These 2 organs are, of course, concerned with respiration, in which carbon dioxide is exchanged for oxygen. The air breathed in is warmed before reaching the lungs via the AIR PASSAGES.
Blood is carried to the lungs by the pulmonary artery, which divides and subdivides into tiny capillaries which lie around the walls of the air cells.
Apart from their main function of gaseous exchange (see AIR), the lungs can release histamine, metabolise noradrenaline, and inactivate prostaglandins. Local immune mechanisms also operate in the lungs.
Lung is composed of very highly elastic tissue which consists of multitudes of tiny sacs arranged at the terminal parts of the smallest of the bronchioles, and which collapse when the balance of pressure between the air in the sacs and on the outside of the lung surface is disturbed. Thus a lung shrinks to about one-third of its normal size when removed from the chest cavity.
The lungs occupy the greater part of the thoracic cavity, and are accurately moulded to the walls of the chest and to the other organs contained within it. The right is considerably larger than the left, owing to the presence of the heart, which lies mostly to the left side of the middle plane of the cavity. In the Equidae the lung is not divided into lobes as it is in some of the other animals. The apex is that portion which occupies the most anterior part of the chest cavity, and just immediately behind it is the deep impression for the heart. Behind this again, and a little above it, is the ‘root’ of the lung, which consists of the blood vessels entering and leaving the lung, lymph vessels, nerves, the bronchus, and here also are situated the bronchial lymph nodes. In cross-section each lung is somewhat triangular in shape, with one of the angles rounded. The rounded angle lies in the uppermost part of the chest, alongside the bodies of the thoracic vertebrae, and the more acute of the remaining angles lies along the floor of the chest.
The lungs are thicker and shorter than in the horse, and there is a greater disproportion in size – the right weighing about half as much again as the left. They are divided into lobes by deep fissures. The left has 3 lobes, and the right 4 or 5. The foot in each case is almost immediately above the impression for the heart. The apical lobe (i.e. the most anterior of the right lung) receives a special small bronchus from the trachea direct.
The lungs show little lobulation.
The left lung is like that of cattle, but the right lung has its apical lobe very often divided into 2 parts. Otherwise there are no great differences. Three bronchi are present, as in cattle.
The lungs are thicker than in either the horse or the ox in conformity with the more barrel-like shape of the chest. There is no cardiac impression in the left lung. Each has 3 large lobes, but the right has a small extra mediastinal lobe, and there may be 1 or more accessory lobes in either lung.
In the perfectly fresh lung from a young unbled animal the colour of the lung is a bright rose-pink with a glistening surface, the pleural membrane; but in the lungs of older animals there is usually a certain amount of deposit of soot, dust, etc., which has been inhaled with the air and collected in the lymph spaces between the air cells.
The lungs are firmly anchored in position by their roots to the heart and trachea, and by the pleura to a longitudinal septum running vertically from front to back, (the mediastinum) (see PLEURA). The pulmonary artery, carrying unoxygenated blood to the lungs, divides into 2 large branches after only a very short course. Each of these branches enters into the formation of the root of the lung, and there begins to divide up into a very large number of smaller vessels. These subdivide many times until the final capillaries are given off around the walls of the air-sacs. From these the blood, after oxygenation, is carried by larger and larger veins, till it eventually leaves the lung by one of the several pulmonary veins. These number 6 or 7 or more, and leave the lungs by the roots. In addition to the blood carried to the lung for aeration a small bronchial artery carries blood to the lung substance for nutritive purposes. This accompanies the bronchi and splits into branches corresponding to the small bronchi and bronchioles. The lymph vessels in the root of the lungs are very numerous, and are all connected with the large bronchial glands for this part.
The main bronchial tube, entering the lung at its root, divides into branches, which subdivide again and again, to be distributed all through the substance of the lung, till the finest tubes, known as ‘bronchioles’ or ‘capillary bronchi’, have a diameter of only about 0.25 mm. In structure, all these tubes consist of a mucous membrane surrounded by a fibrous sheath. The larger and medium bronchi have plates of cartilage in the fibrous layer, and are richly supplied with glands secreting mucus, which is poured out on to the surface of the lining membrane and serves to keep it moist. The surface of this membrane is composed of columnar epithelial cells, provided with little whip-like processes known as ‘cilia’, which have the double function of moving any expectoration upwards towards the throat, and of warming the air as it passes over them. The walls of the bronchial tubes are rich in fibres of elastic tissue, and immediately below the mucous membrane of the small tubes is a layer of plain muscle fibres placed circularly. To this muscular layer belongs the function of altering the lumen of the tube, and, consequently, its air-carrying capacity. It is a spasmodic contraction of the muscular layer that produces the characteristic expiratory ‘cough’ of true asthma.
The smallest divisions of the bronchial tubes open out into a number of dilatations, known as ‘infundibula’, each of which measures about 1.25 mm across, and these are covered with minute sacs, variously known as ‘air-vesicles’, ‘air-alveoli’, or ‘air cells’. An air cell consists of a delicate membrane composed of flattened platelike cells, strengthened by a wide network of elastic fibres, to which the great elasticity of the lung is due; and it is in these thin-walled air cells that the respiratory exchange of gases takes place.
The branches of the pulmonary arteries accompany the bronchial tubes to the farthest recesses of the lung, dividing like the latter into finer and finer branches, and ending in a dense network of capillaries, which lies everywhere between the air vesicles, the capillaries being so closely placed that they occupy a much greater area than the spaces between them. The air in the air vesicles is separated from the blood only by 2 most delicate membranes, the wall of the air cell and the wall of the capillary, and it is through these walls that the respiratory exchange takes place.