Lungs are soft spongy and very elastic in living individuals. If the thoracic cavity were opened, the lungs would shrink to 1/3 of their original size because of their elastic nature and atmospheric forces. Lungs are pink in color in children but in adults they become mottled and dark because of inhalation of dust particles that become trapped in the phagocytes of the lungs. This effect is especially well seen in coal miners and city dwellers. Lungs are placed in the thoracic cavity in such a way so that one lies on each side of the mediastinum and thus the two lungs are separated by heard and large blood vessels besides the other structures in mediastinum.
Shape of lungs:
Each lung is conical in outline and is covered with visceral pleura. It suspends freely in its own pleural cavity and is attached to the mediastinum only through its root (the place where blood vessels and bronchi attach to the lung at hilum).
- Apex: Each lung has a blunt apex projecting upward into the neck for about 1 inch above the clavicle. Base: The base of both lungs is concave, which sits on the diaphragm.
- Costal surface: The costal surface is convex, which corresponds to the convace chest wall.
- Mediastinal surface: The mediastinal surface is concave and is molded to the pericardium and other structures of the mediastinum.
- Hilum: At about the middle of the mediastinal surface, there is a depression where the bronchi, blood vessels and nerves, which form the root, enter of leave the lung.
- Anterior border: The anterior border of the lungs is thin and overlaps the heart. It is here on theleft lung that the cardiac notch is found.
- Posterior border: The posterior border is thick and lies beside the vertebral column.
Lobes and fissures of lungs:
Right lung is slightly larger than the left one and is divided into three lobes, the upper, middle and lower lobes, by oblique and horizontal fissures. The oblique fissure runs from the inferior border upward and backward and cross the mediastinal and costal surface until it cuts the posterior border about 2.5 inches from the apex. The horizontal fissure runs horizontally across the costal surface at the level of fourth costal cartilage and meets the oblique fissure in the midaxillary line. The lobe of the lung above the horizontal fissure is called the upper lobe. One below the oblique fissure is called the lower lobe. The middle lobe is a small triangular lobe bounded by oblique and horizontal fissures.
Left lung contains only one fissure which divides into two lobes: upper and lower. The fissure, which is called the oblique fissure, is similar to the oblique fissure of the right lung. There is not horizontal fissure in the left lungs. The lobe that lies above the oblique fissure is called the upper lobe and one below it is called lower lobe.
Bronchopulmonary segments are the anatomic, functional and surgical units of lungs. The principal bronchi give off the lobar bronchi also known as secondary bronchi. Each lobar bronchus passes to a lobe of the lung and gives off branches called segmental (tertiary) bronchi. Each segmental bronchus then passes to a structurally and functionally independent unit of lung called the bronchopulmonary segment, which is surrounded by connective tissue. The branches of pulmonary artery that have to supply the bronchopulmonary segments accompany the segmental bronchi. The veins, on the other hand, run in connective tissue between adjacent bronchopulmonary segments. Each bronchopulmonary segment has its own lymphatics and autonomic nerve supply.
Characters of bronchopulmonary segments:
- It is an independent subdivision of a lung lobe
- It is pyramid shaped with its apex towards the root of lung
- It is surrounded by connective tissue
- It has its own segmental bronchus, segmental artery, lymph vessels and autonomic nerves.
- The segmental veins lie in connective tissue between adjacent bronchopulmonary segments.
- Any segment can be removed surgically with no effect on the remaining segments.
Airways inside the lungs:
On entering the bronchopulmonary segment, each segmental bronchus divides repeatedly into smaller branches. As the size reduces with each successive division, the U-shaped bars of cartilage disappear and are replaced by irregular plates of cartilage. These plates of cartilage become smaller and fewer in number as the size reduces with each division. The smallest bronchi give rise to bronchioles, which are less than 1 mm in diameter. They possess no cartilage and the submucosa possesses a complete layer of circularly arranges smooth muscles.
The bronchioles then divided into terminal bronchioles. As the division continues, delicate outpouchings start to appear and at this point the terminal bronchioles are called respiratory bronchioles because gaseous exchange takes place between these outpouchings. The diameter of each respiratory bronchiole is about 0.5 mm. The respiratory bronchioles terminate by forming alveolar ducts, which lead into tubular passages with numerous outpouchings called alveolar sacs. These sacs consist of several alveoli that open into a single chamber. These are the actual units of gaseous exchange in lungs. They are richly supplied with blood capillaries and exchange takes place between the air in the alveolar lumen and blood within the surrounding capillaries.
Root of the lungs:
Root is formed by structures that enter or leave the lung. These structures are bronchi, pulmonary arteries, pulmonary veins, lymph vessels, bronchial vessels and nerves. The root is surrounded by tubular sheath of pleura, which joins the mediastinal pleura to the visceral pleura.
Blood supply of lungs:
The bronchi, connective tissue and visceral pleura of lungs receive their blood supply from bronchial arteries, which are branches of descending aorta. The veins that correspond to the bronchial arteries are called bronchial veins. They communicate with pulmonary veins drain into azygous and hemiazygous veins. To be more specific, the right bronchial vein drains into azygous vein while the left one drains into hemiazygous vein.
The alveoli of lungs receive their blood supply from terminal braches of pulmonary arteries. The blood gets oxygenated and drains into the tributaries of pulmonary veins, which follow the intersegmental connective tissue septa to lung root. From each lung root, two pulmonary veins get out and enter the left atrium of the heart.
Nerve supply of lungs:
At the root of each lung is pulmonary plexus composed of efferent and afferent autonomic nerve fibers. This plexus is formed from branches of sympathetic trunk. It receives parasympathetic branches from vagus nerve.
The sympathetic efferent fibers produce bronchodilation and vasoconstriction in the lungs. Conversely, the parasympathetic efferent fibers produce bronchocontriction and vasodilation. They also increase glandular secretion.
Afferent impulses that are derived from stretch receptors in the alveolar wall and bronchial mucous membrane pass to the central nervous system in both sympathetic and parasympathetic nerves.