Describe the gross structure of the human gas exchange system and how we breathe in and out. - AQA - A-Level Biology - Question 10 - 2018 - Paper 1

The human gas exchange system includes several key structures:

1. Trachea: The main airway that transports air from the larynx to the bronchi. It is supported by cartilage rings to keep it open.
2. Bronchi: The trachea divides into two bronchi (left and right) that lead into each lung. These bronchi further branch into smaller bronchioles.
3. Bronchioles: These are smaller branches of the bronchi that lead to the alveoli. They lack cartilage and are lined with smooth muscle, allowing them to constrict or dilate.
4. Alveoli: These tiny air sacs are the site of gas exchange, surrounded by capillaries. Their structure increases surface area for diffusion to facilitate the exchange of oxygen and carbon dioxide.

In terms of the breathing process:

• Inspiration: The diaphragm contracts and moves down, while external intercostal muscles contract, expanding the thoracic cavity, leading to a decrease in pressure and air flowing into the lungs.
• Expiration: The diaphragm relaxes, and the internal intercostal muscles contract, decreasing thoracic cavity volume and increasing pressure, pushing air out.