Describe the structure of the respiratory system and trace the passage of air from the nose to the alveoli

What this dot point is asking

SEAB wants you to describe the structure of the respiratory system and trace the path of air from the nose down to the alveoli, where gas exchange happens. The central idea is that the airways form a branching tree that carries air deep into the lungs, ending in millions of tiny sacs built to swap gases with the blood.

The answer

The pathway of air

Air follows a clear route into the lungs.

• It enters through the nose (or mouth), where it is warmed, moistened and filtered.
• It passes down the trachea (windpipe), a tube held open by rings of cartilage.
• The trachea divides into two bronchi, one entering each lung.
• Each bronchus branches into many smaller bronchioles.
• The bronchioles end in clusters of tiny air sacs called alveoli, where gas exchange occurs.

The structures that support breathing

Several structures move air in and out and protect the lungs.

• The diaphragm is a sheet of muscle below the lungs that contracts and relaxes to change the chest volume.
• The intercostal muscles between the ribs help raise and lower the ribcage.
• The lungs are the two spongy organs that house the airways and alveoli.
• The pleural membranes line the lungs and chest wall, reducing friction as the lungs expand.

The alveoli

The alveoli are where oxygen enters the blood and carbon dioxide leaves it. They are perfectly built for the job:

• a large surface area from millions of sacs, so a lot of gas is exchanged at once;
• walls one cell thick, so gases diffuse over a very short distance;
• a rich capillary network, which keeps the concentration gradient steep;
• a moist lining, so gases dissolve before they diffuse.

Examples in context

Example 1. A swimmer taking a deep breath before a dive. Air rushes in through the mouth, down the trachea and bronchi, into the bronchioles and alveoli, filling the spongy lungs. The large alveolar surface area lets the swimmer load a big store of oxygen into the blood before going under.

Example 2. A runner at altitude. With less oxygen in the air, the same alveolar features still work, but the smaller concentration gradient slows oxygen uptake. This is why athletes find endurance harder at altitude and why training there can boost red blood cell numbers over time.

Try this

• Cue. Put these in the order air reaches them: bronchioles, trachea, alveoli, nose, bronchi. (Nose, trachea, bronchi, bronchioles, alveoli.)

• Cue. Name the main muscle of breathing and where it sits. (The diaphragm, a sheet of muscle below the lungs.)

• Cue. Explain why thin alveolar walls speed up gas exchange. (They give a very short diffusion distance, so oxygen and carbon dioxide cross between air and blood quickly.)