If you're seeing this message, it means we're having trouble loading external resources on our website.
If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.
Main content
Unit 4: Respiratory system anatomy and physiology
About this unit
With every inhale, air rushes through your windpipe to your lungs. After 20 to 30 branch points, oxygen diffuses into the liquid around the alveoli and slips into your blood. Carbon dioxide travels from the blood to the air and leaves the body when you exhale. Learn more about the respiratory system in these videos!