The airways of the gas exchange system are lined with epithelium. Gradual changes in the structural features of this epithelium occur as the airways branch and become increasingly narrow. (a) Table 4.1 shows the changes that occur in the number of goblet cells in the epithelium of the different structures of the gas exchange system. [Table 4.1] gas exchange structure | number of goblet cells in epithelium trachea | many bronchi | many larger bronchioles | very few smaller bronchioles | none alveoli | none Goblet cells produce mucus, which is important in maintaining the health of the airways. The smallest bronchioles closest to the alveoli are known as respiratory bronchioles. Suggest and explain why respiratory bronchioles do not have any goblet cells.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2022). The mark or mark type is shown in brackets.

1. [1] presence of mucus can, hinder / prevent / AW, gas exchange / entry of air (to alveoli) / airflow (to alveoli); A named gases
2. [1] detail; e.g. (lumens of) airways / respiratory bronchioles, are narrow or airways / respiratory bronchioles, are close to alveoli
3. [1] suggestion that respiratory bronchioles have, few / no, cilia present so difficult to move mucus / AW ; R bronchioles do not have cilia as a general statement
4. [1] mucus increases distance for diffusion of (respiratory) gases / oxygen / carbon dioxide
5. [1] R if direction of diffusion of gas is incorrect
6. [1] increased risk of infection from trapped pathogens if mucus present; AW
7. [1] (bronchioles) have columnar, epithelium / cells ; A columnar and cuboidal, epithelium / cells
8. [1] (bronchioles) have ciliated, epithelium / (epithelial) cells ; A cilia (are present) ora
9. [1] (alveoli) have squamous, epithelium / cells ; I endothelium / endothelial cells
10. [1] detail; e.g. (bronchiole epithelial cells) longer / wider / thicker ; R if, mucus makes it thicker / stated as more than one layer or (alveolar cells) flatter / thinner ; allow ideas if stated as epithelium (rather than cells)
11. [1] AVP; e.g. ref. to alveolar type II cells / cells secreting surfactant
12. [1] smooth muscle
13. [1] (muscle) contracts / contraction (and relaxation of bronchiole, lining / wall); R contraction of, bronchiole / lungs / other airways
14. [1] change / decrease, diameter of, bronchiole / lumen / airway; A adjust size of airway ; A constriction of, bronchiole / airway
15. [1] control of air flow (to and from alveoli) / AW; e.g. control volume of air, entering / leaving, alveoli
16. [1] allow ecf max two marks if incorrectly stated as elastic fibres ; stretch/expand, during inhalation; A stretch and recoil ; prevents, overstretching / bursting, during inhalation; recoils to help move air out during exhalation
17. [1] allow ecf to max one mark if incorrectly stated as cartilage ; prevent collapse of, bronchiole / airway; provides support/keeps airway open