1.1.2 Lower respiratory tract

The trachea divides into two branches called bronchi (Video 2). These serve the left and right lungs. Like the trachea, the walls of the bronchi contain cartilage, which prevents their collapse. Each main bronchus divides into smaller and smaller tubes, finally ending in terminal bronchioles. Bronchioles also contain cilia that help to keep the airways clean by moving mucus and particles from the lower respiratory tract up to the pharynx to be expelled.

Download this video clip.Video player: sk299_t7_fig1_03_lungs.mp4

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Video 2 3D model of the trachea, bronchi and bronchioles. (Please note, this video has no audio.)

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This is a “moving” picture, giving a rotating, stereoscopic-effect view of the lungs, bronchi and vertebral column.

First view shows two pink lungs, viewed from the front. Between the lungs at their tops is the trachea (in yellow-grey) with stiffening cartilaginous rings. About one third the way down the lungs the trachea branches into the right and left bronchi. The vertebral column (in grey) lies behind the respiratory organs. The illustration rotates clockwise.

Subsequent views show the lungs from the right and left sides, with the trachea entering between the lungs, and the vertebral column lying behind the respiratory organs.

In a final view the lungs are removed to show the trachea, bronchi and the bronchi branching (in what would be within the lungs) in a ramifying system of bronchioles with smaller and smaller branches.

Video 2 3D model of the trachea, bronchi and bronchioles. (Please note, this video has no audio.)

The lungs are organised into lobes (the left lung comprises two lobes and the right lung has three lobes). Two thin membranes, the visceral and parietal pleura, cover the lungs and keep them attached to the thoracic wall. The base of each lung is concave and rests on the diaphragm (more on that in Section 1.2), whereas the heart sits within the cardiac impressions, or grooves, in each lung (Figure 1, repeated).

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Figure 1 (repeated) Components of the respiratory system.

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Pink silhouette of the lower head and the thorax of a human.

The thoracic cavity is shown bounded by yellow ribs (in cross-section) with intercostal muscles between the ribs. Inside the ribs is the thoracic cavity in pale blue. At the base of the thoracic cavity is the diaphragm, coloured purple, and displayed as an inverted U-shape, with the abdominal cavity below.

Two lungs, coloured pink are on the right and left sides of the thoracic cavity; the lungs are covered with pleurae, shown as a white layer covering the lungs. The lobes of the lung are shown on the right lung.

The mouth and nasal cavities proceed to the pharynx in the neck; this leads to the oesophagus, all coloured pink. From the top of the oesophagus proceeds the trachea (coloured yellow) with darker yellow cartilage rings stiffening and supporting it; at the top of the trachea is shown the box-like larynx in pale yellow. The trachea branches to lead to two bronchi, one leading into each of the right and left lungs. The left bronchus is traced into the left lung, showing extensive branching into ramifying bronchioles, stiffened with cartilage.

The heart is shown in dark red between the base of the lungs; the trunks of major arteries are shown proceeding from the heart.

Figure 1 (repeated) Components of the respiratory system.

Because they act as a conduit for air to move into and out of the lungs, the nasal passages, pharynx, larynx, bronchi and bronchioles are collectively referred to as the conduction zone of the respiratory system.

Air then passes into progressively smaller structures deep in the lungs where gas exchange actually takes place in the respiratory zone, which you will explore in the next section.