The lower part of the airway is also called the bronchial tree because of its resemblance to a trunk and branches (Fig. 18.2). The trachea is a cylindrical tube, 10–12 cm long, made up of 16–20 C-shaped cartilaginous rings joined together by fibrous and muscular tissue. This gives the trachea a firm structure to prevent collapse of the airway during inspiration. The posterior aspect of the cartilaginous rings is absent, facilitating the passage of food down the oesophagus, which lies immediately behind the trachea. The trachea extends from the larynx to the level of the fifth vertebra, where it divides into the two primary bronchi. The right primary bronchus is wider and shorter and more vertical than the left so that inhaled objects tend to enter the right lung rather than the left. The primary bronchi enter the lungs at the hilum, where the right bronchus goes on to divide into three: the right upper, middle and lower bronchi, to serve three lobes of the right lung. The left primary bronchus divides into two: the left upper and lower bronchi, to serve the two lobes of the left lung.

The lower branches of the airway, known as bronchi, still have cartilage in their structure. After this they are known as bronchioles and have smooth muscle in their walls. The smooth muscle is able to respond to stimuli by causing dilatation or constriction of the lumen of the bronchioles. This function is mainly under the control of the autonomic nervous system, with sympathetic impulses causing bronchodilation and parasympathetic impulses causing bronchoconstriction. There are about 8–13 divisions from the trachea to the smallest bronchi and another 3–4 before the terminal bronchioles are reached. Each terminal bronchiole divides into about 50 respiratory bronchioles. About 200 sac-like alveoli are supplied with air by each respiratory bronchiole. Alveoli do not form part of the conducting zone of the respiratory system.

The thoracic cage forms the cavity and contains the two conical lungs and the heart. The organs are separated from each other by the mediastinum and its contents. Each lung is surrounded by a double-layered fluid-filled sac called the pleura, which also attaches them to the inner surface of the thorax. The inner, or visceral, pleura covers the outer surface of the lung and is reflected back to become the outer or parietal pleura which is attached to the inner surface of the thoracic cavity.

The upper airway protects the alveolar tissues by warming, filtering and moistening the air. The structure of the epithelial lining is particularly good as a filter. It contains glands that secrete thick sticky mucus to trap particles and is ciliated to waft excess mucus and foreign particles towards the pharynx where they can be swallowed. The cilia beat about 600–1000 times per minute. Large numbers of phagocytic cells will engulf and destroy debris and bacteria trapped by the mucus.

Coughing is a forceful expiration reflex under the control of the respiratory centre in the medulla, which will expel irritant particles from the larynx. Air rushes out at a speed of 500 miles per hour! It is instigated by messages from a sensitive part of the airway at the bifurcation of the trachea, called the carina. Sneezing is a similar reflex, instigated by irritation of the nasal mucosa. The swallowing reflex is extremely important for respiration. Absence of this reflex, as is seen in uncon-scious or anaesthetised patients, may result in inhalation of particles of food or water into the larynx or lung. The airway may be obstructed or infection and pneumonia may occur.

The terminal bronchioles feed into respiratory bronchioles which branch into the alveolar ducts. These lead into alveolar sacs and the alveoli, where most of the gas exchange occurs (Fig. 18.3). The alveoli are expansions off the alveolar sacs, making the latter resemble bunches of grapes. Alveoli open into a common chamber called the atrium at the terminus of the alveolar duct. There are about 300 million alveoli in the lungs, providing an enormous area for gas exchange.

The alveolar wall (Fig. 18.4) consists of a single layer of flattened squamous epithelial cells called type I cells. The external surface of an alveolus has a few elastic fibres around the opening. There is a dense network of pulmonary capillaries surrounding each alveolus, providing a continuous encircling sheet of blood. Each capillary wall is also only one cell thick so that the interstitial space between the alveolus and its capillary network, forming the air–blood interface, is extremely thin (0.2 μm, compared with the 7 μm diameter of an average red blood cell). This interface is called the respiratory membrane and has blood flowing on one side and gas on the other. Gas exchange occurs by simple diffusion across the respiratory membrane and depends on the existence of pressure gradients between the lungs and the atmosphere. The total surface area of alveoli in contact with capillaries is roughly the size of a tennis court. This extensive area and the thinness of the barrier permit the rapid exchange of large quantities of oxygen and carbon dioxide for diffusion.