The finest divisions of The bronchioles, as stated before, do not contain cartilages in their walls.

Histamine, particularly in sensitive persons. is a powerful bronchoconstrictor i.e. causes spasm of bronchia muscles. Antihitaminic. drugs are also popular in the Treatment of bronchial asthma. There are glands in the trachea. Their secreting parts are in the submucous coat and their ducts open into the lumen of the trachea. The glands secrete mucus and are sup-plied by the parasympathetic nerves Parasympathorytic drugs, e.g. . atropine therefore, produce the so called 'drying up of the cough' Goblet cells also secrete mucus. The finest divisions of The bronchioles, as stated before, do not contain cartilages in their walls. A possibility therefore remains that These finest branches of the bronchiole may undergo collapse. (I will be seen subsequently in this section that forces of inspiration, presence of surfactant, the phenomenon called interdependence and pores of Kohn all help to keep these finest bronchioles and the alveoli open Cilia are present menthes surface of the mucous membrane However, beyond the (i.e. distal to) respiratory bronchioles, the cilia are absent The cilia beat towards the exterior.(i.e. , towards Die laryngeal side). When particles of dust enter the lung through the inspired air. they get stuck by the mucus secreted by the mucous glands and goblet cells and the stuck up mass is pushed upwards by I he beatings of the cilia. this is someone’s called the 'escalator action' of cilia The cilia are damaged by various 'cilitoxins' present in the tobacco smoke. The mucous glands are absent onwards from the respiratory bronchiole Teleological speaking, this is a desirable attar, because, secretions from these glands are likely to plug the very fine bronchioles. The lining epithelium of the trachea is pseudo stratified ciliated columnar So is (he case with the major bronchi But in the bronchioles, in becomes ciliated columnar epithelium. onwards from the respiratory bronchioles, the cilia disappear and the epithelium becomes cubical Mast cells. For a long tune. it was know that histamine causes brcnchoconstriction it was also known that in susceptible persons, release of endogenous histamine or injection of rust amine may produce alarming symptoms But the full picture has become reasonably clear only recently Mast cells are present in the bronchial tree, deeper to the epithelial lining Recall, mast cells, which are morphologically very closely related to the basophile of blood (chap 3 sec II). and yet have different ancestry can and do release histamine when Their cell membranes are perturbed (destabilized ) This released histamine (H) combines with their receptors in the cell membrane of their target cells (i.e. the bronchial smooth muscles) post receptor events final effect ( = bronchi constriction) Recall further. (chap3 sec III), there are two subtypes of H receptors, viz. H1 and H2 These bronchial muscle H receptors belong, mostly, to the H1 variety and hence traditional, anti-histamines like 'pynlamine' can block Their actions [chap 3 sec iii). A recently discovered drug, 'sodium cromoglycate can stabilize the bronchial mast cells (so they no longer release H easily) Inhalation of Na cromoglycate is very popular for prevention of bronchial asthma. Mast cells also produce slow reacting substances. SRSA (now known to be member of eicosanoids) which reinforces the bronchospasm by histamine in bronchial asthma) for details, see chap 6 sec IV, 'asthma') The epithelial Iining of the alveolar wall deserves special mention. The Iining epithelial cells are exceedingly thin, simple squamous type, but the epithetial lining is not incomplete as once suspected. The lining epithelial cebs are of two tyes - (i) squamous cells mentioned just above and also known as type I cells and (ii) type I cells Type II cells (also called granular pneumocytes) are intermingled with the type I cells. Their (type II's) special property is that they secrete 'surfactant', a substance which reduces the surface tension in the alveoli and will be described in greater detail in subsequent chapters In an 'average'lung. The total number of alveolus about 400 x 106 One can well imagine that this arrangement presents a great surface area for exchange of gases between the alveoli and pulmonary capillary blood The total calculated surface area in an 'average' lung is about 80 m2. The alveoli communicate with each other by small pores called pores of Kohn. Accessory communication sometimes occur between fine bronchioles and their adjacent alveoli and are Known as Lamberts sinuses WHEEL'S MODEL A Swiss anatomist. E. R. Weibel, m 1963 introduced his concept of fung anatomy, which in brife. is presented below The whole lung can be divided into two major zones; (i) conducting zone and (ii) respiratory zone. Weibel numbered each generation of tracheobronchial tree Thus the trachea is generation 0 (zero) .The two major dvisions of the trachea. viz. the right and left bronchi, constitute the first generation. and so on in the 16th generation, the bronchi are called, the terminal bronchiole. The 17th generation bronchioles are the respiratory bronchioles There