More information about the lungs

During normal passive breathing at rest, the circumference of the adult chest increases by about five centimetres - in the infant and child it is, of course, correspondingly less. We are stimulated to breathe as a result of the brain registering that the level of carbon dioxide in the blood has increased. Nerve signals tell the muscles of the chest and abdomen to contract, and those that can act to block the pharynx and larynx to relax.

Our main air way, the trachea, divides into two tubes called the main bronchi - the right and left-which then further divide, the right into three and the left into two, which are the air passages to the lobes of the lungs. These further divide and become smaller, eventually branching into bronchioles, which are small tubes which do not have cartilage in their walls. Eventually the air passage terminates in the terminal bronchiole which leads to the alveolus, the air sac where oxygen and carbon dioxide are exchanged across the capillary walls and the membrane of the alveolus. In the adult lung there are about 300 million alveoli, and the total area available for gaseous exchange is about 80 square metres. At birth a baby has about 20 million: the adult number being attained by the age of eight. In order that the alveoli expand at birth the surface tension of the walls must be overcome. This is made possible by the presence of a fluid called surfactant, produced by alveolar cells during the latter part of pregnancy (so it may be deficient in premature babies). Each lung is enclosed in a sac, called the pleural membrane, which contains a small amount of fluid.

The airways are lined by a mucous membrane. The mucous is continually being moved upwards by the action of tiny hairs called cilia. The purpose of this muco-ciliary escalator, as it is called, is to clear the lungs of debris such as dead cells or dust particles. The mucus comes up the trachea and out of the larynx into the pharynx, where it is swallowed. If the air way is irritated or inflamed - as a result of infection, for example - more mucous is produced. This stimulates a cough reflex, which is a mechanism designed to help us clear excess mucus or other foreign matter. During forceful coughing air can be expelled at speeds in excess of 70 miles an hour. The sneeze reflex is a similar response initiated by irritation of the nasal mucosa, rather than the trachea and bronchial tree.

At rest adults normally breathe about 15 times a minute. The young breathe faster - babies about 40 times a minute, 5 year olds 25 times a minute - rates which increase with any respiratory problem, with fever, or during exercise.