1.1.3 Respiratory zone
The terminal bronchioles each divide a further seven times into respiratory bronchioles, then alveolar ducts and finally into alveolar sacs (alveoli; singular, alveolus) that contain holes in their walls called alveolar pores (Figure 3).
Similar to other organs in the body, the organisation of the bronchioles and alveoli allows a large surface area of cells to be contained within the tight space of the thoracic (chest) cavity. This large respiratory surface, which is about 140 m2 in the adult human (roughly the size of a tennis court), enhances the lungs’ capacity to exchange CO2 for O2. This exchange occurs in the respiratory bronchioles, alveolar ducts and alveoli, which collectively form the respiratory zones deep in the lungs.
Alveoli are surrounded by a network of pulmonary capillaries that carry the blood (Video 3). Deoxygenated, CO2-rich blood coming into the lungs from the heart exchanges CO2 for O2 by diffusion, and oxygenated blood then leaves the lungs, returning to the heart to be pumped around the body. The walls of an alveolus and a pulmonary capillary are each only one cell thick, which allows diffusion of O2 and CO2 to occur very quickly (Figure 4).
Because haemoglobin, the molecule that carries O2 in the blood (explored further in Section 4.2), changes colour when bound by O2, oxygenated blood is often depicted as bright red, whereas deoxygenated blood is shown as blue in colour. This course doesn't explore how blood circulates around the body, but it is important to note that the lungs differ from other organs in that deoxygenated blood is carried to the lungs via arterioles (and larger arteries), whereas oxygenated blood leaves the lungs along venules (and larger veins).
Activity 1 Ordering the air flow
To test your understanding so far, order the steps involved in the flow of air from the conduction zone through to the respiration zone.
Using the following two lists, match each numbered item with the correct letter.
Air is taken in through the nasal cavities.
Air passes down the back of the pharynx, past the epiglottis and into the larynx.
From the larynx, air travels into the trachea, and into the bronchi.
Air moves through the terminal bronchioles and into the respiratory bronchioles.
Air passes into the alveolar ducts and into the alveolar sacs.
- 1 = e
- 2 = c
- 3 = a
- 4 = b
- 5 = d
In the next section, you will explore the muscles that are involved in the expansion and contraction of the lungs.