Bronchioles. Alveoli. Type I alveolar cells are very thin simple squamous epithelial cells and form most of the lining of an alveolus.

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1 276 Bronchioles Bronchioles continue on to form bronchi. The primary identifying feature is the loss of hyaline cartilage. The epithelium has become simple ciliated columnar, and there is a complete ring of smooth muscle around the tube. Terminal bronchioles lead to respiratory bronchioles. Terminal bronchioles have shorter epithelium and very few goblet cells. Clara cells, secreting antiproteases and oxidases, are present. Respiratory bronchioles have cuboidal epithelium with alveoli budding out from the walls. Alveoli Respiratory bronchioles lead to alveolar ducts from which arise several alveoli. An alveolus is an air space about 250um in diameter formed by a single layer of cells. The cells forming the lining are of two types: Type I alveolar cells are very thin simple squamous epithelial cells and form most of the lining of an alveolus. Type II alveolar cells are rounded cells containing lamellated bodies. These cells produce surfactant. The type

2 277 II cells are able to divide and may be the precursor of both cell types. Each alveolus is covered by a mesh of capillaries. The endothelial cells of the capillaries are separated from the epithelial cells of the alveoli by a basement membrane. Each alveolus opens into an alveolar sac, duct or respiratory bronchiole, although there are pores between adjacent alveoli. Alveolar macrophages are present on the air side of the epithelium or in the septae between adjacent alveoli. These cells remove material from the alveoli and travel to the bronchi where they are either carried up the conducting airway with the mucus or they enter the lymphatics. The ability of alveoli to inflate and resist collapse is in part due to surfactant and elastin. Surface tension in the wet alveoli is high, tending to cause small alveoli to empty into larger alveoli, and to prevent separation of adherent wet surfaces. Surfactant produced by the type II alveolar cells reduces surface tension and allows small diameter alveoli to remain inflated. Elastin is present throughout the lung. Following expiration the elastin recoils to assist in inflation of the alveoli and preventing bronchiolar and alveolar collapse.

3 278 Pleura The visceral pleura covers the surface of the lung. It is a layer of simple squamous epithelial cells, the mesothelium. It lies on a thin layer of collagen and elastin. Each lobe is wrapped separately with the pleura passing into the fissures. The superficial lymphatic channels lie just below the pleura and normally absorb pleural fluid. The visceral pleural surface is usually not attached to the parietal surface, but adhesions can occur following infection. Lines of Pleural Reflection The parietal pleura lines the body wall and mediastinum and the visceral pleura covers the lung. A thin film of pleural fluid between these two layers keeps them together. When the pleural cavity is expanded by the movements of the body wall, the parietal layer of pleura pulls the visceral layer out with it. At full expiration the diaphragm is raised so that part of the diaphragmatic pleura is apposed to the parietal pleura of the body wall, producing the costodiaphragmatic reflexion. On inspiration the lung is inflated and separates the apposed parietal pleural. Innervation The parietal pleura is derived from somatopleur mesoderm and therefore has somatic innervation. Visceral pleura is derived from splanchnopleur mesoderm and therefore has autonomic innervation.

4 279 The parietal pleura on the body wall is innervated by the intercostal nerves, and that on the diaphragm and mediastinum by the phrenic nerves. Pain from parietal pleura is therefore felt either on the body wall, or in the dermatomes of the phrenic nerve (C3,4,5). The visceral pleura is innervated by the vagus and sympathetic fibers. However it is insensitive to pain and temperature stimuli. Diaphragm The muscle fibers of the diaphragm arise from the periphery to insert into a fibrous central tendon. Muscle fibers arise from the xiphoid process, the ventral rib tips and costal cartilages of the lower six ribs and the upper three lumbar vertebrae. The diaphragm is pierced by several structures: The aorta passes through from the thorax to the abdomen behind the diaphragm, between the crura at the level of the twelfth thoracic vertebra. It lies behind the median arcuate ligament. The thoracic duct accompanies the aorta.

5 280 The esophagus passes through opposite the tenth thoracic vertebra. The fibers of the right crus pass around the esophageal opening. The vagus, left gastric vessels and lymphatics also pass through here. The inferior vena cava passes through the central tendon at about the level of the eight thoracic vertebra. It is accompanied by branches of the right phrenic nerve. The greater and lesser splanchnic nerves pierce the crura, while the sympathetic trunks lie behind the medial arcuate ligaments. The upper surface of the diaphragm is covered by pleura and pericardium. The lower surface is covered by peritoneum, except where it is directly related to the liver. Superior relations of the diaphragm are the lungs and heart, while inferiorly lie the liver, stomach, spleen and kidneys. The diaphragm develops from the septum transversum, the pleuroperitoneal membranes, the dorsal mesentery of the esophagus and the lateral body walls. This complex origin explains its innervation and the clinical problems related to the diaphragm. The septum transversum first appears cranially in the embryo at the end of the third week. By the end of the

6 281 fourth week the septum has migrated cranially and fused with the dorsal mesentery and the pleuroperitoneal membranes to form the primitive diaphragm. The pleuroperitoneal membranes grow from the posterior body wall to separate the pleural and peritoneal cavities. The dorsal mesentery suspends the esophagus from the posterior body wall. This primitive diaphragm goes through further changes with the addition of muscle from the lateral body walls. As growth proceeds the relative proportions contributed by each source change. As the pleural cavities enlarge, the expansion occurs more caudally at the back than the front. The muscle of the diaphragm is derived from the 3.4.5th cervical somites. The myoblasts derived from this source migrate throughout the diaphragm to form all of its muscle. Motor innervation is therefore from C3,4,5 via the phrenic nerve. Sensory innervation is also from the phrenic, with a thin rim around the costal margin being innervated by the lower six intercostal nerves. The most common congenital defect of the diaphragm is congenital diaphragmatic hernia. This results from defective formation and fusion of the pleuroperitoneal membranes with the other three parts of the diaphragm, usually only on one side. Normal fusion occurs by the end of the sixth week. Since this is before return of the gut from the umbilical herniation, part of the gut can pass into the

7 282 thorax. This developmental anomaly is also associated with hypoplastic development of the lung. Lung Mechanics Airways Resistance Measurement at 30 l/min Awake Paralyzed Partially Paralyzed + EFT PEFR Males Females FEV 1 ~ cmh 2 O/l/s~6.0 cmh 2 O/l/ ~ cmh 2 O/l/s~6.0 cmh 2 O/l/s ~10-15 cmh 2 O/l/s (AB says 5-10 cmh 2 O/ls/ ~ l/min ~ l/min ~50-70 ml/kg 3 70% of FVC Factors A. Airway Narrowing Oedema Congestion Inflammation, FB, etc. B. Lung volume expiration > inspiration-closing volume C. Posture