Biom2012 Respiratory Physiology L3 Student PDF

BIOM2012 - Systems Physiology: Respiratory System L3 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonar...

BIOM2012 - Systems Physiology: Respiratory System L3 Dr. Jacky Suen [email protected] Credit: A/Prof Stephen Anderson and Dr. Hardy Ernst Learning Objectives Pulmonary Ventilation and Respiratory mechanics Gas exchange Gas transport Blood pH regulation Control of Respiration  External Respiration External respiration refers to the exchange of oxygen and carbon dioxide between blood and alveoli across the respiratory membrane. Factors influencing gas movement across the respiratory membrane Matching of alveolar ventilation and pulmonary blood perfusion Structural characteristics of the respiratory membrane Partial pressure gradients and gas solubility “borrowed” from CVS lectures Pulmonary Circulation The pulmonary circulation system is the only system through which the entire cardiac output passes. The major role of pulmonary circulation is respiratory gas exchange. “borrowed” from Pulmonary CVS lectures Circulation The pulmonary circuit has the following characteristics: Low pressure and low resistance circuit Relatively short circuit Branches immediately, increases exchange area, and lowers resistance Arteries (less muscle) and therefore higher compliance vessels cf systemic Minimal resting smooth muscle tone – normally near fully dilated 'Passive factors' play an important role in determining flow eg. gravity Receives all cardiac output (from right ventricle) An important difference is local response to hypoxia – vasoconstriction Diversion of blood to regions of better ventilation Is related to minimising ventilation – perfusion (VQ) differences It supplies blood to regions of lung that will most efficiently oxygenate it. ventilation perfusion “matching” or “coupling”  Efficient respiration requires matching ventilation (V) with perfusion (Q) PvO2 40 mm Hg PvCO2 45 mm Hg PAO2 100 mm Hg PACO2 40 mm Hg PcO2 100 mm Hg V/Q = 1 PcCO2 40 mm Hg  V/Q < 1 V/Q > 1 Increased Decreased Increased Decreased CO2 O2 O2 CO2 airway dilation + vasoconstriction airway constriction + vasodilation  Structural Characteristics of the respiratory membrane The respiratory membrane refers to the area between the alveolus and pulmonary capillaries lining the terminal portions of the lungs. Features alveolar walls are lined in thin squamous epithelial cells (type I pneumocytes) pulmonary capillaries tightly encase the external surfaces of the alveoli type II pneumocytes secrete pulmonary surfactant a thin (approximately 0.5 µm) layer of interstitial fluid exists Together, the above allow for O2 and CO2 to easily diffuse between the systems  Structural Characteristics of the respiratory membrane Diffusion is ideal for gas transport over short distances. Recall characteristics of diffusion - hint: what sort of process? - hint: movement occurs how? - short distance - hint: many alveoli = a good what? - solubility of gases matter Over larger distances gases require ‘bulk flow’ (atmosphere to alveoli) or to be carried in circulation (Hb)  Partial pressure gradients & gas solubility Dalton’s law in respiration: In a given system of mixed gases, the partial pressure exerted by a single gas present will be proportional to the amount or percentage of that gas. Atmospheric air is a mixture of gases: nitrogen (N2) at about 78.6% oxygen (O2) at 20.9% most of the remaining 0.5% consists of water vapour (H2O) carbon dioxide (CO2) a mere 0.04% Partial pressure gradients & gas solubility Henry’s law in respiration: The amount of a gas that dissolves in a liquid is directly proportional to the partial pressure of that gas. Also, gases with a higher solubility will have more dissolved molecules than gases with a lower solubility, if they have the same partial pressure. So whilst N2 in the atmosphere is found in high concentrations, only very minute concentrations are dissolved into blood, due to low solubility in plasma. In contrast, CO2 is the most soluble (23x more soluble than O2). Dalton + Henry’s law At a given temperature, the amount of a particular gas in solution is directly proportional to the partial pressure of that gas. Gas exchange occurs down the partial pressure gradient! Quick Quiz Patients with respiratory failure often received oxygen therapy. What’s the PO2 with 50% O2?  Respiration includes 4 events To ensure delivery of O2 to tissues, and the disposal of CO2.  O2 Gas exchange: happens at both the pulmonary capillary and the tissue-capiliary levels Down the partial pressure gradient Gas transport: O2 needs to be moved from lungs to tissues, and vice versa for CO2 CO2  External Respiration Inhaled air is humidified prior to reaching the alveoli. Additionally, the air in the alveoli is comprised of more CO2 and less O2 than atm air. Uptake of O2 into the CVS (from Resp system) Mismatch between the partial pressure of O2 in the air around us (in particular, our alveoli) pO2 = 104 mmHg versus partial pressure of O2 in venous blood pO2 = 40 mmHg drives diffusion of gas down a pressure gradient. Removal of CO2 from the CVS (to Resp system) pCO2 = 46 mmHg in blood to pulmonary capillaries versus pCO2 = 40 mmHg in avleoli But CO2 more soluble in plasma so the the amount of CO2 diffuses between the blood and the alveoli is similar to the amount of O2. The steep O2 gradient allows O2 partial pressures to rapidly reach equilibrium (in 0.25 seconds). Blood can move 3 times as quickly through the pulmonary Adapted from RG Carroll (2007) capillary and still be adequately oxygenated. Elsevier’s Integrated Physiology Factors other than the partial pressure that influence the rate of gas transfer: Surface area Thickness of the membrane diffusion coefficient Factors other than the partial pressure that influence the rate of gas transfer: Changes due to Surface area physiological demands, Thickness of the membrane and diseases Diffusion coefficient - depends on gas and concentration Decreased surface area Emphysema Increased thickness of membrane due to Pulmonary oedema (HAPE) Pneumonia (COVID-19, Influenza A) Pulmonary Fibrosis (post COVID-19) Why does it matter? Acute Respiratory Distress Syndrome (ARDS) 1 in 10 mechanically ventilated patients Up to 45% mortality 3 out of 4 acute COVID-19 death NO TREATMENT! Diffusion Coefficient Diffusion coefficient (D) determines the rate of gas transfer Related to solubility and molecular weight. D for CO2 is ~20 times that of O2 But O2 has a bigger difference in partial pressure Normally equal amounts of O2 and CO2 are exchanged. Quick Quiz What are the 5 factors that affect rate/efficiency of gas transfer? Function Location Influencing factors Ventilation Airways Airway resistance Lung compliance Alveolar surface tension Gas exchange Respiratory membrane Gas characteristics Pressure gradient Diffusion Coefficient Membrane characteristics V/Q matching Gas Transport Respiratory rate V/Q < 1 V/Q > 1