Lungs

• The air you breathe in goes through the trachea, this splits into two tubes called bronchi
• The bronchi split into progressively smaller tubes called the broncioles
• At the end of the bronchioles are tiny sacs called alveoli which have very thin walls to allow gases to diffuse across them easily.
• Alveoli are surrounded by capillaries so that oxygen can diffuse from the lungs into the bloodstream and carbon dioxide can diffuse out of the bloodstream into the lungs.
• The blood passing nexto the alveoli has just returned to the lungs from the rest of the body, so it contains lots of CO2 and very little oxygen.
• Oxygen diffuses out of the alveolus (high concentration) into the blood (low concentration). CO2 diffuses out of the blood into the alveolus to be breathed out
• When the blood reaches body cells, oxygen is released from red blood cells (high concentration) and diffuses into the body cells (low concentration).
• CO2 diffuses out of the body cells (high concentration) into the blood (low concentration). Its then carried back to the lungs.
• The lungs

  
• Bronchioles end in clusters of air sacs called alveoli