Presentation on theme: "Overview of Respiration and Respiratory Mechanics"— Presentation transcript:
1 Overview of Respiration and Respiratory Mechanics Dr Shihab KhogaliNinewells Hospital & Medical School, University of Dundee
2 See blackboard for detailed learning objectives This lecture is the first of four-linked lectures …in this lecture:Understand what is meant by the terms “internal respiration” and “external respiration”Know the four steps of external respirationUnderstand Ventilation - the first step of external respirationWhat isThisLectureAbout?See blackboard for detailed learning objectives
3 Understand ventilation (Step 1 of external respiration). Know that gases move from higher to lower pressure, with the Boyle’s Law.Understand the respiratory mechanics and the relationship between atmospheric, intra-alveolar, and intrapleural pressures.understand the significance of transmural pressure gradient. Know that peumothorax abolishes the transmural pressure gradient.Understand that inspiration is an active process and that normal resting expiration is a passive process.Know the inspiratory muscles and the accessory muscles of respiration (link with anatomy).Describe the role and importance of pulmonary surfactant, with the Law of Laplace and alveolar stability.Know the lung volumes and capacities. Understand the changes in dynamic lung volumes in obstructive and restrictive lung disease.Know the factors which influence airway resistance.Define the compliance of lungs and thorax.Understand what is meant by the term work of breathing.
4 Internal Respiration ‘energy’ + CO2 Our body systems are made of cells These cells need a constant supply of oxygen (O2) to produce energy and functionThe carbon dioxide (CO2) produced by the cellular reactions must continuously be removed from our bodiesThe internal respiration refers to the intracellular mechanisms which consumes O2 and produces CO2Internal Respiration‘food’ O2‘energy’ CO2
5 External Respiration Atmosphere The term external respiration refers to the sequence of events that lead to the exchange of O2 and CO2 between the external environment and the cells of the bodyExternal respiration is the topic for our four-linked physiology lecturesExternal respiration involves four stepsO2CO2Alveoli of lungsCO2O2PulmonarycirculationSystemiccirculationCO2O2Food + O2CO2 + HO2 + HTPTissue cell
6 Steps of external respiration AtmosphereSteps of external respiration1Ventilation or gas exchange betweenthe atmosphere and air sacs (alveoli)in the lungsO2CO2Alveoli of lungs2Exchange of O2 and CO2 between airin the alveoli and the bloodCO2O2Pulmonarycirculation3Transport of O2 and CO2 between thelungs and the tissuesSystemiccirculationCO2O24Exchange of O2 and CO2 between theblood and the tissuesFood + O2CO2 + HO2 + ATPInternal respirationTissue cellFig. 13-1, p. 452
7 The Four Steps of External Respiration VentilationThe mechanical process of moving gas in and out of the lungsGas exchange between alveoli and bloodThe exchange of O2 and CO2 between the air in the alveoli and the blood in the pulmonary capillariesGas transport in the bloodThe binding and transport of of O2 and CO2 in the circulating bloodGas exchange at the tissue levelThe exchange of O2 and CO2 between the blood in the systemic capillaries and the body cells
8 Three body systems are involved in external respiration AtmosphereThree body systems are involvedin external respirationThe Respiratory SystemThe Cardiovascular SystemThe Haematology SystemO2CO2Alveoli of lungsCO2O2PulmonarycirculationSystemiccirculationCO2O2Food + O2CO2 + HO2 + HTPTissue cell
10 Air flow down pressure gradient from a region of high pressure to a region of low pressure The intra-alveolar pressure must become less than atmospheric pressure for air to flow into the lungs during inspiration. How is this achieved?Before inspiration the intra-alveolar pressure is equivalent to atmospheric pressureDuring inspiration the thorax and lungs expand as a result of contraction of inspiratory musclesBut: How the movement of the chest wall expand the lungs as there is no physical connection between the lungs and chest wall?VentilationBoyle’s LawAt any constant temperature thepressure exerted by a gas variesinversely with the volume of the gasas the volume of a gas increases the pressure exerted by the gas decreases
11 Linkage of Lungs to Thorax Two forces hold the thoracic wall and the lungs in close opposition:(1) The intrapleural fluid cohesiveness: The water molecules in the intrapleural fluid are attracted to each other and resist being pulled apart. Hence the pleural membranes tend to stick together.(2) The negative intrapleural pressure: the sub-atmospheric intrapleural pressure create a transmural pressure gradient across the lung wall and across the chest wall. So the lungs are forced to expand outwards while the chest is forced to squeeze inwards.
12 Figure 13.8: Transmural pressure gradient. Across the lung wall, the intra-alveolar pressure of 760 mm Hg pushes outward, while the intrapleural pressure of 756 mm Hg pushes inward. This 4 mm Hg difference in pressure constitutes a transmural pressure gradient that pushes out on the lungs, stretching them to fill the larger thoracic cavity. Across the thoracic wall, the atmospheric pressure of 760 mm Hg pushes inward, while the intrapleural pressure of 756 mm Hg pushes outward. This 4 mm Hg difference in pressure constitutes a transmural pressure gradient that pushes inward and compresses the thoracic wall.
14 Inspiration is an active process depending on muscle contraction The volume of the thorax is increased vertically by contraction of the diaphragm (major inspiratory muscle), flattening out its dome shape.Phrenic nerve from cervical 3,4 and 5The external intercostal muscle contraction lifts the ribs and moves out the sternum.The “bucket handle” mechanism.
15 Figure 13.12: Respiratory muscle activity during inspiration and expiration. (a) Inspiration, during which the diaphragm descends on contraction, increasing the vertical dimension of the thoracic cavity. Contraction of the external intercostal muscles elevates the ribs and subsequently the sternum to enlarge the thoracic cavity from front to back and from side to side.
16 Inspiration is an active process brought about by contraction of inspiratory muscles The chest wall and lungs stretchedThe Increase in the size of the lungs make the intra-alveolar pressure to fallThis is because air molecules become contained in a larger volume (Boyle’s Law)The air then enters the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressureInspiration760Size of thorax oncontraction ofinspiratory muscles759754Size of lungs as theyare stretched to fillthe expanded thorax
17 Normal expiration is a passive process brought about by relaxation of inspiratory muscles The chest wall and stretched lungs recoil to their preinspiratory size because of their elastic propertiesThe recoil of the lungs make the intra-alveolar pressure to riseThis is because air molecules become contained in a smaller volume (Boyle’s Law)The air then leaves the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressureExpiration760Size of thorax onrelaxation ofinspiratorymuscles761756Size of lungs asthey recoil
18 Changes in intra-alveolar and intra-pleural pressures during the respiratory cycle InspirationExpirationIntra-alveolarpressureAtmosphericpressureIntrapleuralpressureTransmural pressuregradient across thelung wallFig , p. 462
19 Pneumothorax (air in the pleural space) abolishes the transmural pressure gradient Figure 13.9: Pneumothorax.(a) Traumatic pneumothorax. A puncture in the chest wall permits air from the atmosphere to flow down its pressure gradient and enter the pleural cavity, abolishing the transmural pressure gradient. (b) Collapsed lung. When the transmural pressure gradient is abolished, the lung collapses to its unstretched size, and the chest wall springs outward. (c) Spontaneous pneumothorax. A hole in the lung wall permits air to move down its pressure gradient and enter the pleural cavity from the lungs, abolishing the transmural pressure gradient. As with traumatic pneumothorax, the lung collapses to its unstretched size.
20 Elastic connective tissue in the lungs What causes the lungs to recoil during expiration? (i.e. what gives the lungs their elastic behaviour)Elastic connective tissue in the lungsThe whole structure bounces back into shapeBut even more important is the alveolar surface tension
21 What is alveolar surface tension? Attraction between water molecules at liquid air interfaceIn the alveoli this produces a force which resists the stretching of the lungsIf the alveoli were lined with water alone the surface tension would be too strong so the alveoli would collapse
22 Surfactant Reduces the Alveolar Surface Tension According to the law of LaPlace: the smaller alveoli (with smaller radius - r) have a higher tendency to collapsePulmonary surfactant is a complex mixture of lipids and proteins secreted by type II alveoliIt lowers alveolar surface tension by interspersing between the water molecules lining the alveoliSurfactant lowers the surface tension of smaller alveoli more than that of large alveoliThis prevent the smaller alveoli from collapsing and emptying their air contents into the larger alveoliSurfactant Reduces the Alveolar Surface TensionSurfactant prevent this happeningIf we regard the alveoli as spherical bubles, then:P = inward directed collapsing pressureT = Surface Tensionr = radius of the buble(LaPlace’s Law)
23 Respiratory Distress Syndrome of the New Born Developing fetal lungs are unable to synthesize surfactant until late in pregnancyPremature babies may not have enough pulmonary surfactantThis causes respiratory distress syndrome of the new bornThe baby makes very strenuous inspiratory efforts in an attempt to overcome the high surface tension and inflate the lungs.
24 Another factor which helps keep the alveoli open is: The Alveolar Interdependence Figure 13.17: Alveolar interdependence.(a) When an alveolus (in pink) in a group of interconnected alveoli starts to collapse, the surrounding alveoli are stretched by the collapsing alveolus. (b) As the neighboring alveoli recoil in resistance to being stretched, they pull outward on the collapsing alveolus. This expanding force pulls the collapsing alveolus open.If an alveolus start to collapse the surrounding alveoli arestretched and then recoil exerting expanding forces in thecollapsing alveolus to open it
26 Figure 13.11: Anatomy of the respiratory muscles. Fig , p. 459
27 Lung Volumes and Capacities See Practical Class and Online Tutorial
28 Predicted normal values vary with age, height, gender,.. Figure 13.18: Variations in lung volume.(b) Normal spirogram of a healthy young adult male. (The residual volume cannot be measured with a spirometer but must be determined by another means.)Predicted normal values vary with age, height, gender,..
29 Lung Volumes and Capacities DescriptionAverage ValueTidal volume (TV)Volume of air entering or leaving lungs during a single breath500 mlInspiratory reserve volume (IRV)Extra volume of air that can be maximally inspired over and above the typical resting tidal volume3000 mlInspiratory capacity (IC)Maximum volume of air that can be inspired at the end of a normal quiet expiration (IC =IRV + TV)3500 mlExpiratory reserve volume (ERV)Extra volume of air that can be actively expired by maximal contraction beyond the normal volume of air after a resting tidal volume1000 mlResidual volume (RV)Minimum volume of air remaining in the lungs even after a maximal expiration1200 ml
30 Lung Volumes and Capacities DescriptionAverage ValueFunctional residual capacity (FRC)Volume of air in lungs at end of normal passive expiration (FRC = ERV + RV)2200 mlVital capacity (VC)Maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IRV + TV + ERV)4500 mlTotal lung capacity (TLC)Maximum volume of air that the lungs can hold (TLC = VC + RV)5700 mlForced expiratory volume in one second (FEV1): Dynamic volumeVolume of air that can be expired during the first second of expiration in an FVC (Forced Vital Capacity) determinationFEV1% = FEV1/FVC ratioNormal >75%
31 Spirometry for Dynamic Lung Volumes Volume time curve - allow you to determine:FVC = Forced Vital Capacity (maximum volume that can be forciblyExpelled from the lungs following a maximum inspiration)FEV1 = Forced Expiratory volume in one secondFEV1% = FEV1/FVC ratio
35 Airway Resistance F: Flow P: Pressure R: Resistance Resistance to flow in the airway normally is very low and therefore air moves with a small pressure gradientPrimary determinant of airway resistance is the radius of the conducting airwayParasympathetic stimulation causes bronchoconstrictionSympathetic stimulation causes bronchodilatationDisease states (e.g. COPD or asthma) can cause significant resistance to airflowExpiration is more difficult than inspiration
36 Dynamic Airway Compression During inspiration the airways are pulled open by the expanding thorax. Therefore in cases of increased airway resistance expiration tends to be more difficult.The transairway pressure tends to compress airways during active expiration -pleural pressure rises during expiration (increases airway resistance)If no obstruction: the increased airway resistance causes an increase inairway pressure upstream. This helps open the airways (i.e. reduce thecompressive transairway pressure)Transairway Pressure = Airway Pressure – Pleural pressureIf there is an obstruction (e.g. COPD), the driving pressure between the alveolus and airway is lost over the obstructed segment. This causes a fall in airway pressure along the airways resulting in airway compression by the transairway pressure during active expiration.
37 Peak Flow Meter Gives an estimate of peak flow rate The peak flow rate assess airway functionThe test is useful in patients with obstructive lung disease (e.g. asthma and COPD)It is measured by the patient giving a short sharp below into the peak flow meterThe average of three attempts is usually takenThe peak flow rate in normal adults vary with age and heightYou will practice taking the peak flow rate in the Clinical Skills CentrePeak Flow Meter
38 Compliance During inspiration the lungs are stretched Compliance is measure of effort that has to go into stretching or distending the lungsVolume change per unit of pressure change across the lungsThe less compliant the lungs are, the more work is required to produce a given degree of inflationDecreased by factors such as pulmonary fibrosis
39 Work of BreathingNormally requires 3% of total energy expenditure for quiet breathingLungs normally operate at about “half full”Work of breathing is increased in the following situationsWhen pulmonary compliance is decreasedWhen airway resistance is increasedWhen elastic recoil is decreasedWhen there is a need for increased ventilation