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).

Figure 3 Terminal bronchioles further divide within the lungs.

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A drawing of a lung (pink) with a bronchus and branching bronchioles within the lung.

The ends of two of the bronchioles are shown in greatly expanded form. A terminal bronchiole is shown with a respiratory bronchiole, branching into two, leading from it. Around the ends of the respiratory bronchioles are alveolar sacs (looking like bunches of grapes), each comprising many spherical alveoli.

An alveolar sac is shown in expanded form. The sac is cut open to shown alveoli, also cut open, spherical units each with an alveolar pore.

Figure 3 Terminal bronchioles further divide within the lungs.

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).

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Video 3 Exchange of CO₂ and O₂ carried by blood in the pulmonary capillaries within the alveoli of the lungs. (Please note, this video has no audio.)

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This is a “moving” picture, giving a rotating, stereoscopic-effect view of a respiratory bronchus, an alveolar sac and associated pulmonary capillaries.

The figure shows a horizontal pink tube, a terminal bronchiole, with a respiratory bronchiole leading down from it into an alveolar sac (looking like a bunch of grapes), each alveolus being spherical. Over one side of the alveolar sacs are branching, bright red capillaries and on the other side (seen as the figure rotates clockwise) are bright blue capillaries.

Video 3 Exchange of CO₂ and O₂ carried by blood in the pulmonary capillaries within the alveoli of the lungs. (Please note, this video ...

Figure 4 (a) Electron micrograph and (b) schematic showing a cross-section of an alveolus (purple), with red blood cells (red) in the surrounding intertwining capillaries and a pulmonary macrophage (green) inside the alveolus.

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(a) shows an artificially coloured electron micrograph of an alveolus with purple walls; cavities in the walls show capillaries in cross-section, some of them containing erythrocytes coloured red. Within the alveolus is a large pulmonary macrophage (coloured pale green) with many processes.

(b) is a diagram based on (a) above. The alveolus is shown as a half of a hollow sphere with pale blue epithelial cells lining it. Darker blue macrophages with branching processes are seen on the alveolar walls. Also in the alveolar walls black alveolar pores are shown. Within the wall of the alveolus, comprising the respiratory membrane, capillaries are seen in cross-section, containing erythrocytes (red blood cells).

Figure 4 (a) Electron micrograph and (b) schematic showing a cross-section of an alveolus (purple), with red blood cells (red) in the ...

In the next section, you will explore the muscles that are involved in the expansion and contraction of the lungs.