharynx and larynx.
- It runs down the trachea and splits into the right and left bronchi, then into ever smaller bronchioles.
- In the alveoli, oxygen diffuses into the surrounding capillaries and carbon dioxide diffuses out.
- The diaphragm relaxes, the lungs recoil, and the used air is breathed out.
Quiet breathing out is mostly passive. During exercise, extra muscles in the neck, chest and belly help to move more air.
What Are the Main Functions of the Respiratory System?
The main function of the respiratory system is gas exchange: delivering oxygen to the blood and removing carbon dioxide. It also does several other jobs that are easy to overlook.
- Ventilation: moving air in and out of the lungs.
- External respiration: oxygen and carbon dioxide trade places between the alveoli and the blood.
- Internal respiration: the blood hands oxygen to the tissues and collects carbon dioxide.
- Voice: air passing the vocal cords in the larynx makes sound.
- Smell: odor receptors sit in the roof of the nasal cavity.
- Protection: mucus, tiny hairs called cilia and the cough reflex trap and remove dust and germs.
- Acid-base balance: breathing faster or slower changes how much carbon dioxide, and therefore acid, stays in the blood.
Each of these is explained in detail in our guide to respiratory system functions.
Which Diseases Affect the Respiratory System?
Respiratory diseases range from short infections such as the common cold and influenza to long-term conditions such as asthma and chronic obstructive pulmonary disease (COPD). Pneumonia, bronchitis and sinusitis are other common examples. Each one affects a particular part of the system, which is why knowing the anatomy helps make sense of them.
Our list of respiratory diseases groups them by the upper and lower tract. It is for learning only; anyone with breathing symptoms should see a doctor. For unfamiliar terms, try the respiratory system glossary.
Key Facts
- The respiratory system is divided into the upper respiratory tract and the lower respiratory tract, with the lower edge of the larynx as the usual boundary.
- Gas exchange happens only in the alveoli and the smallest bronchioles; the rest of the airways just conduct air.
- The right lung has three lobes and the left lung has two, leaving room for the heart.
- A healthy adult at rest breathes about 12 to 20 times per minute.
- The diaphragm is the main breathing muscle; when it contracts, air flows into the lungs.
- Mucus and cilia lining the airways trap dust and germs and sweep them up toward the throat.
Frequently Asked Questions
What are the 3 main parts of the respiratory system?
The three main working parts are the airways, the lungs and the muscles of breathing. The airways (nose, pharynx, larynx, trachea, bronchi and bronchioles) carry air. The lungs contain the alveoli, where gas exchange happens. The diaphragm and intercostal muscles change the size of the chest so that air moves in and out.
What is the main function of the respiratory system?
Its main function is gas exchange: bringing oxygen from the air into the blood and removing carbon dioxide from the blood so it can be breathed out. Oxygen lets cells release energy from food, and removing carbon dioxide keeps the blood from becoming too acidic. The system also helps with speech, smell and protection from inhaled germs.
What is the pathway of air through the respiratory system?
Air enters through the nostrils or mouth, passes through the nasal cavity, then the pharynx and larynx. It travels down the trachea, which splits into the left and right primary bronchi. These divide into smaller bronchi and then bronchioles, which end in clusters of alveoli inside the lungs. Breathed-out air follows the same path in reverse.
How do the respiratory and circulatory systems work together?
The respiratory system brings air into close contact with blood, and the circulatory system carries that blood around the body. In the lungs, oxygen moves from the alveoli into capillaries and carbon dioxide moves the other way. The heart then pumps oxygen-rich blood to the tissues and returns carbon dioxide-rich blood to the lungs.
Is the mouth part of the respiratory system?
Yes. The oral cavity is usually counted as part of the upper respiratory tract because it is a second entrance for air. Breathing through the nose is better for the airways, since the nose warms, moistens and filters the air, but the mouth takes over when the nose is blocked or when we need more air during exercise.
Where does gas exchange take place?
Gas exchange takes place in the alveoli, the tiny air sacs at the ends of the bronchioles inside the lungs. Each alveolus is wrapped in a net of capillaries, and the wall between air and blood is extremely thin. Oxygen diffuses across it into the blood, and carbon dioxide diffuses from the blood into the alveolus to be breathed out.