Air passages

The vital function of gas exchange takes place in the lungs – or to be more precise in the thin-walled air sacs called alveoli. Oxygen is absorbed by, and the waste product carbon dioxide is expelled from, the blood. To keep this process going, the air in the lungs should be continuously refreshed. This succesive drawing in and expulsion of air through the air passages is called breathing.

Besides leading air to and from the alveoli, the air passages also have the function of warming, moisturizing and filtering this air. A person who has had a tracheostomy breathes air through a tube directly into the trachea. In this situation the air passages are hampered in their normal functions. The air that reaches the alveoli is too cold and too dry and lung infections are liable to develop.

Nose and throat

When someone breaths normally, air enters through the nose. The hairs at the entrance of the nostrils block the passage of large particles such as sawdust and prevent the accidental inhalation of insects. Of even greater importance, is the removal of particles as a result of turbulent precipitation, caused by three protrusions known as the conchae nasales at the inside of the nose. As air passes over the obstructing veins, its direction of movement changes. Particles suspended in the air have greater mass and cannot change their direction of travel as quickly. As a result they come into contact with the mucous membranes lining the various air passages. These membranes have mucus cells that secrete a thick, sticky substance to which dust and other small particles adhere. Other cells lining the air passages ha,ve frond-like appendages (cilia) which constantly beat towards the throat. After particles are entrapped in the mucus, this mucus in turn reaches the throat where it is either expectorated or swallowed. The nose is very effective in removing particles from the inhaled air. Only particles smaller than 5 microns (1 micron = 0.000001 m) in diameter manage to enter the lungs. In order to warm the inhaled air as close as possible to the body temperature there is a profuse blood supply to the extensive surfaces of the turbinates and septum. As a result the air that reaches the trachea has been warmed to a temperature of about 35 – 36°C. The air is moistened because of the presence of liquid-producing cells (serous cells) which occur in abundance throughout the nasal passages. At the top of the nose are the olfactory cells, responsible for our sense of smell. Here, the ‘impurities’ that we know as odours come into contact with the olfactory apparatus and we perceive the appropriate smell. This sense should enable us to recognize when the inhaled air is potentially harmful and urge us to go into fresh air. Unfortunately, however, many poisonous gases (for example, carbon monoxide) are odourless in the pure state.

Under certain circumstances inhalation takes place through the mouth. This may occur when the nose has become obstructed as a result of an infection, or when taking strenuous exercise when not enough air can be supplied through the nose. This is not beneficial to the lungs because the air is not warmed, moistened or filtered adequately.

Larynx and trachea

The larynx is situated where the air and food passages separate at the base of the throat. A part of it can be felt in the neck as the ‘Adam’s apple’. The larynx has two main purposes. The first is to stop food and liquid from entering the lungs (in which it is assisted by the epiglottis at the back of the tongue). The second function is to produce noise by means of the vocal chords, enabling us to speak. From the larynx, air passes down into the trachea. This is a tube of 12cm in length and 25mm in width. Incorporated in its wall are 16 to 20 C-shaped cartilages. At the back of the trachea these cartilages are closed by stretchable tissue. This enables the oesophagus – which lies behind the trachea – to expand when a large lump of food passes. The trachea has a further, air conditioning function. In common with all the other air-conducting areas the trachea is lined with serous cells, mucous glands and cilia. Particles not filtered by the nose are removed at the lower air passages, so that only particles smaller than 0.5 microns reach the alveoli. Smoking cigarettes results in a vast amount of particles with average size of 0.3 microns suspended in the air. With each cigarette smoked you directly attack the alveoli. The majority of those particles however, returns from the lungs and is expelled along with the expired air. The thin layer of mucus in the trachea and bronchi is moved by the cilia towards the throat at a speed of about 1cm per minute. In this way – together with the cough reflex – the air passages are kept free of foreign matter. Smoking cigarettes has a damaging influence on the function of the cilia, thus allowing mucus plugs to build up, which can encourage infections. During sleep the cilia are allowed to regain some of their capacity to move the mucus. This is the reason why many smokers have a ‘smokers cough’ in the morning.

The lower air passages

The trachea divides into two bronchi, one going to the right lung and one going to the left lung. The right bronchus is larger and more vertical than the left one, so inhaled foreign bodies – such as peanuts – tend to end up in the right bronchus. The main bronchi divide into smaller ones which subdivide further until the bronchi in the alveoli are just 1mm in diameter. Some people are very sensitive to inhaled substances such as pollen, animal fur, house dust or particles of feathers. This causes swelling of the mucous membranes and immense production of serous fluid. When this occurs in the nose a situation called allergic rhini- tis or hay fever develops. When occurring in the lungs these allergic phenomena are called asthma. In asthma, there is also a constriction of the muscles of the bronchioli (the subdivisions of the bronchi), which can cause extreme breathlessness.

The lungs

The bronchi with all their numerous subdivisions. Terminating in the alveoli, are collectively known as the lungs. They are situated in the chest on both sides of the heart. The lungs of a young child are pink, but with increasing age the colour deepens. The main reason for this change of colour are the polluting substances in the inhaled air. The pulmonary arteries are responsible for the supply of blood. The blood is oxygenated and relieved of its carbon dioxide in the alveoli, after which it leaves the lungs via the pulmonary veins. These veins drain the oxygenated blood into the heart which pumps the blood to the rest of the body.

The lungs are surrounded by and closely attached to a thin membrane, the visceral lung pleura. This membrane lies next to the pleura that covers the inner side of the chest wall and the diaphragm. Between these membranes there is a thin layer of fluid. This acts as a lubricating mechanism, allowing smooth, uniform expansion and contraction of the lung. If the pleurae become inflamed as a result of pneumonia, for example, the gliding process becomes impaired, causing breathing difficulties.