Anatomy and Physiology of Respiratory System PDF

Anatomy & Physiology of Respiratory System Ms. Sammi Chau Senior lecturer  Intended Learning Outcomes At the end of the class, students should be able to: Revisit the basic anatomy of respiratory system Identify the differences between upper and lower respiratory t...

Anatomy & Physiology of Respiratory System Ms. Sammi Chau Senior lecturer  Intended Learning Outcomes At the end of the class, students should be able to: Revisit the basic anatomy of respiratory system Identify the differences between upper and lower respiratory tracts, as well as their primary functions Revisit the physiology of respiratory system Recognize the pressure and mechanism of breathing of respiratory system Comprehend the concept of the transport and exchange of gases Understand the embryology and development of respiratory system  Function of Respiratory System Primary function of respiratory system is responsible for gas exchange through: Supplying the body with oxygen taken from atmospheric air Removing carbon dioxide from the body Respiration is comprised of 4 stages: Ventilation Responsible by respiratory system External respiratory Gas transport Internal respiration  Terminologies Ventilation Movement of air in and out of the lungs is a mechanical process involving inhalation and exhalation Occurs in continuous rhythmic pattern, without any conscious effort depends on: Respiratory muscles Compliance of the lungs and chest wall Gas flow in the airway Controlled by both neural and chemical inputs Homeostasis of oxygen (O2) and carbon dioxide (CO2) [acid–base balance]  Terminologies Ventilation Inspiration is an active process The diaphragm is the main muscle of inspiration External intercostal muscles, scalene muscles and sternocleidomastoids may also be involved Expiration is normally a passive process Relies on elastic recoil of the lungs leading to decrease in lung volume to expel air In certain disease processes, forced expiration may be seen and require energy expenditure (abdominal muscles, internal intercostal muscles, internal and external oblique muscles)  Terminologies Gas Transport Occurs at the respiratory zone of the lung (bronchioles, alveolar ducts, alveolar sacs and alveoli) Exchange of O2 and CO2 takes place at the capillaries at the alveolar level through diffusion Gases pass through alveolar surfactant, alveolar epithelium, basement membrane and capillary endothelium Diffusion barriers: Fluid: tissue fluid, plasma Concentration and solubility of gases (e.g. CO poisoning) Membrane thickness Surface area Surface tension: alveolar epithelium, capillary endothelium  Terminologies External respiration Also called pulmonary gas exchange Transport of O2 and CO2 to and from the alveoli and the blood Diffusion of oxygen (O2) from the lungs to the blood Diffusion of carbon dioxide (CO2) from blood to the lungs  Terminologies Internal respiration Exchange of O2 and CO2 between the blood and the tissue cell Diffusion of O2 from the blood to the tissue Diffusion of CO2 from the tissue to the blood  Transport of Oxygen from Lungs to Tissues O2 is bound with hemoglobin (Hb) and carried along the bloodstream Hb is fully saturated when all 4 binding sites are attached to O2 Small amount of O2 is dissolved in the plasma The term partial pressure of oxygen in arterial blood (PaO2) refers to the amount of O2 dissolved in the blood. It tells how well the O2 gets from your lungs to the bloodstream Transport of Carbon Dioxide from Tissue to Lungs CO2 in the tissue diffuses into the blood Majority of CO2 travels in the form of bicarbonate ion (HCO3-) in the blood Smaller amounts of CO2: Dissolved in plasma and form carbonic acid while reacting with water React with amine radicals and binds to Hb to form carbaminohemoglobin Partial pressure of carbon dioxide (PaCO2) means the amount of CO2 is dissolved in the blood It tells how well the CO2 moves out from the body  Inspiration 1 Expiration 4 External respiration 2 Internal respiration 3 Anatomy of Respiratory System  Anatomy of Respiratory System Anatomically Functionally Upper respiratory tract Conducting zone (nose to bronchioles) Lower respiratory tract Path for conduction of the inhaled gases Respiratory zone (alveolar ducts to alveoli) Exchange of gas takes place Anatomy of Respiratory System Upper respiratory tract Consists of the nose, the pharynx, the larynx and the trachea Functions as warming, humidifying and filtering the air Mucosa lining of the nasal cavity is made of ciliated epithelium Secrete mucus and trap inhaled particles Mobilize the mucus and trapped particles towards the pharynx Nasal conchae increase the mucosal surface area and increase turbulence thus slow airflow Trachea contains cartilaginous C-shaped rings to prevent tracheal collapse  Anatomy of Respiratory System Lower respiratory tract Composed of bronchial tree (trachea, bronchi, bronchioles) and the lungs (alveolar ducts and alveoli; and parenchyma) Bronchial tree is made of the trachea and 23 generations branching bronchioles Gas exchange takes place at the terminal bronchioles and alveoli (respiratory zone) 300 million alveoli present in a healthy adult  Airway Resistance Airway resistance is the pressure difference between the alveoli and the mouth divided by flow rate Using Poiseuille’s law to explain the relationship between airway resistance and the diameter of the airway, given when there is laminar flow: 8𝑛𝑙 𝑅= 4 𝜋𝑟 𝑅 = 𝑎𝑖𝑟𝑤𝑎𝑦 𝑟𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒 ; 𝑛 = 𝑣𝑖𝑠𝑐𝑜𝑠𝑖𝑡𝑦 ; 𝑙 = 𝑙𝑒𝑛𝑔𝑡ℎ ; 𝑟 = radius Airway resistance is inversely proportional to the radius of airway to the power of 4 Smaller airways have higher resistance than larger airways However, the significant downstream branching of the airways would work in parallel, that reduces the total resistance to airflow Therefore, the major resistance to airflow occurs in the upper airway  Respiratory Zone Conducting Zone Respiratory Zone  Anatomy of Respiratory System The lungs Cone-shaped, positioned vertically around the heart in the thoracic cage They are held in contact with the rib cage by negative pressure between the pleural surfaces Each lung is divided into lobes and segments The anatomical position of each bronchopulmonary segment is required for gravity-assisted positioning and for drainage of secretion  Anatomy of Respiratory System The pleurae Serous membrane that folds back on itself to form a two- layered membranous pleural sac Parietal pleura (outer layer) Lines the inner surface of the thoracic wall and the superior surface of the diaphragm Innervated by the phrenic and intercostal nerves (e.g. pleuritis (inflammation of pleura) causes severe sharp and stabbing pain) Visceral pleura (inner layer) Covers the outer surface of the lungs and lines the fissures Innervated by autonomic nervous system, insensitive to pain Pleural cavity Space between the parietal and visceral pleurae Contains a small amount of serous/pleural fluid secreted by the pleurae Acts as lubricant, allowing the pleural layers to glide over each other during breathing Acts to increase surface tension, thus both pleural layers “stick” together and allow the thoracic cavity to expand during inspiration In pneumothorax (air enters the pleural space), the loss in the surface tension cause the lung inability to expand  Muscles of Respiration Inspiration Main: Diaphragm (phrenic nerve, C3-5) External intercostals (T1-11) Accessory: scalenes, SCMs, pectoralis Expiration Passive process by relaxation of diaphragm and external intercostals (elastic recoil) Forced expiration, coughing, sneezing Internal intercostals and abdominals  Movement of the Ribs The dimensions of thorax must change during respiration; vertically, transversely and anterioposteriorly increase in inspiration and decrease in expiration 1st rib Has minimal movement due to anatomically short and rigid attachment to sternum by costal cartilage 2nd – 5th ribs Are raised during inspiration and increase the AP diameter of thorax, known as “pump handle movement” 6th – 7th ribs Have combined features of ribs above and below 8th – 10th ribs Move upwards and outwards during inspiration and increase the transverse diameter of thorax, called “bucket handle movement” 11th – 12th ribs Have no anterior attachment, not contributing to thorax movement  Mechanics of Breathing Boyle’s Law describes the relationship between the pressure (P) and the volume (V) of a gas: 𝑃𝑉 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 The product of the pressure and volume remains constant, given in a fixed temperature 𝑃1𝑉1 = 𝑃2𝑉2 If the volume increases, then the pressure must decreases When the volume of the lungs changes, the pressure of the air in the lungs changes in accordance with Boyle’s law  Mechanics of Breathing During inspiration, the contraction of inspiratory muscles causes increase in volume in thoracic cavity, the pressure inside the lungs decreases below the atmospheric pressure; therefore, the air goes into the lungs During expiration, with the relaxation of respiratory muscles and elastic recoil of the lungs, the volume of lungs decreases, thus the pressure inside the lungs increases above the atmospheric pressure; therefore the air moves out from the lungs  Respiratory Pressures Based on mechanics of breathing, the pulmonary ventilation is dependent on pressure: Atmospheric pressure (Patm) Intrapulmonary or Intra-alveolar pressure (Ppul or Palv) Intrapleural pressure (Pip or Ppl)  Respiratory Pressures Atmospheric pressure (Patm) Amount of force that is exerted by the gases in the air surrounding any given surface (e.g. body) One atm is equal to 760mmHg at sea level; which is the sum of all gases in the air Nitrogen: 78% of 760mmHg O2: 21% of 760mmHg For respiration, the pressure values are discussed in relation to Patm A pressure that is equal to Patm is expressed as zero +ve pressure means the pressure is greater than Patm -ve pressure means the pressure is lesser than Patm  Respiratory Pressures Intrapulmonary pressure (Ppul or Palv) Also referred as intra-alveolar pressure Pressure of the air within the lungs or alveoli Ppul or Palv changes during different phases of breathing Ppul or Palv decreases with inspiration and increases with expiration When Ppul or Palv returns to 0mmHg (equalizes with Patm), it represents the breathing is at the end of inspiration and at the end of expiration  Respiratory Pressures Intrapleural pressure (Pip or Ppl) Refers to the pressure within the pleural cavity Pip or Ppl also changes throughout the breathing cycle Pip increases on inspiration and decreases on expiration Pip is always lower than or negative to Palv Negative Pip is created by two opposing forces: the tendency of the chest wall to expand outward versus the tendency of the lungs to recoil inward The outward pull is slightly greater than the inward pull, creating the –4 mm Hg Pip relative to Palv  Respiratory Pressures Transpulmonary pressure Pressure difference between the pleural space and the alveolar space, representing the distending pressure applied to the lung by contraction of inspiratory muscles or by mechanical (positive pressure) ventilation The greater the transpulmonary pressure, the greater the size of the lungs Provides a guidance to pressure setting (PEEP) and tidal volume in patients with ARDS or obese patients  Gas Exchange Exchange of O2 and CO2 takes place at the capillaries at the alveolar level through diffusion Ventilation (V) refers to the amount of air that enters and leaves the alveoli Perfusion (Q) refers to the amount of blood that flows to the alveolar capillaries  Ventilation Ventilation (V) is commonly referred to as breathing A process in which air is moved in and out of the lungs; and comprises of 2 phases: Inspiration Active process at rest and during exercise requiring muscle contraction During forced or labored breathing, additional accessory muscles are recruited to increase the inspiratory maneuver Expiration Passive process that is achieved through the elastic recoil of the lungs and relaxation of respiratory muscles During forceful expiration, additional muscle work would be required to help expel air out of the lungs  Perfusion Perfusion (Q) refers to the flow of blood to alveolar capillaries that is available for gas exchange The driving pressure in the pulmonary circulation is about much less than the systemic circulation Pulmonary perfusion is influenced by vascular resistance Distribution of perfusion Affected by gravitational forces  Ventilation & Perfusion Ventilation/Perfusion (V/Q) Ratio 𝑉 𝑡ℎ𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑎𝑖𝑟 𝑡ℎ𝑎𝑡 𝑟𝑒𝑎𝑐ℎ𝑒𝑠 𝑡ℎ𝑒 𝑎𝑙𝑣𝑒𝑜𝑙𝑖 𝑟𝑎𝑡𝑖𝑜 = 𝑄 𝑡ℎ𝑒 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑏𝑙𝑜𝑜𝑑 𝑡ℎ𝑎𝑡 𝑟𝑒𝑎𝑐ℎ𝑒𝑠 𝑡ℎ𝑒 𝑎𝑙𝑣𝑒𝑜𝑙𝑖 In optimal respiration or gas exchange, ventilation and perfusion must be matched, with the V/Q ratio = 1.0  Ventilation & Perfusion Bodily position and gravity play a vital role in the distribution of ventilation and perfusion to different aspect of the lung Ventilation Perfusion In upright position, alveoli in the apices of the lung In upright position, gravity allows for a greater have greater residual volume of gas and are amount of blood flow (or perfusion) to the base of subsequently larger. The larger alveoli have greater the lung relative to the apices surface tension and have relatively more difficulty inflating because of less compliance than the smaller alveoli toward the base of the lung  V/Q Imbalance V/Q imbalance High V/Q ratio If there is more ventilation or less perfusion Example: a patient with pulmonary embolism, there is a decreased blood flow in the lungs but a normal ventilation An area with ventilation but no perfusion is known as dead space Low V/Q ratio If there is less ventilation or more perfusion Example: a patient with atelectasis, there is a decreased ventilation but a normal perfusion An area with perfusion but no ventilation is referred to as shunt Most common clinical presentation of V/Q imbalance is hypoxemia (low oxygen level in blood) Embryology & Development of Respiratory System By day 22, the development of the lower respiratory tract begins, then forming the trachea, lungs, bronchi, and alveoli. The development process divides into five stages: Embryonic stage (3-6 weeks) Pseudoglandular stage (5-17 weeks) Canalicular stage (16-25 weeks) Saccular stage (24 weeks-birth) Alveolar stage (36 weeks-8 years) The complete maturation of respiratory system does not take place until the child is approximately 8 years of age In premature babies, their survival is linked to which developmental stage their respiratory tract has reached at the time of birth  Development of Respiratory System Embryonic stage (3-6 weeks) Lower respiratory system (the larynx, trachea, lung primordia, lobe of the lungs and the bronchopulmonary segments and the pleurae) begins to develop until separation of the respiratory tract from the foregut is achieved Pseudoglandular stage (5-17 weeks) Lung buds form and begin to differentiate into the bronchi. The respiratory tree has developed as far as the terminal bronchioles, with the formation of an arterial system, cartilage, and smooth muscle. Larynx is also developed. Infants born at this stage will not be able to facilitate gas exchange and hence unable to survive. Canalicular stage (16-25 weeks) Continual growth of terminal bronchioles with some alveolar ducts and formation of the blood-air barrier (bronchi enlarge and lung tissue becomes highly vascular) Little differentiation of type II pneumocytes into squamous type I pneumocytes (preparation for structural epithelium of alveoli) Survival is slim due to lacking of surface area for gas exchange and limited production of pulmonary surfactant despite intensive care is provided  Development of Respiratory System Saccular stage (24 weeks-birth) Maturation and differentiation of type II pneumocytes into type I pneumocytes results in thin-walled terminal sacs Gas-exchange surface area of the lungs expands significantly and capillary network proliferates around the alveoli Approximately 8%-10% of cardiac output flows through the lung; pulmonary vascular resistance is high Infants born after 32 weeks have a much higher chance of survival Alveolar stage (36 weeks-8 years) The process of alveolar division continues until 3 years of age where enlargement of lungs is a consequence of the increasing number of alveoli; after this point, both the number and size of alveoli increases until the mature lungs form at around 8 years of age.  Intended Learning Outcomes – Achieved! At the end of the class, students should be able to: Revisit the basic anatomy of respiratory system Identify the differences between upper and lower respiratory tracts, as well as their primary functions Revisit the physiology of respiratory system Recognize the pressure and mechanism of breathing of respiratory system Comprehend the concept of the transport and exchange of gases Understand the embryology and development of respiratory system  References Hillegass E.A. (2011) Essentials of Cardiopulmonary Physical Therapy (3rd Ed). Saunders. Main E. & Denely L. (2016) Cardiorespiratory Physiotherapy: Adults and Paediatics (5th Ed). Elsevier