Respiratory Lecture 2.pdf

THE RESPIRATORY SYSTEM Lecture II I. GAS EXCHANGE by DIFFUSION II. PERFUSION of LUNGS III. VENTILATION VOLUMES IV. CONTROL of RESPIRATION Bernard Leung [email protected] HSC1007, AY2024 Meeting ID: 956 7013 2553 Passcode: 375...

THE RESPIRATORY SYSTEM Lecture II I. GAS EXCHANGE by DIFFUSION II. PERFUSION of LUNGS III. VENTILATION VOLUMES IV. CONTROL of RESPIRATION Bernard Leung [email protected] HSC1007, AY2024 Meeting ID: 956 7013 2553 Passcode: 375090  Learning Objectives The concept of ventilation and gas exchange by diffusion. How partial pressure drives the diffusion of O2 and CO2? Factors that influences the rates of gas transfer across the alveolar membrane. Basic concept of ventilation and perfusion. Ventilation terminology and different lung volume measurements. Control of Respiration.  EFFECTIVE OXYGENATION & CO2 REMOVAL IN LUNGS depend on… I. Ventilation & Gas exchange by Diffusion § Do lungs get enough ‘fresh air’ at each breath cycle? § Does the ‘fresh air’ reach the alveoli? § At the alveoli, can the ‘fresh air’ R L DIFFUSE effectively across to the capillary blood? § After gas exchange, can the ‘stale air’ leave the lungs effectively? EFFECTIVE OXYGENATION & CO2 REMOVAL IN LUNGS depend on… II. Perfusion of lungs § Does blood coming from the heart reach the alveoli? § Are all alveoli perfused by blood? 0.5µm capillary § At the alveoli, does the rate of blood flow CO2 give sufficient time O2 for gas exchange? alveolus  GAS EXCHANGE @ ALVEOLI: FACTORS AFFECTING Diffusion across alveolar-capillary barrier – Factors influencing efficiency of diffusion: Partial pressures of gases Thickness of barrier Surface area Blood Flow – Rate of blood flow through alveoli – Perfusion of alveoli  GAS EXCHANGE @ ALVEOLI: DIFFUSION ~ a little maths! Diffusion of gas is from areas of high partial pressure ® low Partial pressure of a gas (Dalton’s Law) : Pressure that gas exerts in a mixture of gases - proportional to % of gas in mixture If gas = O2, and the mixture = dry air @ sea level Then pO2 (partial press. of O2) in air = 0.21 (21% of air is O2) X 760mmHg (total air pressure @ sea level) https://www.youtube.com/watch?v=6qnSsV2syUE Gas Exchange and Partial Pressures, Animation  Partial Pressure and % of Respiratory Gases at Sea Level (Barometric Pressure PB=760 mmHg) ie, 160/760 = 21% ie, 150/760 = 20% Respiratory Zone 600/760 = 79% 47/760 = 6% Gas Exchange 563/760 = 74% (Alveolar Ventilation) (Anatomic Dead Space) Alveolar Ventilation = (Tidal volume – Anatomic dead space) x Breaths per minute Tidal Vol (500ml) – Anatomic Dead Space (150ml) x 12 breaths/min = 4,200ml (4.2L)  GAS EXCHANGE @ ALVEOLI: DIFFUSION Diffusion of gas is from areas of high partial pressure ® low Gas potentially diffuses until partial pressures are equalised in areas High partial pressure low partial pressure Equal partial pressure Diffusion stops Diffusion of oxygen if partial pressure @ alveoli equal capillary blood?  GAS EXCHANGE @ ALVEOLI: DIFFUSION Alveolar Basement Capillary epithelium membranes endothelium capillary CO2 O2 alveolus Diffusion: Gases diffuse down partial pressure (concentration) gradients across epithelial barriers Overview of Respiratory Processes and Partial Pressures in Respiration (For your info!) Pulmonary Venous Admixture - before reaching the left atria, mixing of non- reoxygenated blood with reoxygenated blood distal to the alveoli in the pulmonary veins, reaching the atria with an arterial PO2 of 95 mmHg.  O2 DISSOCIATION CURVE O2@dissociation pH 7.4, 37 curve oC, PCO 40mmHg 2 (pH7.4, 37 degrees Celsius, pCO2 40mmHg) 20 100% O2 concentration for Hb 98% O2 saturation of Hb 16 of 15g/dL (ml/dL) O2-Hb End of pulmonary 12 dissociation capillaries: curve 50% PO2 ~ 8 100mmHg, 4 Dissolved O2 Hb 98*% 0 0% saturated with O2 0 50 100 Rest as dissolved PO2 (mmHg) oxygen! *Most textbook refer to as 97.5% Saturation of Hb  GAS EXCHANGE @ ALVEOLI: DIFFUSION Surface area for diffusion capillary CO2 O2 alveolus Diffusion across many perfused alveoli in both lungs Reduced area for diffusion in eg.? Consequences for gas exchange?  GAS EXCHANGE @ ALVEOLI: RATE OF BLOOD FLOW capillary CO2 O2 alveolus Alveolar gases take time to diffuse & equilibrate with blood Different gases diffuse / equilibrate @ different rates Rate of blood flow across alveolus can change eg. ↑↑↑ in severe exercise: enough time for gas exchange (diffusion)?  GAS EXCHANGE @ ALVEOLI: BLOOD FLOW through ALVEOLI (PERFUSION) capillary CO2 O2 alveolus Gas exchange occurs when blood perfuses capillary Blood flow ceases ® no gas exchange (eg. pulmonary embolism: blood clot in pulmonary artery) Sherwood Human Physiology, Table 13-5, p470.  PERFUSION OF LUNGS Pulmonary circulation route Right Ventricle (RV) ® Main pulmonary artery (PA) branches successively into several pulmonary arteries (accompany bronchioles) PA Arterioles ® LA Capillary bed around alveoli RV Small pulmonary veins ® Large pulmonary veins ® Left Atrium (LA)  PERFUSION OF LUNGS Low pressure system vs. high pressure systemic circulation Pulmonary artery pressure >25/15 mmHg vs Systemic arterial pressure 120/80 mmHg High volume system: lungs receive whole of cardiac output at all times Distribution of blood flow in lungs is not uniform Pulmonary Circulation has Unique Hemodynamic Features Unlike the systemic circulation, the pulmonary circulation is a low- pressure and low resistance system. Pulmonary circulation is characterized as normally dilated, whereas the systemic circulation is characterized as normally constricted. Pressures are given in mm Hg; a bar over the number indicates mean pressure. LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle. Comparison of Systemic vs Pulmonary BP Just a quick visual guide – FYI only! Guyton & Hall Medical Physiology, Ch14, Elsevier.  PERFUSION OF LUNGS Distribution of blood flow in lungs: Influenced by - gravity (hence posture) (eg. upright: more blood flow at bottom) - muscular tone of arterioles distension: pulmonary arterioles have less muscular walls vs. systemic arterioles ® can distend more easily with increased blood flow vasoconstriction: less blood flows through capillaries Ventilation and Perfusion Ratios of the Lungs  Local controls to match airflow and blood flow of the lung Large Air-flow/Small blood flow ie, Apex of the Lungs Sherwood Human Physiology, 9th Ed, Ch13.  Local controls to match airflow and blood flow of the lung Large Blood Flow/Small Air-flow (Base of the Lungs) Sherwood Human Physiology, 9th Ed, Ch13.  LIMITATIONS OF RESPIRATORY RESPONSES IN DISEASE Reduced alveolar ventilation: ↓ Tidal volume &/ or ↑ Dead space What disease conditions may cause ↓ tidal volume Restriction of lung or chest wall movements (ie, Pain) eg. loss of lung elasticity by fibrosis (‘scarring’) of alveolar walls ↑ dead space?  SUMMARY OF GAS EXCHANGE & PERFUSION In order for gas exchange to occur: - Fresh air must reach the alveoli: ventilation (alveolar ventilation vs dead space) - Alveoli must be perfused by blood - Diffusion at alveoli should be efficient (partial gas pressure, rate of blood flow, diffusion barrier & surface area)  SUMMARY OF PARTS I & II: GAS EXCHANGE & PERFUSION Special properties of pulmonary circulation - Low pressure, high volume system (100% cardiac output) - Gravity-dependent - Blood flow through capillaries can be regulated by arteriolar tone Time for a Break!  PART III. VENTILATION TERMINOLOGY Not all air taken in undergoes gas exchange: Dead Space (usually mainly = air in conducting airways) More forceful inspiration & expiration (vs. normal quiet breathing) are possible Various volumes of air can move in & out of lungs: defined by different terms AT REST Tidal volume (TV; VT) – Volume of air entering lungs @ each resting breath (or exiting lungs on passive expiration) Inspiratory reserve volume (IRV) - Extra air entering lungs with RV maximal inspiration (on top of TV) Expiratory reserve volume (ERV) ERV - Extra air expelled from lungs with TV maximum expiration (after passive IRV expiration) Residual volume (RV) - Volume of air left in lungs after maximum expiration  VENTILATION VOLUMES: normal range of values vary with size, age & physical fitness of person Men (Ave Litre per Women breath) RV: 1.2 1.1 ERV: 1.0 0.7 TV: 0.5 0.5 IRV: 3.3 1.9 Total: Total: 6 4.2 VENTILATION VOLUMES can change Tidal volume (TV; VT) Volume of air that moves into lungs with each inspiration (or exits with each expiration) during a respiratory cycle RV Depends on eg. activity ERV Resting VT < Exercising VT TV IRV Exercising VT recruits other lung volumes (IRV,ERV)  VENTILATION VOLUMES can change Ability to ventilate depends on properties of chest wall & lungs & other factors which affect breathing Some egs. Chest wall : Muscle power? Skeletal deformities? RV Lungs: Resistance to airflow? Areas of ‘stiffness’ ERV (loss of elasticity)? Areas of Vital collapse? TV capacity Others: Abdominal IRV movement restricted eg. pain?  Minute ventilation: TV X Respiratory rate (breaths/ minute) What is the normal range of minute ventilation in the adult human at rest? TV ~ 0.5L, respiratory rate = ? Breaths /min Hence minute vent. = ? L /min During exercise? Some deviations from ‘normal’ ventilation Hyperventilation: ↑ ventilation Hypoventilation: ↓ ventilation Tachypnoea: ↑ rate of respiration Dyspnoea: distressful sensation of breathing  ALVEOLAR VENTILATION Not all air breathed in gets to alveoli VD Alveolar ventilation (VA): volume of air that reaches alveoli / minute RV ERV VA (L/min) = [ TV - VD ] TV (VT;tidal volume) (dead space) IRV X Respiratory rate (breaths per minute)  SUMMARY OF PART III. VENTILATION Alveolar ventilation is key to gas exchange in the lungs Control of Respiration  CO2 TRANSPORT IN BLOOD Carbamino Hb (23%) Red blood cell + Hb HHb (buffers pH) Carbonic Hb Cl- shift anhydrase + maintains CO2 + H2O ® H2CO3 ® H+ + HCO3- electrical neutrality CO2 Dissolved HCO3 - Cl- CO2 (7%) (~70%) plasma diffusion CO2 (higher PCO2) tissue  Chemoreceptor Control Peripheral Chemoreceptors Carotid body & Aortic bodies O2 & H+ sensor medulla Blood Pressure Baroreceptor  Chemical Control of Breathing PCO2 – main respiratory regulator – mainly affect on central chemoreceptors – CO2 can pass blood-brain barrier – H+ cannot pass the barrier [H+] – monitored by carotid & aortic bodies PO2 – monitored by carotid & aortic bodies – arterial PO2< 60 mmHg to stimulate peripheral chemoreceptors  Influence of Chemical Factors on Respiration Sherwood Human Physiology Ch13 Chemoreceptor Response to Changes in PCO2 Martini Anatomy & Physiology Ch23, Fig23-26.  Learning Objectives The concept of ventilation and gas exchange by diffusion. How partial pressure drives the diffusion of O2 and CO2? Factors that influences the rates of gas transfer across the alveolar membrane. Basic concept of ventilation and perfusion. Ventilation terminology and different lung volume measurements. Control of Respiration.