The trachea

The trachea is 10-12 cm in length. It lies slightly to the right of the midline and divides at the carina into right and left main bronchi. The carina lies under the junction of the manubrium sternum and the second right costal cartilage. The right main bronchus is more vertical than the left and, hence, inhaled material is more likely to end up in the right lung.
The right main bronchus divides into the upper lobe bronchus and the intermediate bronchus, which further subdivides into the middle and lower lobe bronchi. On the left the main bronchus divides into upper and lower lobe bronchi only. Each lobar bronchus further divides into segmental and subsegmental bronchi. There are about 25 divisions in all between the trachea and the alveoli.
The first seven divisions are bronchi that have:

• walls consisting of cartilage and smooth muscle
• epithelial lining with cilia and goblet cells
• submucosal mucus-secreting glands
• endocrine cells - Kulchitsky or APUD (amine precursor and uptake decarboxylation) containing 5-hydroxytryptamine.

The next 16-18 divisions are bronchioles that have:

• no cartilage and a muscular layer that progressively becomes thinner
• a single layer of ciliated cells but very few goblet cells
• granulated Clara cells that produce a surfactant-like substancs

The ciliated epithelium is a key defence mechanism. Each cell bears approximately 200 cilia beating at 1000 beats per minute in organized waves of contraction. Each cilium consists of nine peripheral parts and two inner  longitudinal fibrils in a cytoplasmic matrix. Nexin links join the peripheral pairs. Dynein arms consisting of ATPase protein project towards the adjacent pairs. Bending of the cilia results from a sliding movement between adjacent fibrils powered by an ATP-dependent shearing force developed by the dynein arms. Absence of dynein arms leads to immotile cilia. Mucus, which contains macrophages, cell debris, inhaled particles and bacteria, is moved by the cilia towards the larynx at about 1.5 cm/min.

The bronchioles finally divide within the acinus into smaller respiratory bronchioles that have alveoli arising from the surface. Each respiratory bronchiole supplies approximately 200 alveoli via alveolar ducts. The term 'small airways' refers to bronchioles of less than 2 mm; there are 30 000 of these in the average lung.

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