The structure and function of the human respiratory system, the mechanism of breathing in and out, the adaptations of the alveoli for gas exchange, and the effect of exercise on breathing.

What this dot point is asking

WJEC wants you to name the parts of the respiratory system and their functions, describe how breathing in and out happens in terms of volume and pressure, explain how the alveoli are adapted for gas exchange, and describe the effect of exercise on breathing.

The parts of the respiratory system

Air travels through several structures on its way to the gas exchange surface.

• Trachea: the windpipe, held open by rings of cartilage, carrying air from the throat towards the lungs.
• Bronchi: the two tubes (one to each lung) that branch from the trachea.
• Bronchioles: smaller tubes that branch from the bronchi within each lung.
• Alveoli: tiny air sacs at the ends of the bronchioles, where gas exchange takes place.
• Diaphragm: a sheet of muscle below the lungs that changes the volume of the chest.
• Intercostal muscles: muscles between the ribs that move the rib cage.
• Ribs: bones that protect the lungs and move during breathing.

The mechanism of breathing

Breathing (ventilation) works by changing the volume of the chest, which changes the pressure inside the lungs.

Inhalation (breathing in). The diaphragm contracts and flattens (moves down), and the external intercostal muscles contract, pulling the rib cage up and out. This increases the volume of the chest, which lowers the pressure inside the lungs below the outside air pressure, so air moves into the lungs.

Exhalation (breathing out). The diaphragm relaxes and domes upward, and the intercostal muscles relax so the rib cage moves down and in. This decreases the volume of the chest, which raises the pressure inside the lungs, so air moves out.

Gas exchange at the alveoli

The alveoli are where oxygen enters the blood and carbon dioxide leaves it, by diffusion.

• Oxygen diffuses from the air in the alveoli (high concentration) into the blood in the capillaries (low concentration).
• Carbon dioxide diffuses from the blood (high concentration) into the alveoli (low concentration) to be breathed out.

The alveoli are superbly adapted for this exchange, sharing the same features as other exchange surfaces.

• Large surface area: there are millions of alveoli, giving a huge total area for diffusion.
• Thin walls: each alveolus wall is only one cell thick, so the diffusion distance is very short.
• Moist lining: gases dissolve in the moisture before diffusing across.
• Rich blood supply: a dense network of capillaries carries gases away quickly, keeping a steep concentration gradient.

The effect of exercise

During exercise, the muscles respire faster and need more oxygen and produce more carbon dioxide. To meet this demand, breathing becomes faster and deeper, moving more air in and out per minute. This increases the rate of gas exchange so more oxygen reaches the blood and more carbon dioxide is removed.