The main function of the respiratory organs is to provide a constant supply of oxygen to the tissues and to remove carbondioxide from them through the lungs. Ultimately this gas exchange occurs between the alveolar air and mixed venous blood in the capillaries across the alveolo-capillary membrane.
The alveolocapillary membrane has a total area of 75 meter square in an adult. Air is taken in through the air passages comprising the nose, pharynx, larynx, trachea, bronchi, and the bronchioles. The terminal portions of the air passages- the respiratory bronchioles and alveolar ducts- sub-serve the function of gas exchange. The part above the vocal cords is termed the upper respiratory tract and the parts below are called the lower respiratory tract.
The trachea which is 11cm long is kept permanently open by the presence of C-shaped cartilages on its wall. Several mucous glands present in the mucous membrane provide mucus which moistens the surface and takes part in ciliary action. The trachea divides into the right and left bronchi. The bronchi are similar to the trachea in structure. The right main bronchus is 1-2.5cm in length and it is in direct line with the trachea. This fact makes it more vulnerable for obstruction by foreign bodies entering through the trachea. The right main bronchus divides into branches which supply the right upper lobe, middle lobe, and lower lobe. The left main bronchus is longer (5cm) and it forms an angle of 50-100 degrees with the right main bronchus. It divides into two branches which supply the upper and lower lobes. Further division of the lobar bronchi gives rise to segmental bronchi which supply bronchopulmonary segments.
Bronchopulmonary segments
The bronchopulmonary segment is a wedge of lung tissue supplied by each segmental bronchus along with the corresponding branches of the pulmonary artery and vein. The bronchopulmonary segments act as independent units and are separated by fibrous septa.
Divisions of the bronchial tree:
After 8-13 successive divisions the segmental bronchi break up into the smallest bronchi. They continue further as bronchioles. The bronchioles have no cartilage and mucous glands on their walls. The bronchioles divide further and the terminal bronchioles divide further and the terminal bronchioles are formed after the fourth division. The terminal bronchioles give rise to respiratory bronchioles. Alveoli begin to appear on the walls of the respiratory bronchioles. As the respiratory bronchioles divide further, the number of alveoli arising from them progressively increases. Normal adult lung contains about 300 million alveoli. Rapid division of the respiratory bronchioles results in enormous increase in surface area. The terminal portions of the respiratory bronchioles divide into alveolar ducts and sacs. Alveoli sacs. Alveoli are 0.1-0.2mm in diameter. Up to the respiratory bronchioles the airways only conduct air passively, but beyond this they, also take part in gaseous exchange. The part supplied by a single terminal bronchiole is called an "acinus". An alveolar duct with its distal connections is called a "primary lobule". A group of primary lobules separated by connective tissue septa form a "secondary lobule".
Pores of Kohn and Canals of Lembert:
Pores of Kohn are openings connecting alveoli, which allow communication between them and sometimes even between adjacent segments. Canals of Lembert are short communications lines by epithelium which exist between distal bronchioles and some of the neighboring alveoli. These take part in collateral ventilation between different regions of the lung.
The Lining of the trachea, bronchi, and bronchioles consists of ciliated columnar epithelium containing goblet cells. The respiratory bronchioles are lined by non-ciliated cuboidal epithelium. The lining epithelium of the alveoli is flattened and it comprises of two types of cells- type I and type II pneumonocytes-arranged on a basement membrane. Type I pneumonocytes are numerous and they cover most of the inner surface of the alveoli. Gas exchange occurs mainly across these cells. Type II pneumonocytes are smaller in number. They contain lamelleated osmiophilic inclusion bodies which are thought to be of lysosomal nature. Surfactant is produced or stored in them.
Secretions of the airways and ciliary action:
Mucus is secreted by the mucous glands and goblet cells. Mucous glands are seen all along from the trachea to the smallest bronchi. They are most numerous in the medium-sized bronchi and are absent from bronchioles. In the bronchioles there are only a few goblet cells. Vagus is secretomotor for the mucous glands. The goblet cells respond to direct irritation. The mucous contains acid and neutral polysaccharides mainly, and variable quantities of sodium, potassium, albumin, globulin, specific antibodies, lysozyme and transferring. In addition to its antibacterial action, the mucus provides a milieu for the cilia to function. Ciliary action helps in removing particulate matter. Each cell contains about 200 cilia, each being 6-7 micrometer long. By successive rhythmic movement, they produce a wave motion passing regularly from cell to cell. As optimum amount of mucus of the correct thickness (5 micrometer) and optimum viscosity is essential for proper ciliary function. Drying up of tracheal secretions, increase in thickness, and viscosity of the mucous layer, inhalation of irritants, excessive intake of alcohol and certain drugs like cocaine impair ciliary function and predispose infection of the respiratory tract. Ciliary function is impaired in inherited disorders such as Kartagener's syndrome.
Surfactant
This is a substance produced by type II pneumonocytes from the 30th week of intrauterine life. It lines the alveoli. It contains an insoluble lipoprotein (dipalmitoyl lecithin) which forms a thin layer at the air-fluid interface and lowers surface tension. Surfactant prevents the alveoli from collapsing by reducing surface tension within the alveoli. Absence of surfactant results in the collapse of small alveoli during expiration and hyperinflation of the larger alveoli during inspiration. In addition, increase in surface tension leads to transudation of fluid from capillaries into the alveoli. Absence of surfactant leads to the formation of hyaline membrane disease in the newborn and adult respiratory distress syndrome in adults. Impairment of pulmonary blood flow and prolonged administration of dry oxygen or air leads to reduction in surfactant.
The Pleura:
The Lung is covered by visceral pleura on its surface and the thoracic cavity is lined by the parietal pleura. The space between them contains 10-20 ml of serous fluid having a protein content of 1.77g/dl. Pleural cavity is only a potential space. During inspiration the lung fills the pleural space. The pleural space is under negative pressure so that the lung is kept in apposition with the parietal pleura. At the end of a quiet expiration the pleural pressure is about 5cm of water. The pressure inside the pleural cavity is not uniform throughout. The negative pressure is higher at the apices than at the bases. The pleural fluid is formed at the parietal pleura and absorbed at the visceral pleura.
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