These questions ask you to place respiratory structures in order, explain how air moves, and decide where each gas goes. Work from the whole route to the local mechanism instead of memorising isolated organ names.
Trace the route and identify the driving difference
Follow inhaled air from the larynx into the trachea, then through the two main bronchi, smaller bronchioles and finally the alveoli. C-shaped cartilage supports the trachea without turning it into a rigid bone tube. At the end of the branches, many alveoli provide a large area with thin, moist walls beside capillaries. This short, damp boundary allows gases to diffuse down their concentration differences.
| Part of the mechanism | What enters, leaves or controls it |
|---|---|
| Conducting airways | Air enters through larger tubes and moves towards progressively smaller branches; food belongs in the oesophagus, not this route. |
| Alveolar surface | Oxygen moves from alveolar air into blood, while carbon dioxide moves from blood into the alveoli for removal. |
| Quiet inhalation | The diaphragm contracts, chest volume increases, pressure inside falls, and air moves inward. Intercostal muscles help move the rib cage. |
| Quiet exhalation | Inspiratory muscles relax, elastic recoil reduces chest volume, and air moves outward. |
| Airway protection | Mucus traps many particles, cilia sweep the trapped material towards the throat, and the epiglottis helps cover the airway entrance during swallowing. |
Use two different driving differences. Pressure differences move a bulk flow of air into or out of the lungs. Concentration differences move individual gases across the alveolar wall in opposite directions. Alveoli do not pump either air or gas; their structure makes passive exchange efficient.
Where the mistakes come from
| Tempting wrong route | How to reject it |
|---|---|
| Reversing the main gases or claiming that the lungs manufacture oxygen | Track their sources. Oxygen arrives from inhaled air; carbon dioxide arrives in blood after being made by cells. |
| Sending air through the oesophagus or heart, or starting with alveoli before larger branches | Keep the conducting chain continuous: trachea, bronchi, bronchioles, then alveoli. |
| Making each main bronchus enter both lungs or carry swallowed food | The trachea divides once into two main branches, with one leading into each lung. These are airways, not food tubes. |
| Choosing thick, dry or muscular alveolar walls | Diffusion needs a short moist path and ample area. Thickness, dryness and active pumping work against that design. |
| Contracting the diaphragm to shrink the chest, or asking alveoli to pump air out | Link contraction to increased chest volume during inhalation. Ordinary quiet exhalation follows relaxation and elastic recoil. |
| Giving cilia the job of gas exchange | Cilia clear conducting airways by sweeping trapped material towards the throat. Diffusion belongs at alveolar and capillary surfaces. |
Try it
The trachea stays open even when pressure around it changes. Which structure provides this support?
- A layer of alveolar fluid
- C-shaped cartilage rings
- A chain of small bones
- Contractions that pump air
At an alveolus, blood arriving from the body has less oxygen and more carbon dioxide than alveolar air. Which movement occurs?
- Both gases diffuse from alveolar air into blood.
- Oxygen enters blood, while carbon dioxide enters the alveolus.
- Oxygen leaves blood, while carbon dioxide also leaves the alveolus.
- The alveolus actively pumps both gases into blood.
While a person swallows, which structure helps cover the entrance to the airway?
- A main bronchus
- The diaphragm
- An alveolus
- The epiglottis