Respiration - RESPIRATION A-M Lauzon Ph.D Class notes 2008...

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Unformatted text preview: RESPIRATION A.-M. Lauzon, Ph.D. Class notes 2008 Reference textbooks: “Human Physiology” by Widmaier, Raff, Strang “Respiratory Physiology- the essentials” by West "The Normal Lung" by Murray I STRUCTURE OF THE LUNGS AND CHEST WALL A. Function of Respiration The primary function of respiration is gas exchange. In mammals, gas exchange occurs in the lungs. During inspiration, air rich in O2 is inhaled into the lungs. During expiration, CO2 produced during the oxidative processes of the body is exhaled from the lungs. Both gases are transported by the blood. Therefore, both the cardiovascular system and the respiratory system are involved with supplying body cells with O2 and eliminating their waste product, CO2. B. The respiratory tract Air flows through a series of air passages that connect the lungs to the nose and mouth. Inhaled air passes over a complex series of surfaces when it goes through the nose: the nasal septum and the nasal turbinates. These surfaces clean the air of big dust particles. From the nose, warmed and moistened air flows through the common passages for air and food, the pharynx, and then continues through the larynx. Air finally reaches the periphery of the lungs via the trachea and bronchi (figure 1). The lungs and the airways share the chest cavity with the heart, the great vessels, and the esophagus. The airways consist of a series of tubes that branch and become narrower, shorter and more numerous as they penetrate into the lungs. The trachea divides into 2 main bronchi, each of which divides into lobar and segmental bronchi. The right main bronchus has 3 lobar bronchi (the right lung has 3 lobes), while the left main bronchus divides into only 2 bronchi (the left lung has only 2 lobes). The segmental bronchi divide further into smaller branches. The smallest airways without alveoli are the terminal bronchioles (figure 2). The respiratory tract Figure 1 turbinates clean air pharynx larynx trachea two main bronchi one bronchi for each of three lobes on right side. 3 lobar bronchi left hand side has two lobes. 2 lobar bronchi then segmental bronchi....finally alvioli surrounded by capillaries * pleural space filled with a small amount of fluid. - couples the lungs with the chest * Subdivisions of the conducting airways and terminal respiratory units Figure 2 * Notes: Pleural surfaces • A way to visualize the apposition of the two pleural surfaces is to put a drop of water between two glass microscope slides. The two slides can easily slide over each other but are very difficult to pull apart. lubrication between the lungs (“Human Physiology” by Widmaier, Raff, Strang) and the chest. but they are coupled • The pressure in the pleural space is negative. This will become clearer after the discussion of pneumothorax (figure 35). *Note: These figures will be discussed in detail in section VII on the mechanics of breathing. They are included here because in class, as we discuss the anatomy of the respiratory system, we will briefly address the function of the pleural space. The pleural - couples the lungs to the chest space when you pull on the chest, the lungs will move with it as well The pump analogy The pneumothorax - the pressure in the pleural space is negative w.r.t outside - as you breathe, the pressure in the pleural space becomes more negative - pleural space - similar concept to having two microscope slides together with water in between them - water is non-compressable - if a hole is made in the pleural space, air will flow in, and you lose the coupling of the chest and the lungs C. Conducting and Respiratory Zones The airways are divided into 2 zones: the conducting zone (made up of the conducting airways) and the respiratory zone (figure 2). The conducting airways consist of the airways from the mouth and nose openings, all the way down to the terminal bronchioles. These airways conduct air from the atmosphere to the respiratory part of the lungs. The conductive airways do not contribute to gas exchange, and are thus said to compose the anatomical dead space. The respiratory part of the lungs (the respiratory zone) begins where the terminal bronchioles divide into respiratory bronchioles, which have some alveoli opening into their lumena (figure 3). Beyond the respiratory bronchioles are the alveolar ducts lined with alveoli. The alveolated region of the lungs is the site of gas exchange, and is called the respiratory zone. Because of such abundant branching of the airways, the respiratory zone makes up most of the lungs. The smallest physiological unit of the lungs (distal to the terminal bronchiole) is the acinus (figure 3). smooth muscle Acinus Figure 3 the smooth muscles around the airways mean trouble. don't know why they're there. these are the muscles that contract during asthma -decreasing the luma DON'T HELP TO BREATHE AT ALL. Acinus (Reproduced from Netter, F.H.: The CIBA Collection of Medical Illustrations, vol. 7). D. Functions of the Conducting Airways The conducting airways have 4 main functions: 1. Defense against bacterial infection and foreign particles: the epithelial lining of the bronchi has hair-like projections called cilia. The epithelial glands secrete a thick substance, mucous, which lines the respiratory passages as far down as the bronchioles. Foreign particles stick to the mucous and the cilia constantly sweep the mucous up into the pharynx. - mucoual cillial elevator - paralyzed by nicotine 2. Warm and moisten inhaled air. 3. Sound and speech are produced by the movement of air passing over the vocal cords. 4. Regulation of air flow: smooth muscle around the airways may contract or relax to alter resistance to air flow. E. Function of the Respiratory Zone The respiratory zone is the site of gas exchange between the air in the alveoli and the blood in the pulmonary capillaries. There are roughly 300 million alveoli in the human lungs, and each alveolus may be associated with as many as 1000 capillaries. F. Blood Supply The lungs have two circulations: the pulmonary circulation (figures 4 & 5), bringing mixed venous blood (blood that comes from different body organs with different metabolic activities) to the lungs (figure 4), allowing for the blood to get oxygenated, and then back to the left heart, and the bronchial circulation (figure 5), supplying oxygenated blood from the systemic circulation to the tracheobronchial tree (this circulation allows for the airways to get oxygenated). 1) Pulmonary circulation: Blood to the pulmonary capillaries is supplied via the pulmonary artery, which originates from the right ventricle (figures 4 and 5). Branches of the pulmonary artery run with the airways. When the alveoli are reached, arterioles divide into a capillary bed. The pulmonary arteries supply with blood all capillaries within the alveolar walls, which constitute the respiratory surface of the lungs where the gas exchange takes place. Oxygenated blood (from the alveolar capillaries) leaves the lungs and returns to the left heart via the pulmonary veins (figures 4 and 5). 2) Bronchial circulation: Bronchial arteries from the aorta supply the airway walls (figure 4). The bronchial circulation is part of the systemic circulation. pulmonary circulation pulmonary artery pulmonary vein to left heart Figure 4 The pulmonary and bronchial circulations. (Reproduced from Netter, F.H.: The CIBA Collection of Medical Illustrations, vol. 7). Figure 5 G. Alveolar Cell Types There are three alveolar cell types (Figure 6): 1. Epithelial type I and II cells: Alveoli are lined by epithelial type I and II cells. Together, all the alveolar epithelial cells form a complete epithelial layer sealed by tight junctions. Little is known about the specific metabolic activities of type I cells. Type II cells produce pulmonary surfactant, a substance that decreases the surface tension of the alveoli (see sections VII- F & G). 2. Endothelial cells: Endothelial cells constitute the walls of the pulmonary capillaries. These cells may be as thin as 0.1 micron. 3. Alveolar macrophages: These remove foreign particles that may have escaped the mucociliary defense system of the airways and found their way into the alveoli. Alveolar-capillary membrane Figure 6 H. Respiratory Muscles The lung tissue is elastic, but it is unable to expand or contract by itself. Air has to be sucked into the lungs (see figure 40). This function is powered by the respiratory muscles of the chest wall. There are 2 types of respiratory muscles: inspiratory and expiratory (figure 7). everything on this slide is important Figure 7 passive, chest relaxes increases longitudinal length, elevates rib cage and sternum depress ribs abdominal muscles pushes the abdomen in, lifting the diaphragm Reproduced from Netter, F.H.: The CIBA Collection of Medical Illustrations, vol. 7. Inspiratory muscles: The main inspiratory muscle is the diaphragm. It is innervated by the phrenic nerves from cervical segments 3, 4 and 5. Contraction of the diaphragm causes its dome to descend and the chest to expand longitudinally. At the same time, its contraction elevates the lower ribs because of the vertically oriented attachments of the diaphragm to the costal margins. Contraction of the external intercostal muscles also raises the ribs during inspiration. As the ribs are elevated, the anterior-posterior and transverse dimensions of the chest enlarge (figure 7). In addition to the diaphragm, the external intercostal muscles, and the parasternal intercartilaginous muscles, the neck muscles (sternocleidomastoid and scalenus muscles) may assist in inspiration. Their major contribution is during high levels of ventilation. Contraction of these muscles is also apparent during severe asthma and other disorders that obstruct the movements of air into the lungs. The neck muscles elevate and fix the uppermost part of the rib cage, elevate the sternum, and slightly enlarge the anterio-posterior and longitudinal dimensions of the chest. Expiratory muscles: In contrast to inspiration, expiration is passive during quiet breathing as a result of the recoil of the lungs and the chest wall. It becomes active at higher levels of ventilation (exercise), or in pathological states when expiratory resistance increases and the movement of airflow out of the lungs is impeded. Muscles involved in active expiration include the internal intercostal muscles and the abdominal muscles. Contractions of these muscles compress the abdominal content, depress the lower ribs, and pull down the anterior part of the lower chest (Figure 7). In effect, they force the diaphragm upwards. They are essential for several body functions: e.g. coughing, singing, talking, vomiting. Forced maximal contraction of the expiratory muscles against a closed glottis (Valsalva's maneuver) can result in an enormous increase in pressure (up to 100 mm Hg) in the thoracic cage and abdomen. If sustained, this would lead to a decrease in venous return to the heart, thus a decrease in cardiac output. Summary of events during inspiration: Inspiration: Diaphragm and intercostal muscles contract ↓ Thoracic cage expands ↓ Intrapleural pressure becomes more subatmospheric - even more negative pressure created in ↓ the pleural space Transpulmonary pressure increases - pressure difference across difference in pressure ↓ the lungs increases from inside to outside the lungs Lungs expands ↓ Alveolar pressure becomes subatmospheric ↓ Air flows into alveoli - air flows along the pressure gradient *Note: This figure will be discussed in detail in section VII on the mechanics of breathing. Summary of events during expiration: Expiration: Diaphragm and external intercostal muscles stop contracting ↓ Chest wall moves inwards ↓ Intrapleural pressure goes back towards preinspiratory value ↓ Transpulmonary pressure goes back towards preinspiratory value ↓ Lung recoil towards preinspiratory volume ↓ Air in lungs is compressed ↓ Alveolar pressure becomes greater than atmospheric pressure ↓ Air flows out of the lungs canister with water with an inverted canister with air in it I. Spirometry: Measuring lung volumes can only measure volumes of air that you breathe in and out Clinically, it is useful to be able to measure the volume of air inhaled during inspiration under a number of different circumstances. Subdivisions of the lung volumes can be determined by means of a spirometer (Figure 8): a spirometer measures volumes of inhaled or exhaled gas, so can be used to measure tidal volume, vital capacity, inspiratory capacity, expiratory reserve volume, and inspiratory reserve volume (Figures 8 & 9). It cannot be used to measure functional residual capacity, total lung capacity, or residual volume. Figure 8 you cannot measure functional residual capacity with a spirometer - it can only be used to measure volumes that you can breathe in an out Lung volumes. (Reproduced from West, J.B.: Respiratory Physiology- the essentials). - at rest, one is taking a tidal breath - functional residual capacity is the volume of air that is left in the lungs during tidal breathing, makes sure that there is air left in the lungs at the end of a breath, maximize the amount of gas exchange, maintain air equilibrium - residual volume, there is Figure 9 always some air that is kept in your lungs even if you breath as hard as you can necessary to maintain equilibrium and prevent collapsing of the lungs - total lung capacity = all the air that you can hold in your lungs functional residual capacity J. Measurement of FRC FRC can be measured by helium dilution (Figure 10). Let C1 be the helium concentration in a spirometer of volume V1 and let the subject breath out to FRC. Then, open the valve and ask the subject to breath in and out from the spirometer. After equilibration with the subject's lungs, the concentration in the spirometer is C2. Since the total amount of helium is conserved, we have: C1 x V1 = C2 x (V1 + FRC ) so that: FRC = (C1 x V1 / C2) - V1 Figure 10 C1 = concentration of helium V1 = volume of air in the tube - helium does not dissolve in air, but rather will equilibrate in the spirometer and the lungs C2 = concentration of helium after diluted in the total volume of lungs (V2) (Reproduced from West: Respiratory Physiology- the essentials). *Note: Think of the lung as a balloon not as a jar. II VENTILATION A. Minute ventilation vs Alveolar ventilation The amount of air inspired into the lungs over some period of time is called ventilation. Usually, it is measured for one minute, and therefore we call it minute ventilation (VE). Therefore, VE is the amount of air inspired (or expired) during one minute: . . VE = VT x f where VT is the tidal volume, and f is the number of breaths per minute. (A dot above a symbol means a change with respect to time). . Figure 11 air that does reach the lungs on inhalation In a normal adult male, VT = 500ml, and f =12 breaths/minute. Therefore, VE = 6000ml/min. However, not all the air inhaled into the lungs reaches the gas exchanging area (the respiratory zone). Some of the air remains in the conducting airways, i.e. in the anatomical dead space (Figure 12). The volume of the anatomical dead space in the adult subject is about 150ml. Thus, the amount of air that reaches the respiratory zone per minute and available for gas exchange, the alveolar ventilation (VA), is: VA= (500-150 ml) x 12= 4200 ml/min. . . . removal of air contributed by anatomic dead space fresh air Figure 12 - not all the air inhaled into the lungs reaches the gas exchanging area. some of the air remains in the conduction airways = anatomical dead space (VD) The volume of the anatomical dead space in the adult subject is about 150mL (anatomical dead space is difficult to measure, but a close approximation is a subject's weight in pounds). for 150lb person Physiological dead-space Physiological dead space = alveolar dead space + anatomical dead space Under some pathological conditions, a certain amount of inspired air, although reaching the respiratory zone, does not take part in gas exchange. In this case, alveoli either receive a decreased blood supply or no blood supply at all. These alveoli represent alveolar dead space. The sum of alveolar and anatomical dead space is called the physiological dead space (VD). Therefore, the difference between minute and alveolar ventilation is the dead space ventilation that is wasted from the gas exchange point of view, i.e. VD=VE –VA, or in terms of volume, VD=VT-VA. ... P = total pressure Px = partial pressure of gas x Fx = fractional concentration in dry gas Px = Fx*P FO2 = 21% FCO2 = 0.03% Barometric P = 760mmHg Contribution of water vapour to barometric pressure = 47mmHg Px = (P-47mmHg)*Fx PO2 = (760-47mmHg)*21/100 = 150mmHg PCO2 = (760-47mmHg)*0.03/100 -alveolar dead space can occur under pathological conditions, e.g. blood clot - even if the alveoli have gas flow, there is no blood flow to the area and thus no gas exchange B.Types of Alveolar Ventilation . Normal Alveolar Ventilation: V matches CO2 and keeps PaCO2 at a constant level (figure 13). A - partial pressures on the air when it reaches the lungs decreases on AVERAGE - on first breath, the partial pressure of oxygen will also be 150mmHg - however there is immediate diffusion of oxygen and carbon dioxide (a large pressure gradient is present) - the movement of gasses leads to a decrease in the average pressure Figure 13 Types of Alveolar Ventilation (Cont’ed) ventilation exceeds the body's needs - must be taken w.r.t the body's requirements Alveolar hyperventilation: This occurs when more O2 is supplied and more CO2 removed than the metabolic rate requires (VE exceeds the needs of the body). As a consequence, alveolar and arterial partial pressures of O2 rise and those of CO2 decrease. Note that this is with respect to the metabolism so ventilation during exercise is not hyperventilation. Alveolar hypoventilation: A fall in the overall level of ventilation can reduce alveolar ventilation below that required by the metabolic activity of the body. Under the condition of alveolar hypoventilation, the rate at which O2 is added to alveolar gas, and the rate at which CO2 is eliminated, is lowered, so that the alveolar partial pressure of O2 (PAO2) falls and PACO2 rises. As a result of this, the blood in the pulmonary capillary is less oxygenated, and PaO2 falls below normal values. Similarly, PaCO2 rises above the normal value. Alveolar hypoventilation may occur during severe disorders of the lungs (e.g. chronic obstructive lung disease), or when there is damage to the respiratory muscles. It can also occur when the chest cage is injured and the lungs collapse, or when the central nervous system is depressed. PACO2 (alveolar) is not the same as PaCO2 (arterial) . III GAS DIFFUSION A.Diffusion Rate In order to fulfill the purpose of ventilation, oxygen from the alveolar gas must be transferred across the alveolar-capillary membrane. This process occurs by passive diffusion. Diffusion is governed by Fick's Law, which states that the rate of diffusion of a gas through a tissue is proportional to the tissue area and the difference in gas partial pressure between the 2 sides, and is inversely proportional to the tissue thickness. Thus, Diffusion rate is proportional to: surface area (50-100 m2) partial pressure gradient 1/thickness (~0.2 mm) alveolar capillary membrane can thicken in the case of edema, accumulation of fluid in the ISF Diffusion Rate (Cont’ned) In the venous blood reaching the alveoli, because of the O2 consumption and CO2 production by the metabolic processes, PO2 is lower and PCO2 is higher than in the alveoli. Since the diffusion direction is from higher to lower pressure, O2 diffuses from the alveolar gas to the blood, and CO2 diffuses in the opposite direction. O2 from a gaseous medium diffuses through the alveolar-capillary membrane to a liquid plasma in the pulmonary capillaries (Figure 14). In order...
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