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Showing posts with label Respiratory. Show all posts
Showing posts with label Respiratory. Show all posts

Wednesday, May 5, 2021

INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE

INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE


INNERVATION OF THE LUNGS AND TRACHEOBRONCHIAL TREE
The tracheobronchial tree and lungs are innervated by the autonomic nervous system. Three types of pathways are involved: autonomic afferent, parasympathetic efferent, and sympathetic efferent. Each type of fiber is discussed here; the neurochemical control of respiration is covered later in the section on physiology (see Plates 2-25 and 2-26).
 
INNERVATION OF TRACHEOBRONCHIAL TREE: SCHEMA
INNERVATION OF TRACHEOBRONCHIAL TREE: SCHEMA
Autonomic Afferent Fibers
Afferent fibers from stretch receptors in the alveoli and from irritant receptors in the airways travel via the pulmonary plexus (located around the tracheal bifurcation and hila of the lungs) to the vagus nerve. Similarly, fibers from irritant receptors in the trachea and from cough receptors in the larynx reach the central nervous system via the vagus nerve. Chemoreceptors in the carotid and aortic bodies and pressor receptors in the carotid sinus and aortic arch also give rise to afferent autonomic fibers. Whereas the fibers from the carotid sinus and carotid body travel via the glossopharyngeal nerve, those from the aortic body and aortic arch travel via the vagus nerve. Other receptors in the nose and nasal sinuses give rise to afferent fibers that form parts of the trigeminal and glossopharyngeal nerves. In addition, the respiratory centers are controlled to some extent by impulses from the hypothalamus and higher centers as well as from the reticular activating system.
STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI

STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI


STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI
The trachea or windpipe passes from the larynx to the level of the upper border of the fifth thoracic vertebra, where it divides into the two main bronchi that enter the right and left lungs. About 20 C-shaped plates of cartilage support the anterior and lateral walls of the trachea and main bronchi. The posterior wall, or membranous trachea, is free of cartilage but does have interlacing bundles of muscle fibers that insert into the posterior ends of the cartilage plates. The external diameter of the trachea is approximately 2.0 cm in men and 1.5 cm in women. The tracheal length is approxi- mately 10 to 11 cm.

STRUCTURE OF THE TRACHEA AND MAJOR BRONCHI

Mucous glands are particularly numerous in the posterior aspect of the tracheal mucosa. Throughout the trachea and large airways, some of these glands lie between the cartilage plates, and others are external to the muscle layers with ducts that penetrate this layer to open on the mucosal surface. Posteriorly, elastic fibers are grouped in longitudinal bundles immediately beneath the basement membrane of the tracheal epithelium, and these appear to the naked eye as broad, flat bands that give a rigid effect to the inner lining of the trachea; they are not so obvious anteriorly. More distally, the bands of elastic fibers are thinner and surround the entire circumference of the airways.

Monday, May 3, 2021

INTRAPULMONARY AIRWAYS

INTRAPULMONARY AIRWAYS


INTRAPULMONARY AIRWAYS
According to the distribution of cartilage, airways are divided into bronchi and bronchioles. Bronchi have cartilage plates as discussed earlier. Bronchioles are distal to the bronchi beyond the last plate of cartilage and proximal to the alveolar region. Cartilage plates become sparser toward the periphery of the lung, and in the last generations of bronchi, plates are found only at the points of branching. The large bronchi have enough inherent rigidity to sustain patency even during massive lung collapse; the small bronchi collapse along with the bronchioles and alveoli. Small and large bronchi have submucosal mucous glands within their walls.

INTRAPULMONARY AIRWAYS

When any airway is pursued to its distal limit, the terminal bronchiole is reached. Three to five terminal bronchioles make up a lobule. The acinus, or respiratory unit, of the lung is defined as the lung tissue supplied by a terminal bronchiole. Acini vary in size and shape. In adults, the acinus may be up to 1 cm in diameter. Within the acinus, three to eight generations of respiratory bronchioles may be found. Respiratory bronchioles have the structure of bronchioles in part of their walls but have alveoli opening directly to their lumina as well. Beyond these lie the alveolar ducts and alveolar sacs before the alveoli proper are reached.
STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY

STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY


STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY
The airways are the hollow tubes that conduct air to the respiratory regions of the lung. They are lined throughout their length by pseudostratified, ciliated, columnar epithelium (also referred to as respiratory epithelium) supported by a basement membrane (see Plate 1-24 for details of cell types and their arrangement). The remainder of the wall includes a muscle coat and accessory structures such as submucosal glands, together with connective tissue. In the bronchi, cartilage provides additional support.
In adults, the diameter of the main bronchus is similar to that of the trachea (-2 cm), and the diameter of a terminal bronchiole is about 1 mm. These measurements vary with age and the size of the individual and with the functional state of the airway. For reference purposes, it is helpful to designate airways by their order or generation along an axial pathway. The epithelium is thicker in the larger airways and gradually thins toward the periphery of the lung.
Immediately beneath the basement membrane, elastic  fibers are collected into fine  bands that form longitudinal ridges. In cross-section, the fiber bundles are at the apices of the bronchial folds. The rest of the wall is made up of loose connective tissue containing blood vessels, nerves, capillaries, and lymphatics.

STRUCTURE OF BRONCHI AND BRONCHIOLES LIGHT MICROSCOPY

Blood Supply
The bronchial arteries supply the capillary bed in the airway wall, forming one plexus internal and another external to the muscle layer (see also Plate 1-26).
ULTRASTRUCTURE OF THE TRACHEAL, BRONCHIAL, AND BRONCHIOLAR EPITHELIUM

ULTRASTRUCTURE OF THE TRACHEAL, BRONCHIAL, AND BRONCHIOLAR EPITHELIUM


ULTRASTRUCTURE OF THE TRACHEAL, BRONCHIAL, AND BRONCHIOLAR EPITHELIUM
The lining of the respiratory airways is predominantly a pseudostratified, ciliated, columnar epithelium in which all cells are attached to the basement membrane but not all reach the lumen. In the smaller peripheral airways, the epithelium may be only a single layer thick and cuboidal rather than columnar because basal cells are absent at this level.
Ciliated cells are present in even the smallest airways and respiratory bronchioles, where they are adjacent to alveolar lining cells. The “ciliary escalator” starts at the most distal point of the airway epithelium. In smaller airways, the cilia are not as tall as in the more central airways. Eight epithelial cell types can be identified in humans, although ultrastructural features and cell kinetics have been studied mainly in animals. The following classification is based on studies in the rat: the (1) basal and (2) pulmonary neuroendocrine cells are attached to the basement membrane but do not reach the lumen; (3) the intermediate cell is probably the precursor that differentiates into (4) the ciliated cell, (5) the brush cell, or one of the secretory cells (6) the mucous (goblet) cell, (7) the serous cell, or (8) the Clara cell.
The Clara cell, The mucous (goblet) cell, The brush cell,  The ciliated cell, The basal cell

The basal cell divides and daughter cells pass to the superficial layer.
The pulmonary neuroendocrine cell (PNEC), previously referred to as the Kulchitsky cell, contains numerous neurosecretory granules and is a rare, but likely important, functional cell of the airway epithelium. The PNEC neurosecretory granules contain serotonin and other bioactive peptides such as gastrin-releasing peptide (GRP). PNECs are more numerous before birth and may play a role in the innate immune system. The intermediate cell is columnar. It has electronlucent cytoplasm and no special features. It is probably the cell that differentiates into the others.
BRONCHIAL SUBMUCOSAL GLANDS

BRONCHIAL SUBMUCOSAL GLANDS


BRONCHIAL SUBMUCOSAL GLANDS
The submucosal glands of the human airways are of the branched tubuloacinar type: tubulo refers to the main part of the secretory tubule and acinar to the blind end of such a tubule.
Three-dimensional reconstruction of the gland reveals its various zones:
1.  The origin is referred to as the ciliated duct and is lined by bronchial epithelium with its mixed population of cells. With the naked eye, the origin of the gland is seen as a hole of pinpoint size in the surface epithelium of the bronchus.
2.  The second part of the duct expands to form the collecting duct and is lined by a columnar epithelium in which the cells are eosinophilic after staining with hematoxylin and eosin. Ultrastructural examination shows these cells to be packed with mitochondria, resembling the cells of the striated duct of the salivary gland (except that they lack the folds of membrane responsible for the appearance of striation). The collecting duct may be up to 0.25 mm in diameter and 1 mm long. It passes obliquely from the airway lumen, so the usual macroscopic section does not include the full length of the duct. It is usually seen as a rather large “acinus” composed of cells without secretory granules.
3.  About 13 tubules rise from each collecting duct. These may branch several times and are closely intertwined with each other. The secretory cells lining these tubules are of two types: mucous and serous. Mucous cells line the central or proximal part of a tubule; serous cells line the distal part. Outpouchings or short-sided tubules may arise from the sides of the mucous tubules, and these are lined by serous cells. The peripheral portion of a tubule usually branches several times, and each of the final blind endings is lined with serous cells.

BRONCHIAL SUBMUCOSAL GLANDS

The gland tissue is internal to a basement membrane. In addition to the cell types described above, the following are found: (1) myoepithelial cells; (2) “clear” cells; and (3) nerve fibers, including motor fibers. Outside the basement membrane, there are rich vascular and lymphatic networks and the nerve plexus.

Tuesday, April 20, 2021

INTRAPULMONARY BLOOD CIRCULATION

INTRAPULMONARY BLOOD CIRCULATION


INTRAPULMONARY BLOOD CIRCULATION
The human lung is supplied by two arterial systems referred to as pulmonary and bronchial, each originating from a different side of the heart. Blood from the lungs is drained by two venous systems, pulmonary and true bronchial. The pulmonary veins drain oxygenated blood from the regions supplied by the pulmonary artery and deoxygenated blood from the airways within the lung that are supplied by the  bronchial  artery. The true bronchial veins serve only the perihilar region, supplied mainly by the bronchial artery, and this blood drains to the azygous system and right atrium.


Arteries
The bronchial arteries arise from the aorta and supply the capillary plexus of the airway walls from the hilum to the respiratory bronchiole.
The pulmonary artery branches run with airways and their accompanying bronchial arteries in a single connective tissue sheath referred to as the bronchoarterial or bronchovascular bundle. The pulmonary artery transforms into a capillary bed only when it reaches the alveoli of the respiratory bronchiole. It supplies all capillaries in the alveolar walls that constitute the respiratory surface of the lung.
FINE STRUCTURE OF ALVEOLAR CAPILLARY UNIT

FINE STRUCTURE OF ALVEOLAR CAPILLARY UNIT


FINE STRUCTURE OF ALVEOLAR CAPILLARY UNIT
The cellular composition of the alveolar capillary unit was not recognized until the era of electron microscopy. Before that time, it was thought that a single membrane separated blood and air at the level of the terminal airspace. We now know that, even at its narrowest, the boundary between blood and air is composed of at least two cell types (the type I alveolar epithelial cell and the endothelial cell) and extracellular material, namely, the surfactant lining of the alveolar surface, the basement membranes, and the so-called “endothelial fuzz.” The last is composed of mucopolysaccharides and proteoglycans (or glycocalyx) that may be involved in signal transduction, including mechanotransduction or shear stress at the endothelial surface. Plate 1-27 shows part of a terminal airspace and cross sections of surrounding capillaries. In humans, the diameter of the alveoli varies from 100 to 300 μm. The capillary segments are much smaller in diameter (10-14 μm) and may be separated from each other by even smaller distances. Each alveolus (there are 300 million alveoli in the adult human lungs) may be associated with as many as 1000 capillary segments.
The thinness of the cellular boundary between the blood and the air presents enormous surface area to air on one side and to blood on the other (  ̴70 m2 for both lungs). Given the paucity of organelles, the cells at this location likely play mainly passive roles in physiologic and metabolic events involved in the management of airborne or bloodborne substrates.
Ninety-five percent of the alveolus is lined by epithelial type I cells. The remaining cells are larger polygonal type II cells. These two cell types form a complete epithelial layer sealed by tight junctions. The cellular layer lining the alveoli is remarkably impermeable to salt-containing solutions, but little is known about specific metabolic activities of type I alveolar cells. Growing evidence suggests a more important role in the maintenance of alveolar homeostasis than previously thought, evidenced by the expression a large number of proteins such as aquaporin (AQP-5), T1α, functional ion channels, caveolins, adenosine receptors, and multidrug-resistant genes. Type II cells and endothelial cells have long been known to play active roles in the metabolic function of the lung by producing surfactant and processing circulating vasoactive substances, respectively. In addition, recent research suggests more complex roles for both of these cell types.
ULTRASTRUCTURE OF PULMONARY ALVEOLI AND CAPILLARIES
ULTRASTRUCTURE OF PULMONARY ALVEOLI AND CAPILLARIES

Alveolar Cells And Surface-Active Layer
As illustrated in Plate 1-28, in addition to being larger, the type II alveolar cell is distinguished from the type I alveolar cell by having short, blunt projections on the free alveolar surface and lamellar inclusion bodies. The intracellular origins of the lamellar bodies (LBs) and the exact mechanism for lipid transport into them are not known with certainty, although lipid translocation across the LB membrane is facilitated by the ABCA subfamily of adenosine triphosphate binding cassette transporters. The LB contains the phospholipid component of surfactant and two small hydrophobic surfactant polypeptide proteins (SP-B and SP-C) that are coreleased from the type II cell by a process similar to exocytosis. Two additional components of surfactant (large hydrophilic proteins SP-A and SP-D) are synthesized and released independent of LBs.

ANATOMY PHYSIOLOGY

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