The Big Picture
Your body needs oxygen to survive. The respiratory system brings oxygen into the lungs and removes carbon dioxide from the body.
Air moves through a series of passages before reaching tiny air sacs in the lungs. This is where oxygen enters the blood and carbon dioxide leaves the blood.
Breathing also depends on the diaphragm. The lungs cannot move by themselves, so movement of this muscle drives air movement in and out.
Quick tip: Tap, click, or press Enter on any term with a dotted underline and an ⓘ icon to see its plain-English definition.
Start Here If You’re Overwhelmed
Focus on these core ideas:
- The respiratory system brings oxygen into the lungs and removes carbon dioxide.
- Air travels through the nose, pharynx, larynx, trachea, bronchi, bronchioles, and finally the alveoli.
- Gas exchange happens between the alveoli and surrounding capillaries.
- The right lung has three lobes. The left lung has two lobes.
- The diaphragm provides the driving force for ventilation.
- The pleura and its thin layer of fluid help prevent rubbing during breathing.
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Course Notes: Key Ideas You Need to Know
What the Respiratory System Does
Every cell needs oxygen. Air contains about 21% oxygen, which meets the body’s needs under normal conditions.
The respiratory system has two main jobs:
- Bring oxygen into the lungs.
- Remove carbon dioxide from the body.
Respiration is the process of supplying oxygen to cells and removing carbon dioxide from them.
Inhalation, Exhalation, and Ventilation
Inhalation means breathing in.
Exhalation means breathing out.
You might also see two other terms:
- Inspiration means breathing in.
- Expiration means breathing out.
Together, inspiration and expiration make up ventilation.
Upper and Lower Respiratory Tracts
The respiratory system has an upper respiratory tract and a lower respiratory tract.
Upper Respiratory Tract
The upper respiratory tract includes:
- nose and nasal cavities
- pharynx
- larynx
Lower Respiratory Tract
The lower respiratory tract includes:
- trachea
- bronchi
- bronchioles
- alveoli
How Air Travels Through the Respiratory System
Air first enters through the nose.
It then passes into the pharynx, also called the throat. The pharynx is a tube-shaped passageway used by both air and nutrients.
From the pharynx, air moves into the larynx. The larynx contains the vocal cords and is also called the voice box.
The Epiglottis
The epiglottis is a piece of cartilage that acts like a lid over the larynx.
During swallowing, it prevents food and liquids from entering the airway.
During inhalation, the epiglottis lifts so air can pass over the larynx and enter the trachea, also called the windpipe.
From the Trachea to the Alveoli
The trachea divides at its lower end into two branches:
- right bronchus
- left bronchus
Each bronchus enters one lung.
Inside the lungs, each bronchus divides several times into smaller branches called bronchioles.
The bronchioles keep dividing until they end in tiny air sacs called alveoli.
Alveoli and Gas Exchange
The alveoli have an extremely thin membrane that is only one cell layer thick.
They look like small clusters of grapes and have large numbers of pulmonary capillaries around them.
Capillaries are the smallest blood vessels. They are sites where gases and other substances are exchanged between cells and the bloodstream.
Gas exchange between the alveoli and capillaries involves oxygen and carbon dioxide.
What Happens to Oxygen
Blood in the capillaries picks up oxygen from the alveoli.
The blood then returns to the left side of the heart. From there, the heart pumps it to the rest of the body.
What Happens to Carbon Dioxide
The alveoli pick up carbon dioxide from the capillaries.
The body then removes the carbon dioxide through breathing out.
The Lungs
The lungs are spongy tissues.
They contain:
- alveoli
- blood vessels
- connective tissue
- a few nerves
Each lung is divided into sections called lobes.
Number to remember:
- The right lung has three lobes.
- The left lung has two lobes.
The Diaphragm and Ventilation
The lungs and airways sit inside the thoracic cavity .
A muscle called the diaphragm separates the thoracic cavity from the abdominal cavity.
The lungs do not contain actual skeletal muscle tissue. Because of this, they cannot move by themselves.
Movement of the diaphragm provides the driving force for ventilation.
The Pleura
Each lung has a two-layered sac around it called the pleura.
One layer attaches to the lung. The other layer attaches to the chest wall.
The pleura produces a thin fluid that fills the space between these layers.
Function: This fluid prevents the two layers from rubbing together during inhalation and exhalation.
Fast Review
Use these points for a quick review before a quiz or exam.
- The respiratory system brings oxygen into the lungs and removes carbon dioxide.
- Respiration supplies oxygen to cells and removes carbon dioxide from them.
- Inhalation and inspiration mean breathing in. Exhalation and expiration mean breathing out.
- Inspiration and expiration together are called ventilation.
- Air moves through the nose, pharynx, larynx, trachea, bronchi, bronchioles, and alveoli.
- The epiglottis prevents food and liquids from entering the airway during swallowing.
- Oxygen and carbon dioxide move between the alveoli and surrounding capillaries.
- Blood picks up oxygen from the alveoli and returns to the left side of the heart before being pumped through the body.
- The right lung has three lobes. The left lung has two.
- The diaphragm provides the driving force for ventilation because the lungs cannot move by themselves.
- The pleura has two layers and produces fluid that prevents rubbing during breathing.
Final Pep Talk
There are several respiratory structures and new terms to remember, so focus first on the path air follows and what happens at the alveoli. Once that path feels familiar, the other pieces become easier to organize during review.