Pulmonary Anatomy and Physiology



Pulmonary Anatomy and Physiology


Kathleen M. Stacy


The pulmonary system consists of the thorax, conducting airways, respiratory airways, and pulmonary blood and lymph supply. The primary functions of the pulmonary system are ventilation and respiration. Ventilation is the movement of air in and out of the lungs. Respiration is the process of gas exchange by means of movement of oxygen from the atmosphere into the bloodstream and movement of carbon dioxide from the bloodstream into the atmosphere. The anatomic structures that constitute the pulmonary system are intimately related to function, and structural abnormalities can readily translate into pulmonary disorders. An applicable knowledge of anatomy and physiology is imperative in caring for the patient with pulmonary dysfunction.



Thorax


The thorax contains the major organs of respiration. It consists of the thoracic cage, lungs, pleura, and muscles of ventilation. Together, these structures form the ventilatory pump, which performs the work of breathing.



Thoracic Cage


The thoracic cage is a cone-shaped structure that is rigid but flexible. It must be somewhat rigid to protect the underlying structures, but it must be flexible to accommodate inhalation and exhalation. The cage consists of 12 thoracic vertebrae, each with a pair of ribs. Posteriorly, each rib is attached to its own vertebra, but anteriorly, attachment varies (Fig. 17-1). The first seven pairs of ribs are attached directly to the sternum. The 8th, 9th, and 10th pairs are attached by cartilage to the ribs above. Because the 11th and 12th ribs have no anterior attachment, they sometimes are referred to as floating ribs. The second rib is attached to the sternum at the angle of Louis, which is the raised ridge that can be felt just below the suprasternal notch.1




Lungs


The lungs are cone-shaped organs that have a total volume of approximately 3.5 to 8.5 liters. The superior portion is known as the apex, and the inferior portion is known as the base. The apical portion of each lung rises a few centimeters above the clavicle (see Fig. 17-1). Each lung is firmly attached to the thoracic cavity at the hilum and at the pulmonary ligament.2



Lobes and Segments


The lungs are divided into lobes and segments (Fig. 17-2), with the lobes being separated by pleural membrane-covered fissures. The right lung, which is larger and heavier than the left, is divided into upper, middle, and lower lobes. The left lung is divided into only an upper and a lower lobe.2 A portion of the left lung, the lingula, corresponds anatomically with the right middle lobe. The horizontal fissure divides the right upper lobe from the right middle lobe. The oblique fissure divides the right upper and middle lobes from the lower lobe and the left upper lobe from the lower lobe. The lobes are divided into 18 segments, each of which has its own bronchus branching immediately off a lobar bronchus. Ten segments are located in the right lung and eight in the left lung.1





Pleura


The pleura is a thin membrane that lines the outside of the lungs and the inside of the chest wall. The visceral pleura adheres to the lungs, extending onto the hilar bronchi and into the major fissures. The parietal pleura lines the inner surface of the chest wall and mediastinum.2 The two pleural surfaces are separated by an airtight space, which contains a thin layer of lubricating fluid. Pleural fluid allows the visceral and parietal pleural membranes to glide against each other during inhalation and exhalation.1,3 The pleural space has the capacity to hold much more fluid than its normal volume of a few milliliters.1



Intrapleural Pressure


The pleural space has a pressure within it called the intrapleural pressure, which differs from the intrapulmonary (pressure within the lungs) and atmospheric pressures.4 Under normal conditions, intrapleural pressure is less than intrapulmonary pressure and less than atmospheric pressure, with a normal range of −4 to −10 cm H2O during exhalation and inhalation, respectively.3 A deep inhalation can generate intrapleural pressures of −12 to −18 cm H2O. This negative intrapleural pressure results from forces within the chest wall that exert pressure to pull the parietal pleura outward and away from the visceral pleura, while the elastic fibers within the lungs exert pressure to pull the visceral pleura inward away from the parietal pleura. The constant pull of the two pleural membranes in opposite directions causes the pressure within the space to be subatmospheric.4 The negative pressure in the pleural space keeps the lungs inflated (Box 17-1). If atmospheric pressure enters the pleural space, all or part of a lung will collapse, producing a pneumothorax.1



Box 17-1   Why the Lungs Stay Inflated


The lungs stay inflated because the pressure surrounding them (intrapleural) is always less than the pressure within them (intrapulmonary).




Why Do the Two Pleural Membranes Pull in Opposite Directions?


The parietal pleura, attached to the chest, is pulled outward because the elastic fibers within the intercostal muscles exert outward pressure on the ribs. These fibers are in a relaxed state when the rib cage is fully expanded, such as during a deep inhalation. The visceral pleura is attached to the lungs and is pulled inward because the elastic fibers within the lungs that are responsible for elastic recoil exert pressure to make the lungs smaller. Elastic fibers in the lung are in a relaxed position only when the lung is at its smallest configuration, as occurs with a pneumothorax. Because of the opposite pull of the chest wall and the lung and because the pleural membranes are attached to these structures, there is a constant pull of the two membranes in opposite directions. The subatmospheric pressure that results within the pleural space and the greater-than-atmospheric intrapulmonary pressure within the lungs allows the lungs to remain inflated. Anything that causes the pressure within the pleural space to rise to atmospheric pressure or above will cause the lung to collapse—a pneumothorax.



Muscles of Ventilation


The muscles of ventilation (Fig. 17-3) are governed by the regulatory activity of the central nervous system, which sends messages to the muscles to stimulate contraction and relaxation. This muscular activity controls inhalation and exhalation. Muscles that increase the size of the chest are called muscles of inhalation; those that decrease the size of the chest are called muscles of exhalation.5




Inhalation


The main muscle of inhalation is the diaphragm. The diaphragm is a dome-shaped, fibromuscular septum that separates the thoracic and abdominal cavities. It is connected to the sternum, ribs, and vertebrae. During normal, quiet breathing, the diaphragm does approximately 80% of the work of breathing. On inhalation, the diaphragm contracts and flattens, pushes down on the viscera, and displaces the abdomen outward. Diaphragmatic contraction also lifts and expands the rib cage to some extent.1,5,6


The action of the diaphragm is governed by the medulla, which sends its impulses through the phrenic nerve. The phrenic nerve arises from the cervical plexus through the fourth cervical nerve, with secondary contributions by the third and fifth cervical nerves. For this reason and because the diaphragm does most of the work of inhalation, trauma involving levels C3 to C5 causes ventilatory dysfunction.5


Other muscles of inhalation include those that lift the rib cage. The most important of these are the external intercostal muscles, which elevate the ribs and expand the chest cage outward. The scalene, anterior serratus, and sternocleidomastoid muscles also participate to elevate the first two ribs and sternum.1,5,6





Conducting Airways


The conducting airways consist of the upper airways, the trachea, and the bronchial tree. The purposes of the conducting airways are to warm and humidify the inhaled air, to act as a protective mechanism that prevents the entrance of foreign matter into the gas exchange areas, and to serve as a passageway for air entering and leaving the gas exchange regions of the lungs.1-3



Upper Airways


The upper airways consist of the nasal and oral cavities, the pharynx, and the larynx (see Fig. 17-4). Their main contribution to ventilation is the conditioning of inspired air. Conditioned air is air that has been warmed, humidified, and cleansed of some irritants. Warming and humidifying, which are essential to prevent irritation of the lower airways, occur mainly within the nose by means of a dense vascular network that lines the nasal passages. The air is cleansed by the coarse hairs that line the nasal passages and filter large inhaled particles.1,3





Trachea


The trachea is a hollow tube approximately 11 cm (4.5 inches) long and 2.5 cm (1 inch) in diameter (Fig. 17-5). It begins at the cricoid cartilage and ends at the bifurcation (major carina) from which the two main stem bronchi arise. The carina is positioned approximately at the level of the aortic arch, the fifth thoracic vertebra,7 or just below the level of the angle of Louis.1 The trachea consists of smooth muscle supported anteriorly by 16 to 20 C-shaped, cartilaginous rings. They prevent tracheal collapse during bronchoconstriction and strong coughing. The posterior wall of the trachea lies contiguous with the anterior wall of the esophagus. Having no cartilaginous support, this wall is composed only of muscle tissue, which is separated from the anterior esophageal wall by loose connective tissue (see Fig. 17-5, inset).1




Bronchial Tree


The two main stem bronchi are structurally different (see Fig. 17-5). The left bronchus is slightly narrower than the right, and because of its position above the heart, the left bronchus angles directly toward the left lung at approximately 45 to 55 degrees from the midline. The right bronchus is wider and angles at 20 to 30 degrees from the midline. Because of this angulation and the forces of gravity, the most common site of aspiration of foreign objects is through the right main stem bronchus into the lower lobe of the right lung.2,3



Bronchi


Each branching of the tracheobronchial tree produces a new generation of tubes (Fig. 17-6). The main stem bronchi are the first generation; the next branch, the five lobar bronchi, is the second generation. The third generation includes the 18 segmental bronchi. The fourth through approximately the ninth generations are referred to as the small bronchi, beginning with the subsegmental bronchi. In these bronchi, diameters decrease; however, because the number of bronchi increases with each generation, the total cross-sectional area increases with each generation. This great increase in the cross-sectional area of the lung is significant because it allows easy ventilation despite decreasing airway lumens.1




Bronchioles


The final subdivision of the conducting airways is the bronchioles. These tubes have a diameter less than 1 mm and have no connective tissue and cartilage within their walls. Their walls do, however, contain smooth muscle.2 When smooth muscle constriction occurs, these airways may close completely because of the lack of structural support. The terminal bronchioles form the last branch of the conducting airways, after which the gas exchange areas of the lungs begin. There are more than 32,000 terminal bronchioles total.1



Defense System


The main defense system within the airways is the mucociliary escalator, or mucous blanket, a combination of mucus and cilia. The mucus, which floats atop the cilia (Fig. 17-7), traps foreign particles. Ciliary movement then propels the entire mucous blanket and any trapped particles upward toward the pharynx at an average speed of 1 mm/min in the smaller bronchioles and 12 mm/min in the larger airways and trachea. After the pharynx is reached, the mucus is swallowed or cleared. The submucous glands of the airways produce approximately 100 mL of mucus per day, with all but about 10 mL resorbed through the bronchial lining. The mucociliary escalator is so efficient that almost no particles larger than 3 microns reach the alveoli.8



The cough reflex is another protective mechanism in the lungs. Excessive amounts of foreign particles in the trachea and bronchi can initiate the cough reflex. Once initiated, the rapid expulsion of air carries away any foreign particles with it.9



Respiratory Airways


The respiratory airways consist of the respiratory bronchioles and the alveoli. The respiratory airways also are known as the terminal respiratory units, or the acini. Gas exchange takes place in these areas of the lungs.



Respiratory Bronchioles


Each terminal bronchiole gives rise to two respiratory bronchioles, with each branching two to four more times.2 The respiratory bronchioles form the transition zone of the lungs, acting as conducting airways and gas exchange units. While air is moving through them, alveolar outpouchings on their surfaces allow gas exchange to take place (see Fig. 17-6).1



Alveoli


Each respiratory bronchiole gives rise to several alveolar ducts, which terminate in clusters of 10 to 16 alveoli (see Fig. 17-6). Each terminal respiratory unit contains approximately 100 alveolar ducts and 2000 alveoli.2 The alveolus is the primary site of gas exchange and the end point in the respiratory tract. Approximately 300 million alveoli are in the two lungs. The alveoli are composed of several types of cells, including type I and II alveolar epithelial cells and alveolar macrophages.1,3



Type I Alveolar Epithelial Cells


Type I alveolar epithelial cells comprise approximately 90% of the total alveolar surface within the lungs (Fig. 17-8). They are the chief structural cells of the alveolar wall and play a major role in the maintenance of the gas-blood barrier and gas exchange. Type I cells are extremely susceptible to injury and become inflamed when exposed to inhaled toxins.1




Collateral Air Passages. 

A variety of collateral air passages are located within the lower regions of the lungs. Within the walls of the type I cells are the pores of Kohn (Fig. 17-9), which allow collateral movement of air between alveoli. The canals of Lambert are collateral air pathways that exist between the alveoli and the respiratory and terminal bronchioles. They are of particular benefit when a respiratory bronchiole is blocked or collapsed, because they allow gas to pass into alveoli distal to the blockage. Collateral air passages are of significant benefit in any pathologic condition of the lung that results in obstruction of airflow into a portion of the lungs. However, these pores and canals also allow the movement of microorganisms through lung tissue.1,2




Type II Alveolar Epithelial Cells


Type II alveolar epithelial cells occur in much greater numbers than type I cells, but because of their minute size, they comprise a smaller portion of the total alveolar wall. After injury to the alveolar wall, type II cells rapidly divide to line the surface; later, they transform into type I cells. The most important function of the type II cells is their ability to produce, store, and secrete pulmonary surfactant (Fig. 17-10).1,3



Only gold members can continue reading. Log In or Register to continue

Oct 29, 2016 | Posted by in NURSING | Comments Off on Pulmonary Anatomy and Physiology
Premium Wordpress Themes by UFO Themes