W4.1_Intro_mechanics of pulmonary and alveolar ventilation PDF

Introduction to the organs and structures of the respiratory system Membrane Transport & Membrane Potentials (1) Violetta Raffay, MD, PhD Asst. Professor - School of Medicine EUC Feb 2021...

Introduction to the organs and structures of the respiratory system Membrane Transport & Membrane Potentials (1) Violetta Raffay, MD, PhD Asst. Professor - School of Medicine EUC Feb 2021 Dr Elina Psara The slides were adapted from Dr Raffay’s slides. [email protected] The respiratory system is composed of: The lungs The conducting airways The parts of the CNS concerned with the control of the muscles of respiration The chest wall: ―The chest wall consists of the muscles of respiration (diaphragm, intercostal muscles, and the abdominal muscles), and the rib cage Main functions of the respiratory system To obtain O2 from the external environment To supply O2 to the cells and to remove from the body the CO2 produced by cellular metabolism Portions of the respiratory system are also used for non-vital functions: ― Sensing odors ― Speech production ― Straining (e.g. during childbirth or coughing)  2 major areas of the respiratory system The conducting zone ― Consists of all the structures that provide passageways for air to travel into and out of the lungs: the nasal cavity, pharynx, trachea, bronchi, and most bronchioles ― Includes the organs and structures not directly involved in gas exchange The respiratory zone ― Includes the structures of the lung that are directly involved in gas exchange: the respiratory bronchioles and alveoli ― It is where gas exchange takes place Organs and structures of the respiratory system  The conducting zone Major functions ― Provides a route for incoming and outgoing air ― Removes debris and pathogens from the incoming air ― Warms and humidifies the incoming air Several structures within the conducting zone perform other functions as well ― E.g. the epithelium of the nasal passages is essential for sensing odors ― E.g. the bronchial epithelium that lines the lungs can metabolise some airborne carcinogens The conducting zone  The respiratory zone It begins where a terminal bronchiole gives rise to respiratory bronchioles They are the smallest type of bronchiole Each respiratory bronchiole gives rise to 3 alveolar ducts Alveolar ducts open into a cluster of alveoli Bronchioles lead to alveolar sacs in the respiratory zone, where gas exchange occurs  Alveoli An alveolar duct is a tube composed of smooth muscle and connective tissue, which opens into a cluster of alveoli An alveolus is one of the many small, grape-like sacs that are attached to the alveolar ducts An alveolar sac is a cluster of many individual alveoli that are responsible for gas exchange An alveolus is approximately 200 μm in diameter with elastic walls that allow the alveolus to stretch during air intake, which greatly increases the surface area available for gas exchange Alveoli are connected to their neighbors by alveolar pores, which help maintain equal air pressure throughout the alveoli and lung Structures of the respiratory zone (a) The alveolus is responsible for gas exchange (b) A micrograph shows the alveolar structures within lung tissue. LM × 178. (Micrograph provided by the Regents of University of Michigan Medical School © 2012) The alveolar wall consists of 3 major cell types: Type I alveolar cells ― A type I alveolar cell is a squamous epithelial cell of the alveoli, which constitute up to 97 percent of the alveolar surface area. These cells are about 25 nm thick and are highly permeable to gases Type II alveolar cells ― A type II alveolar cell is interspersed among the type I cells and secretes pulmonary surfactant, a substance composed of phospholipids and proteins that reduces the surface tension of the alveoli Alveolar macrophages ― Roaming around the alveolar wall is the alveolar macrophage, a phagocytic cell of the immune system that removes debris and pathogens that have reached the alveoli  The respiratory membrane The simple squamous epithelium formed by type I alveolar cells is attached to a thin, elastic basement membrane. This epithelium is extremely thin and borders the endothelial membrane of capillaries Taken together, the alveoli and capillary membranes form a respiratory membrane that is approximately 0.5 mm thick The respiratory membrane allows gases to cross by simple diffusion, allowing O2 to be picked up by the blood for transport and CO2 to be released into the air of the alveoli  The lungs A major organ of the respiratory system Each lung houses structures of both the conducting and respiratory zones The main function of the lungs is to perform the exchange of O2 and CO2 with air from the atmosphere To this end, the lungs exchange respiratory gases across a very large epithelial surface area—about 70 square meters—that is highly permeable to gases  Review of the lungs (1/2) The lungs are the major organs of the respiratory system responsible for performing gas exchange The lungs are paired and separated into lobes: ― The left lung consists of two lobes, whereas the right lung consists of three lobes Blood circulation is very important, as blood is required to transport oxygen from the lungs to other tissues throughout the body The function of the pulmonary circulation is to aid in gas exchange The pulmonary artery provides deoxygenated blood to the capillaries that form respiratory membranes with the alveoli, and the pulmonary veins return newly oxygenated blood to the heart for further transport throughout the body  Review of the lungs (2/2) The lungs are innervated by the sympathetic and parasympathetic nervous systems ― The autonomic nervous system coordinates the bronchodilation and bronchoconstriction of the airways The lungs are enclosed by the pleura, a membrane that is composed of visceral and parietal pleural layers The space between these two layers is called the pleural cavity. The mesothelial cells of the pleural membrane create pleural fluid Pleural fluid serves as both a lubricant (to reduce friction during breathing) and as an adhesive to adhere the lungs to the thoracic wall (to facilitate movement of the lungs during ventilation)  Gross anatomy of the lungs (1/4) The lungs are pyramid-shaped, paired organs that are connected to the trachea by the right and left bronchi; on the inferior surface, the lungs are bordered by the diaphragm The diaphragm is the flat, dome-shaped muscle located at the base of the lungs and thoracic cavity The lungs are enclosed by the pleurae, which are attached to the mediastinum The right lung is shorter and wider than the left lung, and the left lung occupies a smaller volume than the right  Gross anatomy of the lungs (2/4) The cardiac notch is an indentation on the surface of the left lung ― It allows space for the heart The apex of the lung is the superior region, whereas the base is the opposite region near the diaphragm The costal surface of the lung borders the ribs. The mediastinal surface faces the midline  Apex Apex Base Base  Gross anatomy of the lungs (3/4) Each lung is composed of smaller units called lobes ― Fissures separate these lobes from each other The right lung consists of three lobes: ― The superior lobe ― The middle lobe ― The inferior lobe The left lung consists of two lobes: ― The superior lobe ― The inferior lobe  Gross anatomy of the lungs (4/4) A bronchopulmonary segment is a division of a lobe, and each lobe houses multiple bronchopulmonary segments Each segment receives air from its own tertiary bronchus and is supplied with blood by its own artery Some diseases of the lungs typically affect one or more bronchopulmonary segments, and in some cases, the diseased segments can be surgically removed with little influence on neighboring segments A pulmonary lobule is a subdivision formed as the bronchi branch into bronchioles Each lobule receives its own large bronchiole that has multiple branches An interlobular septum is a wall, composed of connective tissue ― It separates lobules from one another  Blood supply and nervous innervation of the lungs The blood supply of the lungs plays an important role in gas exchange and serves as a transport system for gases throughout the body Innervation by both the sympathetic and parasympathetic nervous systems provides an important level of control through dilation and constriction of the airway  Blood supply of the lungs (1/2) The major function of the lungs is to perform gas exchange, which requires blood from the pulmonary circulation This blood supply contains deoxygenated blood and travels to the lungs where erythrocytes, also known as RBCs, pick up O2 to be transported to tissues throughout the body The pulmonary artery is an artery that arises from the pulmonary trunk and carries deoxygenated, arterial blood to the alveoli ― It branches multiple times as it follows the bronchi, and each branch becomes progressively smaller in diameter  Blood supply of the lungs (2/2) One arteriole and an accompanying venule supply and drain one pulmonary lobule As they near the alveoli, the pulmonary arteries become the pulmonary capillary network ― The pulmonary capillary network consists of tiny vessels with very thin walls that lack smooth muscle fibers The capillaries branch and follow the bronchioles and structure of the alveoli ― It is at this point that the capillary wall meets the alveolar wall, creating the respiratory membrane Once the blood is oxygenated, it drains from the alveoli by way of multiple pulmonary veins, which exit the lungs through the hilum  Nervous innervation of the lungs Dilation and constriction of the airway are achieved through nervous control by the sympathetic and parasympathetic nervous systems ― The sympathetic nervous system stimulates bronchodilation ― The parasympathetic system causes bronchoconstriction Reflexes (e.g. coughing), and the ability of the lungs to regulate O2 and CO2 levels, also result from this ANS control Sensory nerve fibers arise from the vagus nerve, and from the second to fifth thoracic ganglia The pulmonary plexus is a region on the lung root formed by the entrance of the nerves at the hilum ― These nerves then follow the bronchi in the lungs and branch to innervate muscle fibers, glands, and blood vessels  Pleura of the lungs Each lung is enclosed within a cavity that is surrounded by the pleura The pleura (plural = pleurae) is a serous membrane that surrounds the lung The right and left pleurae, which enclose the right and left lungs, respectively, are separated by the mediastinum The pleurae consist of two layers: The visceral pleura is the inner layer that is superficial to the lungs and extends into and lines the lung fissures The parietal pleura is the outer layer that connects to the thoracic wall, the mediastinum, and the diaphragm The visceral and parietal pleurae connect to each other at the hilum. The pleural cavity is the space between the visceral and parietal layers  2 major functions of the pleurae Production of pleural fluid Creation of cavities that separate the major organs ― The pleurae create a division between major organs that prevents interference due to the movement of the organs ― This division also prevents the spread of infection  Pleural fluid Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces This lubrication ↓ friction between the two layers to prevent trauma during breathing, and it creates surface tension that helps maintain the position of the lungs against the thoracic wall This adhesive characteristic of the pleural fluid causes the lungs to enlarge when the thoracic wall expands during ventilation, allowing the lungs to fill with air  The process of breathing Pulmonary ventilation is the act of breathing ― It can be described as the movement of air into and out of the lungs The major mechanisms that drive pulmonary ventilation are: ― Atmospheric pressure (Patm) ― The air pressure within the alveoli, called intra-alveolar pressure (Palv) aka intrapulmonary pressure ― The pressure within the pleural cavity, called intrapleural pressure (Pip)  Review of pulmonary ventilation (1/2) Pulmonary ventilation is the process of breathing, which is driven by pressure differences between the lungs and the atmosphere Atmospheric pressure is the force exerted by gases present in the atmosphere ― Expressed in the unit of atm or mmHg ― 1 atm=760 mmHg, which is the atmospheric pressure at sea level Intra-alveolar (intrapulmonary) pressure is the force exerted by gases within the alveoli Intra-alveolar pressure will equalise with the atmospheric pressure Intrapleural pressure is the force exerted by gases in the pleural cavity Typically, intrapleural pressure is lower, or negative to intra-alveolar pressure The difference in pressure between intrapleural and intra-alveolar pressures is called transpulmonary pressure Transpulmonary pressure determines the size of the lungs A higher transpulmonary pressure corresponds to a larger lung  Review of pulmonary ventilation (2/2) Pressure is determined by the volume of the space occupied by a gas and is influenced by resistance Air flows when a pressure gradient is created, from a space of higher pressure to a space of lower pressure Boyle’s law describes the relationship between volume and pressure ― A gas is at lower pressure in a larger volume because the gas molecules have more space to in which to move ― The same quantity of gas in a smaller volume results in gas molecules crowding together, producing increased pressure  Mechanisms of breathing The intra-alveolar and intrapleural pressures are dependent on certain physical features of the lung However, the ability to breathe—to have air enter the lungs during inspiration and air leave the lungs during expiration—is dependent on the air pressure of the atmosphere and the air pressure within the lungs  Pressure relationships Inspiration (or inhalation) and expiration (or exhalation) are dependent on the differences in pressure between the atmosphere and the lungs In a gas, pressure is a force created by the movement of gas molecules that are confined. For example, a certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one- liter container. In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Likewise, if volume decreases, pressure increases. Pressure and volume are inversely related (P = k/V) Therefore, the pressure in the one-liter container (one-half the volume of the two- liter container) would be twice the pressure in the two-liter container. Boyle’s law is expressed by the following formula: P1V1 = P2V2  Pressure relationships P1V1 = P2V2 In this formula, P1 represents the initial pressure and V1 represents the initial volume, whereas the final pressure and volume are represented by P2 and V2, respectively If the two- and one-liter containers were connected by a tube and the volume of one of the containers were changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume)  Boyle’s Law In a gas, pressure increases as volume decreases  More on atmospheric pressure Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure ― Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure ― A pressure that is equal to the atmospheric pressure is expressed as 0 mmHg  More on intra-alveolar pressure Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing Because the alveoli are connected to the atmosphere via the tubing of the airways (similar to the two- and one-liter containers in the earlier example), the intrapulmonary pressure of the alveoli always equalises with the atmospheric pressure  More on intrapleural pressure Intrapleural pressure is the pressure of the air within the pleural cavity, between the visceral and parietal pleurae Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure) Although it fluctuates during inspiration and expiration, intrapleural pressure remains approximately –4 mmHg throughout the breathing cycle  Competing forces within the thorax cause the formation of the negative intrapleural pressure (1/2) Lung elasticity & surface tension of alveolar fluid Vs surface tension of pleural fluid & thoracic wall One of these forces relates to the elasticity of the lungs themselves—elastic tissue pulls the lungs inward, away from the thoracic wall Surface tension of alveolar fluid, which is mostly water, also creates an inward pull of the lung tissue  Competing forces within the thorax cause the formation of the negative intrapleural pressure (2/2) This inward tension from the lungs is countered by opposing forces from the pleural fluid and thoracic wall Surface tension within the pleural cavity pulls the lungs outward Too much or too little pleural fluid would hinder the creation of the negative intrapleural pressure Therefore, the level must be closely monitored by the mesothelial cells and drained by the lymphatic system Since the parietal pleura is attached to the thoracic wall, the natural elasticity of the chest wall opposes the inward pull of the lungs Ultimately, the outward pull is slightly greater than the inward pull, creating the –4 mmHg intrapleural pressure relative to the intra-alveolar pressure  Intra-alveolar and intrapleural pressure relationships Intra-alveolar pressure changes during the different phases of the cycle. It equalizes at 760 mmHg, but it does not remain at 760 mmHg  Summary of pressure changes The difference in pressures drives pulmonary ventilation because air flows down a pressure gradient Air flows from an area of higher pressure to an area of lower pressure Air flows into the lungs largely due to a difference in pressure Atmospheric pressure > intra-alveolar pressure Intra-alveolar pressure > intrapleural pressure Air flows out of the lungs during expiration based on the same principle Pressure within the lungs becomes > the atmospheric pressure  Physical factors affecting ventilation (1/2) In addition to the differences in pressures, breathing is also dependent upon the contraction and relaxation of muscle fibers of both the diaphragm and thorax The lungs themselves are passive during breathing, meaning they are not involved in creating the movement that helps inspiration and expiration ― This is because of the adhesive nature of the pleural fluid, which allows the lungs to be pulled outward when the thoracic wall moves during inspiration ― The recoil of the thoracic wall during expiration causes compression of the lungs Contraction and relaxation of the diaphragm and intercostal muscles cause most of the pressure changes that result in inspiration and expiration These muscle movements and subsequent pressure changes cause air to either rush in or be forced out of the lungs  Physical factors affecting ventilation (2/2) Other characteristics of the lungs influence the effort that must be expended to ventilate Resistance is a force that slows motion, in this case, the flow of gases The size of the airway is the primary factor affecting resistance and pressure changes A small tubular diameter forces air through a smaller space, causing more collisions of air molecules with the walls of the airways F = ∆P / R  Importance of pulmonary surfactant As highlighted earlier, there is surface tension within the alveoli caused by water present in the lining of the alveoli ― This surface tension tends to inhibit expansion of the alveoli However, pulmonary surfactant secreted by type II alveolar cells mixes with that water and helps reduce this surface tension Without pulmonary surfactant, the alveoli would collapse during expiration  Thoracic wall compliance It is the ability of the thoracic wall to stretch while under pressure ― This can also affect the effort expended in the process of breathing In order for inspiration to occur, the thoracic cavity must expand The expansion of the thoracic cavity directly influences the capacity of the lungs to expand ― If the tissues of the thoracic wall are not very compliant, it will be difficult to expand the thorax to increase the size of the lungs  Question 1 What is the definition of transpulmonary pressure? A. The difference between intra-alveolar pressure and atmospheric pressure B. The difference between intrapleural pressure and intra- alveolar pressure C. The difference between atmospheric pressure and intrapleural pressure D. The difference between intrapleural pressure and atmospheric pressure  Question 2 What is the primary function of pulmonary surfactant? A. To increase surface tension in the alveoli B. To decrease surface tension in the alveoli C. To increase the rate of gas diffusion in the alveoli D. To decrease the rate of gas diffusion in the alveoli To be continued… Thank you!