Presentation on theme: "Respiratory Physiology: Gas Exchange"— Presentation transcript:
1Respiratory Physiology: Gas Exchange Dr Shihab KhogaliNinewells Hospital & Medical School, University of Dundee
2See blackboard for detailed learning objectives Understand the difference between pulmonary ventilation and alveolar ventilation, and the significance of anatomical dead spaceUnderstand the basic principles of ventilation perfusion matchingUnderstand the significance of alveolar dead spaceKnow that the physiological dead space = anatomical + alveolar dead spaceUnderstand the four factors which influence the gas transfer across the alveolar membraneKnow the non-respiratory functions of the respiratory systemWhat isThisLectureAbout?See blackboard for detailed learning objectives
3Understand the followings in relation to the four factors which influence gas transfer across membranes:The Dalton’s Law of partial pressures. Know that gases move across membranes by partial pressure gradientThe role of diffusion coefficient on gas transfer across membranesThe effect of membrane surface area and membrane thickness on gas transfer, with the Fick’s Law of diffusion
5Pulmonary Ventilation & Alveolar Ventilation Some inspired air remains in the airways (anatomical dead space) where it is not available for gas exchangePulmonary Ventilation = tidal volume (ml/ breath) x Respiratory Rate (breath/min)= 0.5 L X 12 breath/min = 6 L/min under resting conditionsAlveolar Ventilation is less than pulmonary ventilation because of the presence of anatomical dead space.Alveolar Ventilation = (tidal volume – dead space volume) x Respiratory Rate = (0.5 – 0.15) x 12 = 4.2 L/min under resting conditions.Pulmonary Ventilation & Alveolar VentilationFresh airfrom inspirationAirway dead-spacevolume (150 ml)Alveolar airAfter inspiration,before expiration
6Figure 13.22: Effect of dead space volume on exchange of tidal volume between the atmosphere and the alveoli.Even though 500 ml of air move in and out between the atmosphere and the respiratory system and 500 ml move in and out of the alveoli with each breath, only 350 ml are actually exchanged between the atmosphere and the alveoli because of the anatomic dead space (the volume of air in the respiratory airways).Fig , p. 472
7Pulmonary Ventilation: Is the volume of air breathed in and out per minute Alveolar Ventilation: Is the volume of air exchanged between the atmosphere and alveoli per minute This is more important as it represent new air available for gas exchange with blood.
8Pulmonary Ventilation & Alveolar Ventilation To increase pulmonary ventilation (e.g. during exercise) both the depth (tidal volume) and rate of breathing (RR) increase.because of dead space:It is more advantageous to increase the depth of breathingPulmonary Ventilation & Alveolar VentilationFresh airfrom inspirationAirway dead-spacevolume (150 ml)Alveolar airAfter inspiration,before expiration
9It is more advantageous to increase the Depth of Breathing
10Ventilation Perfusion The transfer of gases between the body and atmosphere depends upon:Ventilation: the rate at which gas is passing through the lungs.Perfusion: the rate at which blood is passing through the lungs
11Ventilation Perfusion Both blood flow and ventilation vary from bottom to top of the lungThe result is that the average arterial and alveolar partial pressures of O2 are not exactly the same. Normally this effect is not significant but it can be in disease.V/Q Ratio12FlowLung PositionBottomTopBlood FlowVentilation
12Alveolar Dead SpaceThe match between air in the alveoli and the blood in the pulmonary capillaries is not always perfectVentilated alveoli which are not adequately perfused with blood are considered as alveolar dead spaceIn healthy people, the alveolar dead space is very small and of little importance (note: the physiological dead space = the anatomical dead space + the alveolar dead space)The alveolar dead space could increase significantly in disease
13Ventilation Perfusion Match in the Lungs Local controls act on the smooth muscles of airways and arterioles to match airflow to blood flowAccumulation of CO2 in alveoli as a result of increased perfusion decreases airway resistance leading to increased airflowIncrease in alveolar O2 concentration as a result of increased ventilation causes pulmonary vasodilation which increases blood flow to match larger airflow
14Dilation of local airways Area in which blood flow (perfusion)is greater than airflow (ventilation)HelpsbalanceHelpsbalanceLarge blood flowSmall airflowCO2 in areaO2 in areaRelaxation of local-airwaysmooth muscleContraction of local pulmon-ary arteriolar smooth muscleDilation of local airwaysConstriction of local blood vesselsAirway resistanceVascular resistanceAirflowBlood flow
15CO2 in area O2 in area Airway resistance Vascular resistance Airflow Area in which airflow (ventilation)is greater than blood flow (perfusion)HelpsbalanceHelpsbalanceLarge airflowSmall blood flowCO2 in areaO2 in areaContraction of local-airwaysmooth muscleRelaxation of local pulmonaryarteriolar smooth muscleConstriction of local airwaysDilation of local blood vesselsAirway resistanceVascular resistanceAirflowBlood flow
17Partial Pressure Gradient of O2 and CO2 Diffusion Coefficient for O2 and CO2Surface Area of Alveolar MembraneThickness of Alveolar MembraneFour Factors Influence The Rate of Gas Exchange Across Alveolar Membrane
18Dalton’s Law of Partial Pressures Gases move across cell membranes etc by pressure gradientThe partial pressure of a gas determines the pressure gradient for that gasThe partial pressure of gas (1) in a mixture of gases that don’t react with each other is:The pressure that gas (1) would exert if it occupied the total volume for the mixture in the absence of other componentsThus if the total pressure of the gas mixture is 100 kPa; and half of the mixture is gas (1): the partial pressure for gas (1) is 50 kPaWhat is Partial Pressureof Gas?Ptotal =P1 + P2 +…+ PnDalton’s Law of Partial PressuresThe Total Pressure exerted bya gaseous mixture =The sum of the partial pressures ofeach individual component inthe gas mixture
19The partial pressure of gas is: The pressure that one gas in a mixture of gases would exert if it were the only gas present in the whole volume occupied by the mixture at a given temperature.
20Figure 13.24: Concept of partial pressures. The partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of the gas in the mixture.
21Overview of Respiration Gases move from higher to lower partial pressures (partial pressure gradient)Note units in the diagram (mmHg). Here in UK you we use kPa (kilopascals) but Americans and American texts use mmHg.To convert divide mmHg by 7.5.
22Across Pulmonary Capillaries: O2 partial pressure gradient Atmospheric airAcross Pulmonary Capillaries:O2 partial pressure gradientfrom alveoli toblood =60 mm Hg (8 kP)100 – 40 mmHg i.e. ( kP)CO2 partial pressure gradientfrom blood toalveoli =6 mm Hg (0.8 kP)46 – 40 mmHg i.e. (6.1 – 5.3 kP)InspirationExpirationNet diffusion gradientsfor O2 and CO2 betweenthe lungs and tissuesAlveoliPulmonarycirculationAcross Systemic Capillaries:O2 partial pressure gradientfrom blood totissue cell => 60 mm Hg (8 kP)100 – < 40 mmHg i.e. (13.3- < 5.3 kP)CO2 partial pressure gradientfrom tissue cell toblood => 6 mm Hg (0.8 kP)> 46 – 40 mm Hg i.e. (> 6.1 – 5.3 kP)SystemiccirculationTissues
23The diffusion coefficient for CO2 is 20 times that of O2 But the partial pressure gradient for CO2 is much smaller than the partial pressure gradient for O2???What offset the difference in partial pressure gradient for CO2 and O2?CO2 is more soluble in membranes than O2. The solubility of gas in membranes is known as the Diffusion Coefficient for the gas.The diffusion coefficient for CO2 is 20 times that of O2
24Effect of surface Area & Membrane Thickness on Gas Diffusion The lungs provide a very large surface area with thin membranes to facilitate effective gas exchangeThe airways divides repeatedly to increase the surface area for gas exchangeThe small airways form outpockets (the alveoli). This help increase the surface area for gas exchange in the lungsThe lungs have a very extensive pulmonary capillary networkRemember: the pulmonary circulation receives the entire cardiac outputEffect of surface Area & MembraneThickness on Gas DiffusionFick’s Law of diffusionThe amount of gas that movesacross a sheet of tissue in unittime is proportional to the area ofthe sheet but inversely proportionalto its thickness
26Respiratory Membranes Alveoli: Thin-walled inflatable sacsFunction in gas exchangeWalls consist of a single layer of flattened Type I alveolar cellsPulmonary capillaries encircle each alveolusNarrow interstitial space
27Four Factors Influence the Rate of Gas Transfer Across The Alveolar Membrane
28Nonrespiratory Functions of Respiratory System Route for water loss and heat eliminationEnhances venous return (Cardiovascular Physiology)Helps maintain normal acid-base balance (Respiratory and Renal Physiology)Enables speech, singing, and other vocalizationsDefends against inhaled foreign matterRemoves, modifies, activates, or inactivates various materials passing through the pulmonary circulationNose serves as the organ of smell