Lungs

Lungs. The two lungs lie within the cavity of the thorax, the heart being situated between them. During life the lungs are always more or less on the stretch, and the lung surface lies close beneath the chest wall, the lungs filling up the cavity of the thorax. The lungs are surrounded by a kind of bag or sac, composed of a serous membrane called the pleura, but this pleural sac is practically empty, the layer which covers the lung surface lying close to the outer layer, which is adherent to the chest wall.

The air in the lungs is, in fact, under pressure, the atmospheric pressure being transmitted from the mouth to the ultimate extremities of the air passages, by which the lung is permeated; there is thus a continual distending force acting upon the lung from within, and there is no counterbalancing pressure applied from without, for the lungs lie in the air-tight chest, shielded, as it were, from the pressure of the atmosphere outside, by the walls of the thorax. The lung is thus blown up, so to speak, by pressure from within, and, containing as it does a large amount of elastic tissue, it is distended and made to fill the thoracic cavity. If an opening is made in the chest wall of an animal after death, the lungs at once collapse, for the outer surface of the lung is now exposed to atmospheric pressure as well as the inner surface, and these pressures counterbalancing one another the stretched elastic tissues of the lungs are no longer maintained in a state of tension, but contract and produce a shrinking up of the lung. When the lung collapses, the two layers of the pleura part company, the layer covering the lung shrinking with the lung, and the outer layer remaining adherent to the chest wall.

It has been stated that the lung is permeated by air passages. The trachea (q.v.) is the main channel of communication between all these air passages, on the one hand, and the larynx, mouth, and outer air, on the other.

The trachea divides into two tubes, the two bronchi, one for each lung; each bronchus subdivides, and each division of the bronchi again subdivides and so on. These subdivisions bring us at length to the minute tubes which are known as the ultimate bronchioles. Each of these bronchioles expands into a funnel-shaped sac, into which open a number of minute pouches. These pouches are the air cells, and the sac with its pouches is called an infundibulum. The lungs are divided into lobes the right lung having three lobes, and the left lung two lobes. These lobes or divisions of the lungs are again divided into lobules, and each lobule may be regarded as made up of a number of the infundibula already alluded to.

The entire inner surface of the air passages of the lung is lined by a mucous membrane, covered internally with epithelium. External to the mucous membrane is a supporting framework containing elastic tissue, glandular tissue, and, in the case of the bronchi and trachea, cartilage. The epithelium of the larger air passages comprises several layers of cells, the innermost layer consisting of ciliated epithelial cells. In the smaller tubes, the epithelium and the outer supporting framework become attenuated, the number of layers of cells diminishes until only one layer is left, and finally in the infundibula and air cell-3 this single layer of cells is no longer ciliated but consists of much-flattened epithelial cells. These ultimate air passages consist of nothing more than a delicate membrane which has elastic fibres coursing over it, is lined internally with flattened cells and supports the rich network of capillary blood-vessels distributed upon its outer aspect. The blood in the capillaries of the lung is thus spread out over a large aerating surface, for while each air cell is a minute microscopical object, the combined area of the surface of all the air cells is considerable, and there is, therefore, abundant opportunity for the interchange of gases between the blood and the air in the air cells.

Venous blood is conveyed from the right ventricle of the heart to the lungs by the pulmonary artery. It is there distributed throughout the capillaries, takes up oxygen and loses carbonic acid by exposure to the air contained in the air cells, and is returned by means of the pulmonary veins to the left auricle of the heart. [Respibation.]

Diseases of the lung, see Beonchitis, Consumption, Emphysema, Empyema, Pleueisy, Pneumonia.