3 Gas Exchange in Humans

3.1 Gas exchange in organisms

• Small organisms (e.g. bacteria which is unicellular, or yeast which is multicellular) with a small organism volume have a large surface area to volume ratio. Thus, oxygen can diffuse into and carbon dioxide gas can diffuse out of these organisms quickly.

• Large, complex organisms with a large organism volume consist of numerous cells packed closely together, layer upon layer. Thus, there is a need for specialised organ system for oxygen to diffuse in and carbon dioxide gas to diffuse out of the organism quickly. This movement of gases between the organism and the surrounding air is known as the gaseous exchange. In the case of the lungs in animals and gills in fishes, the lungs and gills have a large surface area to volume ratio for fast exchange of gases.

• In humans and most animals, the organ system responsible for gaseous exchange is known as the respiratory system.

3.2 Components of the respiratory system

Diagram of the respiratory system.

1. Nostrils

2. Nasal passage

3. Pharynx

4. Larynx

5. Trachea

6. Bronchi (Singular: Bronchus)

7. Bronchioles

8. Alveoli

3.2.1 Nostrils

• Hair in the nostril acts as a filter for dust particles and they remain in the nostrils instead of travelling down the respiratory system.

• The sticky mucus trap the dust particles and foreign particles.

• Olfactory sensory cells lining the mucous membrane of the nasal passage detect harmful chemicals.

3.2.2 Nasal passage to the trachea

• Air in the nasal passage enters the pharynx and passes into the larynx and then into the trachea (Note spelling of trachea.)

3.2.3 Trachea

• The trachea is supported by cartilage that keeps the lumen of the trachea open at all times. This is significant as when a person coughs, the trachea still remains open to allow smooth passage of air. This prevents choking.

• The lining on the surface of the trachea is known as the epithelium.

• The epithelium of the trachea consists of (i) gland cells that secrete mucus to trap dust particles and bacteria and (ii) ciliated cells that have hair-like structures known as cilia that sweep the dust-trapped mucus up the trachea.

• Air is warmed by the blood vessels near the epithelial cells of the trachea and the movement of the trachea during the process of inhalation and exhalation.

• The mucus lining the trachea serves to moisten the air as it passes through the respiratory system.

3.2.4 Bronchi and Bronchioles

3.2.5 Alveoli

• Singular: alveolus, plural: alveoli

• Each alveolus is an air sac that is well surrounded by blood capillaries.

• Gas exchange takes place through the alveoli.