Science · Body foundations
Respiratory System and Gas Exchange
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Air travels through conducting passages to the alveoli.
- Pressure changes caused by chest volume move air.
- Oxygen and carbon dioxide diffuse down their concentration gradients.
- Rising carbon dioxide is a major signal to increase breathing.
What you’ll be able to do
- Trace air to the alveoli.
- Explain breathing pressure and gas diffusion.
- Connect carbon dioxide, pH, and breathing control.
Follow air inward, then follow oxygen into blood and carbon dioxide out.
Optional review
If you’d like to review the basics, visit Anatomical Orientation, Organization, and Homeostasis. You can start this lesson without completing those first.
From atmosphere to alveoli
Air enters through the nose or mouth, then passes the pharynx (throat), larynx (voice box), trachea (windpipe), bronchi (large branches), and smaller bronchioles. It ends in tiny air sacs called alveoli. Mucus traps particles, and cilia move the mucus upward. Cartilage helps keep larger airways open.
The epiglottis helps direct swallowed material away from the airway. The trachea divides into a right and left bronchus. Each bronchus branches into bronchioles. Bronchioles can change diameter, altering resistance to airflow. Alveoli are the exchange surface, not just the end of a tube.
From nose to lungs
Follow the airway from the nose into the lungs and down to the tiny air sacs. The diaphragm is the muscle below the lungs. When it contracts, chest space increases and air moves in. Air travels through airways, not blood vessels.
From nose to lungs — OpenStax College, Anatomy & Physiology (2014 CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
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