Respiratory Physiology Lec 1 & 2 PDF

Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 Lung Physiology Page | 1  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 By the end of lecture 1 ; you wil...

Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 Lung Physiology Page | 1  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 By the end of lecture 1 ; you will be able to:  Outline the anatomy of the lung  Compare between the upper and the lower airway Identification the importance of conducting zone and respiratory zone Draw and explain the major component of the respiratory epitheliumand the differences between brochi ,bronchioles and alveolus  Explaining the major Blood supply of the lung. By the end of lecture 2; you will be able to:  Ennumerate the main functions of the lungs  Differences between external and internal respiration Definition of the respiratory unit and respiratory membrane. Surfactant(definition,function,pathological condtions associated with its deficincy) The major organs of the respiratory system function primarily to provide oxygen to body tissues for cellular respiration,remove the waste product carbon dioxide, and help to maintain acid-base balance. Portions of the respiratory system are also used for non-vital functions, such as sensing odors, speech production, and for straining, such as during coughing. Page | 2  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 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 oxygen and carbon dioxide 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. Gross Anatomy of the Lungs 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. The cardiac notch is an indentation on the surface of the left lung, and 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. 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, middle, and inferior lobes. The left lung consists of two lobes: the superior and inferior lobes. 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 Page | 3  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 multiple branches. An interlobular septum is a wall, composed of connective tissue, which separates lobules from one another. Lung Innervation Dilation and constriction of the airway are achieved through nervous control by the parasympathetic and sympathetic nervous systems. The parasympathetic system causes bronchoconstriction, whereas the sympathetic nervous system stimulates bronchodilation. Reflexes such as coughing, and the ability of the lungs to regulate oxygen and carbon dioxide levels, also result from this autonomic nervous system 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. The 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 layer that is superficial to the lungs, and extends into and lines the lung fissures. In contrast, 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.  The pleurae perform two major functions: They produce pleural fluid and create cavities that separate the major organs. Pleural fluid is secreted by mesothelial cells from both pleural layers and acts to lubricate their surfaces. This lubrication reduces friction between the two layers to prevent trauma during breathing, and 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. Page | 4  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 The pleurae also create a division between major organs that prevents interference due to the movement of the organs, while preventing the spread of infection. REGIONS OF THE RESPIRATORY TRACT Parts of the upper respiratory tract:  The upper respiratory tract, can refer to the parts of the respiratory system lying above the sternal angle(outside of the thorax), above the vocal folds, or above the cricoid cartilage.  The larynx is sometimes included in both the upper and lower airways.  The tract consists of the nasal cavity and paranasal sinuses, the pharynx (nasopharynx, oropharynx and laryngopharynx).  The larynx extends from the lower part of the pharynx to complete the upper airway. 1. The Nose and its Adjacent Structures 2. Pharynx The pharynx is a tube formed by skeletal muscle and lined by mucous membrane Page | 5  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 that is continuous with that of the nasal cavities. The pharynx is divided into three major regions: the nasopharynx, the oropharynx, and the laryngopharynx. 3. Larynx The larynx is a cartilaginous structure inferior to the laryngopharynx that connects the pharynx to the trachea and helps regulate the volume of air that enters and leaves the lungs. The structure of the larynx is formed by several pieces of cartilage. Three large cartilage pieces—the thyroid cartilage (anterior), epiglottis (superior), and cricoid cartilage (inferior)—form the major structure of the larynx. Parts of the lower respiratory tract: 1. Trachea :The trachea bifurcates at the levels of the 4th-6th intercostal space, approximately halfway between the thoracic inlet and the diaphragm 2. Main stem bronchus 3. Lobar bronchus 4. Segmental bronchus 5. Bronchiole 6. Alveolar duct 7. Alveolus Trachea The trachea (windpipe) extends from the larynx toward the lungs. The trachea is formed by 16 to 20 stacked, C-shaped pieces of hyaline cartilage that are connected by dense connective tissue. The trachealis muscle and elastic connective tissue together form the fibroelastic membrane, a flexible membrane that closes the posterior surface of the trachea, connecting the C-shaped cartilages. The trachea is lined with pseudostratified ciliated Page | 6  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 columnar epithelium, which is continuous with the larynx. The esophagus borders the trachea posteriorly. The lower respiratory tract or lower airway is derived from the developing foregut and consists of the trachea, bronchi (primary, secondary and tertiary), bronchioles (including terminal and respiratory), and lungs (including alveoli). It also sometimes includes the larynx. The lower respiratory tract is also called the respiratory tree or tracheobronchial tree From the trachea to the alveoli, there are 23 branching generations of airways: The first 16 : constitute the conducting zone, which is an anatomic dead space, because no gas exchange takes place. The 17-23 : generations form the respiratory zone. Advantages: These multiple divisions increase the total cross-sectional area of airways (from 2.5 to 11800 cm2) in the alveoli. As a result the velocity of airflow in small airways is greatly reduced allowing better gaseous exchange. The alveoli are surrounded by pulmonary capillaries.The major respiratory structures span the nasal cavity to the diaphragm. Functionally, the respiratory system can be divided into a conducting zone and a respiratory zone. Page | 7 Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 A. The conducting zone branches are made up of : 1. Bronchi 2. bronchioles 3. terminal bronchioles.  The conducting airway is made up of a variety of specialized cells that provide more than simply a conduit for air to reach the lungs.The mucosal epithelium is attached to a thin basement membrane, and beneath this, the lamina propria. Collectively these are referred to as the “airway mucosa’.  Smooth muscle cells are found beneath the epithelium and an enveloping connective tissue is likewise interspersed with cartilage that is more predominant in the portions of the conducting airway of greater caliber.  The epithelium is organized as a pseudostratified epithelium and contains several cell types, including: 1. ciliated 2. secretory cells (eg, goblet cells and glandular acini) that provide key components for airway innate immunity 3. basal cells that can serve as progenitor cells  Secretory glands are absent from the epithelium of the bronchioles and terminal bronchioles, smooth muscle plays a more prominent role and cartilage is largely absent from the underlying tissue.  Club cells (formerly termed “Clara cells”), non ciliated cuboidal epithelial cells that secrete important defense markers and serve as progenitor cells after injury, make up a large portion of the epithelial lining in the latter portions of the conducting airway.  The walls of the bronchi and bronchioles are innervated by the autonomic nervous system.  These neurons can signal the respiratory centers to contract the respiratory muscles and initiate sneeze or cough reflexes.  The receptors show rapid adaptation when they are continuously stimulated to limit sneeze and cough under normal conditions.  The β2-adrenergic receptors help mediate bronchodilation. They also increase bronchial secretions (eg,mucus), while α1-adrenergic receptors inhibit secretions. Terminal bronchioles The terminal bronchiole is the most distal segment of the conducting zone. Page | 8 Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 Terminal bronchioles are lined with simple cuboidal epithelium containing club cells. Each of the terminal bronchioles divides to form respiratory bronchioles which contain a small number of alveoli. Terminal bronchioles contain a limited number of ciliated cells and no goblet cells. Club cells are non-ciliated, rounded protein-secreting cells. Their secretions are a non-sticky, proteinaceous compound to maintain the airway in the smallest bronchioles. secretion of anti inflammatory and immunomodulatory proteins, and Participitate in airway repair after injury. Club cells, a stem cell of the respiratory system, produce enzymes that detoxify substances dissolved in the respiratory fluid. Page | 9  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 B. The exchange effective respiratory zone comprises of the: 1. respiratory bronchioles 2. alveolar ducts 3. alveolar sacs The total area between pulmonary capillary blood and alveolar air ranges from 70- 140 m2 in adult humans (increased during exercise) through recruitment of new capillaries in particular in the apical parts of the lungs. Vagal stimulation (by smoke, dust, cold air, and irritants) leads to airway constriction, whereas sympathetic stimulation dilatates the airways. These multiple divisions greatly increase the total cross-sectional area of the airways, from 2.5 cm2 in the trachea to 11,800 cm2 in the alveoli. Consequently, the velocity of airflow in the small airways declines to very low values. The alveoli are lined by 3 types of epithelial cells: I. Type I cells : are flat cells with large cytoplasmic extensions and are the primary lining cells of the alveoli,they involved in the process of gas exchange between the alveoli and blood , covering approximately 95% of the alveolar epithelial surface area. Page | 10  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 II. Type II cells (granular pneumocytes) : are thicker and contain numerous lamellar inclusion bodies. they represent approximately 60% of the epithelial cells in the alveoli. Type II cells are important in alveolar repair as well as other lung cellular functions. One prime function of the type II cell is the production of surfactant This surfactant layer plays an important role in maintaining alveolar structure by reducing surface tension. I. during inspiration the surfactant molecules move further apart as the alveoli enlarge, and surface tension increases. II. during expiration the surfactant molecules decreases when they move closer together. Typical lamellar bodies, membrane-bound organelles containing whorls of phospholipid, are formed in type II cells and secreted into the alveolar lumen by exocytosis. Tubes of lipid called tubular myelin form from the extruded bodies, and the tubular myelin in turn forms a phospholipid film Following secretion, the phospholipids of surfactant line up in the alveoli with their hydrophobic fatty acid tails facing the alveolar lumen. This surfactant layer plays an important role in maintaining alveolar structure by reducing surface tension. Surface tension is inversely proportional to the surfactant concentration per unit area. The alveoli are surrounded by pulmonary capillaries. In most areas, air and blood are separated only by the alveolar epithelium and the capillary endothelium, so they are about 0.5 μm apart. The alveoli also contain other specialized cells, including pulmonary alveolar macrophages(PAMs, or AMs), lymphocytes, plasma cells, neuroendocrine cells, and mast cells. III. Type III cells : PAMs are an important component of the pulmonary defense system. Like other macrophages, these cells come originally from the bone marrow. PAMs are actively phagocytic and ingest small particles that evade the mucociliary escalator and reach the alveoli. process inhaled antigens for immunologic attack, they secrete substances that attract granulocytes to the lungs as well as substances that stimulate granulocyte and monocyte formation in the bone marrow. PAM function can also be detrimental—when they ingest large amounts of the substances in cigarette smoke or other irritants, they may release lysosomal products into the extracellular space to cause inflammation. Page | 11  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 Blood and Lymph in the Lung Both the pulmonary circulation and the bronchial circulation contribute to blood flow in the lung.  pulmonary circulation almost all the blood in the body passes via the pulmonary artery to the pulmonary capillary bed, where it is oxygenated and returned to the left atrium via the pulmonary veins. I. The pulmonary arteries strictly follow the branching of the bronchi down to the respiratory bronchioles. II. The pulmonary veins, however, are spaced between the bronchi on their return to the heart.  bronchial circulation This is much smaller,it includes the : I. bronchial arteries that come from systemic arteries. They form capillaries, which drain into bronchial veins or anastomose with pulmonary capillaries or veins. II. The bronchial veins drain into the azygos vein. The bronchial circulation nourishes the trachea down to the terminal bronchioles and also supplies the pleura and hilar lymph nodes. It should be noted that lymphatic channels are more abundant in the lungs than in any other organ. The lymph nodes are connected by lymph vessels and allow for unidirectional flow of lymph to the subclavian veins. Page | 12  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 A:Respiration To provide oxygen to the tissues and remove carbon dioxide.  The four major components of respiration are : (1) pulmonary ventilation,which means the inflow and outflow of air between the atmosphere and the lung alveoli. (2) diffusion of oxygen (O2) and carbon dioxide (CO2) between the alveoli and the Blood. (3) transport of oxygen and carbon dioxide in the blood and body fluids to and from the body’s tissue cells (4) regulation of ventilation and other facets of respiration. B: Non-respiratory Functions of Lungs 1. Lung defense mechanisms  The respiratory passages not only serve as gas conduits, but also humidify and cool or warm the inspired air to make it attain the body temperature by the time it reaches the alveoli.  Bronchial secretions contain secretory immunoglobulin (IgA) and other substances that help to resist infections and maintain mucosal integrity.  Pulmonary epithelium contains an interesting group of protease activated receptors (PARs), which when activated triggers the release of PGE2, which in turn protect the epithelial cells.  Pulmonary alveolar macrophages (PAMs): These cells come originally from bone marrow. They are important component of pulmonary defense mechanisms. They are actively phagocytic and ingest inhaled bacteria and small particles. They help in processing the inhaled antigens for immunological attack. They secrete substances that attract granulocyte to the lung as well as substances that stimulate granulocyte and monocyte formation in bone marrow.  Prevents foreign body from reaching alveoli. i. Particles ≥ 10 µm in diameter: The hair in the nostrils strain out many particles and they settle down on mucous membrane in the nose and pharynx. ii. Particles 2–10 µm diameter: These particles when fall on the walls of the bronchi as airflow slows in smaller passages, initiates reflex bronchoconstriction and coughing, thereby the particles are moved away from lungs by ciliary escalator action. iii. Particles ≤ 2 µm in diameter: Generally reach the alveoli, where they are ingested by the macrophages. Epithelial cells in the conducting airway can secrete a variety of molecules that aid in lung defense: I. Secretory immunoglobulins (IgA) Page | 13  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 II. collectins (including surfactant protein (SP) A and SP-D) III. defensins and other peptides and proteases IV. reactive oxygen species (ROS), and reactive nitrogen species are all generated by airway epithelial cells. V. These secretions can act directly as antimicrobials to help keep the airway free of infection. VI. Airway epithelial cells also secrete a variety of chemokines and cytokines that recruit traditional immune cells and other immune effector cells to site of infections. C:Functions of pulmonary circulation Reservoir for left ventricle: If the LV output becomes transiently greater than systemic venous return, LV output.can be maintained by drawing out blood stored in the pulmonary circulation. Role as a filter: Pulmonary circulation act as a filter for many substances like fibrin clots, fat cells, detached cancer cells, agglutinated RBCs. Fluid exchange and drug absorption: Low pulmonary hydrostatic pressure tends to pull fluid from alveoli into pulmonary capillaries, thereby keeping the alveolar surface free from liquids. This facilitates the rapid entry of drugs into systemic circulation which rapidly pass through the alveolar-capillary barrier by diffusion. For example: (i) anesthetic gases (ii) aerosols D:Metabolic and endocrine functions of the lungs 1. Manufactures surfactant for local use, which prevents the development of surface tension between the fluid lining the alveoli and the alveolar air. 2. Substances synthesized or stored and released into the blood. For example, prostaglandins (esp. PGE2 and PGF2a) 3. Contain a fibrinolytic system that lyses clots in the pulmonary vessels. 4. The lungs also activate one hormone in the pulmonary circulation. (This reaction occurs in other tissues also, but prominent in lungs Angiotensin I—Inactive decapeptide Angiotensin II—A pressor, aldosterone stimulating octapeptide. Large amounts of ACE is located on endothelial cells of the pulmonary capillaries. Angiotensin converting enzyme (ACE) inactivates bradykinin. 5. It helps in the removal of serotonin and norepinephrine, thereby reducing the amounts of vasoactive substances reaching the systemic circulation. However, many other Page | 14  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 vasoactive substances like epinephrine, dopamine, oxytocin pass through the lungs without being metabolized Respiratory functions of the lung: The main function of respiratory system is to extract O2 from the atmosphere, to deliver it to the tissues and to take CO2 from the tissues and discharge it to the atmosphere. The entire respiration can be divided into two main divisions: External respiration: It deals with the absorption of O2 and removal of CO 2 from the body as a whole. Internal respiration: It is the utilization of the O2 and production of CO2 by cells and the gaseous exchange between the cells in their fluid medium. External Respiration: The pulmonary artery carries deoxygenated blood into the lungs from the heart, where it branches and eventually becomes the capillary network composed of pulmonary capillaries. These pulmonary capillaries create the respiratory membrane with the alveoli. As the blood is pumped through this capillary network, gas exchange occurs. Although a small amount of the oxygen is able to dissolve directly into plasma from the alveoli, most of the oxygen is picked up by erythrocytes (red blood cells) and binds to a protein called hemoglobin. Oxygenated hemoglobin is red, causing the overall appearance of bright red oxygenated blood, which returns to the heart through the pulmonary veins. Carbon dioxide is released in the opposite direction of oxygen, from the blood to the alveoli. External respiration occurs as a function of partial pressure differences in oxygen and carbon dioxide between the alveoli and the blood in the pulmonary capillaries. The partial pressure of carbon dioxide is also different between the alveolar air and the blood of the capillary. However, the partial pressure difference is less than that of oxygen, about 5 mm Hg. The partial pressure of carbon dioxide in the blood of the capillary is about 45 mm Hg, whereas its partial pressure in the alveoli is about 40 mm Hg. However, the solubility of carbon dioxide is much greater than that of oxygen—by a factor of about 20—in both blood and alveolar fluids. As a result, the relative concentrations of oxygen and carbon dioxide that diffuse across the respiratory membrane are similar Internal Respiration: Internal respiration is gas exchange that occurs at the level of body tissues Similar to external respiration, internal respiration also occurs as simple diffusion due to a partial pressure gradient. However, the partial pressure gradients are opposite of those present at the respiratory membrane. The partial pressure of oxygen in tissues is low, about 40 mm Hg, because oxygen is continuously used for cellular respiration. In contrast, the partial pressure of oxygen in the blood is about 100 mm Hg. This creates a pressure gradient that causes oxygen to dissociate from hemoglobin, diffuse out of the blood, cross the interstitial Page | 15  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 space, and enter the tissue. Hemoglobin that has little oxygen bound to it loses much of its brightness, so that blood returning to the heart is more burgundy in color. Considering that cellular respiration continuously produces carbon dioxide, the partial pressure of carbon dioxide is lower in the blood than it is in the tissue, causing carbon dioxide to diffuse out of the tissue, cross the interstitial fluid, and enter the blood. It is then carried back to the lungs either bound to hemoglobin, dissolved in plasma, or in a converted form. By the time blood returns to the heart, the partial pressure of oxygen has returned to about 40 mm Hg, and the partial pressure of carbon dioxide has returned to about 45 mm Hg. The blood is then pumped back to the lungs to be oxygenated once again during external respiration.  External respiration  Internal respitation Page | 16  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 The air in the alveoli is separated from the blood in the pulmonary capillaries by a wall called “respiratory membrane”. Basically, it consists of alveolar wall and the capillary wall. The following are the six layers of the respiratory membrane: 1. A layer of surfactant lining the alveolus. 2. The alveolar epithelium. 3. Basal lamina of alveolar epithelial cells. 4. A thin interstitial space between the alveolar epithelium and the capillary membrane 5.Basal membrane of endothelial cells. 6. A capillary endothelial membrane. Despite these large number of layers, the overall thickness of the membrane in some areas is as little as 0.2 micrometer (average about 0.6 micrometer), except where there are cell nuclei. The total surface area of the respiratory membrane is about 70 m2 in the normal adult human male, which facilitates the rapidity of respiratory exchange of O2 and CO2. There are about 300 million alveoli in the two lungs. Page | 17  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 The factors that determine how rapidly a gas will pass through the membrane are:  the thickness of the membrane  the surface area of the membrane  the diffusion coefficient of the gas in the substance of the membrane  the partial pressure difference of the gas between the two sides of the membrane DIFFUSING CAPACITY OF THE RESPIRATORY MEMBRANE The ability of the respiratory membrane to exchange a gas between the alveoli and the pulmonary blood is expressed in quantitative terms by the respiratory membrane’s diffusing capacity, which is defined as : the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mm Hg. All the factors discussed earlier that affect diffusion through the respiratory membrane can affect this diffusing capacity. Increased Oxygen Diffusing Capacity during Exercise During strenuous exercise or other conditions that greatly increase pulmonary blood flow and alveolar ventilation, the diffusing capacity for O2 increases in young men to a maximum of about 65 ml/min/mm Hg, which is three times the diffusing capacity under resting conditions. This increase is caused by several factors, among which are I. Distention and recruitment II. a better match between the ventilation of the alveoli and the perfusion of the alveolar capillaries with blood, called the ventilation perfusion ratio III. Therefore, during exercise, oxygenation of the blood is increased not only by increased alveolar ventilation but also by greater diffusing capacity of the respiratory membrane for transporting O2 into the blood It is now known that some of the cells lining the alveoli secrete a material that profoundly lowers the surface tension of the alveolar lining fluid. Page | 18  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 Surfactant is a phospholipid, and dipalmitoyl phosphatidylcholine (DPPC) is an important constituent How does surfactant reduce the surface tension so much? Apparently the molecules of DPPC are hydrophobic at one end and hydrophilic at the other, and they align themselves in the surface. When this occurs, their intermolecular repulsive forces oppose the normal attracting forces between the liquid surface molecules that are responsible for surface tension. The reduction in surface tension is greater when the film is compressed because the molecules of DPPC are then crowded closer together and repel each other more. What are the physiological advantages of surfactant? I. First, a low surface tension in the alveoli increases the compliance of the lung and reduces the work of expanding it with each breath. II. Next, stability of the alveoli is promoted. III. A third role of surfactant is to help to keep the alveoli dry. Just as the surface tension forces tend to collapse alveoli, they also tend to suck fluid out of the capillaries. the surface tension of the curved alveolar surface reduces the hydrostatic pressure in the tissue outside the capillaries. By reducing these surface forces, surfactant prevents the transudation of fluid. stability of the alveoli is promoted,how? 1.The 500 million alveoli appear to be inherently unstable because areas of atelectasis (collapse) often form in the presence of disease.the pressure generated by a given surface force in a bubble is inversely proportional to its radius, with the result that if the surface tensions are the same, the pressure inside a small bubble exceeds that in a large bubble ,a small surface area is associated with a small surface tension. Thus, the tendency for small alveoli to empty into large alveoli is apparently reduced. 2. There is another mechanism that apparently contributes to the stability of the alveoli in the lung. that all the alveoli (except those immediately adjacent to the pleural surface) are surrounded by other alveoli and are therefore supported by one another. In a structure such as this with many connecting links, any tendency for one group of units to reduce or increase its volume relative to the rest of the structure is opposed. For example, if a group of alveoli has a tendency to collapse, large expanding forces will be developed on them because the surrounding parenchyma is expanded What are the consequences of loss of surfactant? 1. This will stiff lungs (low compliance). 2. Areas of atelectasis 3. Alveoli tend to be filled with transudate Page | 19  Respiratory Physiology.LEC.1&2 Dr.Zainab Ali Altufailie \2024 surfactant does not normally begin to be secreted into the alveoli until between the sixth and seventh months of gestation, and in some cases, even later. Therefore, many premature babies have little or no surfactant in the alveoli when they are born, and their lungs have an extreme tendency to collapse, sometimes as great as six to eight times that in a normal adult person. This situation causes the condition called respiratory distress syndrome of the newborn Its also called hyaline membrane disease many of these infants die of suffocation when large portions of the lungs become atelectatic. **Home work: Q:Define atelectasis? Q:Define Respiratory distress syndrome(RDS)? Q:Deine pulmonary edema? References: Good luck Page | 20