Breathing and circulation
How does inhaled oxygen reach body cells, and how does carbon dioxide leave?
Follow air, alveoli, and blood to explain how ventilation, gas exchange, and circulation work together.
Learning goals
- Describe the route of air to the alveoli in order.
- Link diaphragm motion, chest volume, and pressure in the lungs.
- Explain how alveolar exchange and circulatory transport are connected but distinct.
Before you begin
- Know that cells need oxygen and produce carbon dioxide.
- Use arrows to represent the direction of movement.
Interactive science experiment
- 1Airflow route
- 2Diaphragm & pressure
- 3Alveolar gas exchange
- 4Circulation transport
Preparing the interactive model…
Qualitative teaching model: values and diagrams compare trends; they are not precision measurements.
Observe, then explain
Work through four steps. Each button moves the interactive model to the matching observation state.
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Air enters through the nose or mouth, travels down the trachea and bronchi into smaller branches, and reaches the alveoli. Airways do not carry air directly to every cell.
Your task: Point out the route in order: nose/mouth, trachea, bronchi, alveoli.
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During inhalation the diaphragm contracts and moves down. Chest volume increases, pressure in the lungs falls below outside pressure, and air flows in. Exhalation reverses the trend.
Your task: Move the breathing-phase control and compare diaphragm position and airflow during inhalation and exhalation.
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Where oxygen partial pressure is higher in alveolar air, oxygen diffuses across the thin alveolar–capillary barrier into blood. Carbon dioxide diffuses the other way and is exhaled.
Your task: State the direction in which oxygen and carbon dioxide cross the alveolar wall.
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The heart sends oxygen-rich blood to body tissues. Blood returns carbon dioxide made by cells to the lungs. Ventilation refreshes air; circulation transports gases between lungs and tissues.
Your task: Trace one portion of blood around lungs → heart → body → heart → lungs.
Build an explanation
Ventilation, exchange, and cellular respiration
Everyday “breathing” can blur several processes. Ventilation moves air in and out; gas exchange is diffusion across membranes; cellular respiration is a set of chemical reactions that releases usable energy inside cells.
How structure supports function
Many alveoli create a large exchange area. A thin moist barrier and dense capillaries shorten diffusion distance and help maintain gradients. No single feature works alone.
Check your understanding
Printable worksheet
Complete the worksheet first. Answers and explanations are on a separate page.
Biology
Breathing and circulation
How does inhaled oxygen reach body cells, and how does carbon dioxide leave?
- Which route takes air to the gas-exchange surface?
- Which changes begin a normal inhalation?
- How do oxygen and carbon dioxide usually move at the alveoli?
- Why is getting air into the lungs not enough to supply cells in the foot?
Answer page
Breathing and circulation
- Nose/mouth → trachea → bronchi → alveoli
Airways lead from nose or mouth through the trachea and bronchi to alveolar exchange surfaces.
- Diaphragm down, chest volume up, lung pressure down
Increasing chest volume makes lung pressure temporarily lower than outside pressure, so air flows in.
- Oxygen into blood; carbon dioxide into alveoli
Each gas diffuses down its own partial-pressure gradient, in opposite directions.
- Gas exchange and blood circulation are also needed
Oxygen first crosses from alveoli into blood, then heart-driven circulation carries it to tissues.