Presentation on theme: "Overview of Respiration and Respiratory Mechanics Dr Shihab Khogali Ninewells Hospital & Medical School, University of Dundee."— Presentation transcript:
Overview of Respiration and Respiratory Mechanics Dr Shihab Khogali Ninewells Hospital & Medical School, University of Dundee
This lecture is the first of four-linked lectures …in this lecture: Understand what is meant by the termsinternal respiration and external respiration Know the four steps of external respiration Understand Ventilation - the first step of external respiration What is This Lecture About? See blackboard for detailed learning objectives
Know that gases move from higher to lower pressure, with the Boyles 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. Understand ventilation (Step 1 of external respiration).
Our body systems are made of cells These cells need a constant supply of oxygen (O 2 ) to produce energy and function The carbon dioxide (CO 2 ) produced by the cellular reactions must continuously be removed from our bodies The internal respiration refers to the intracellular mechanisms which consumes O 2 and produces CO 2 food + O 2 energy + CO 2 Internal Respiration
The term external respiration refers to the sequence of events that lead to the exchange of O 2 and CO 2 between the external environment and the cells of the body External respiration is the topic for our four- linked physiology lectures External respiration involves four steps Atmosphere Tissue cell Alveoli of lungs Pulmonary circulation Systemic circulation CO 2 O2O2 Food + O 2 CO 2 + HO 2 + HTP O2O2 CO 2 O2O2 External Respiration
Atmosphere Tissue cell Alveoli of lungs Pulmonary circulation Systemic circulation CO 2 O2O2 Food + O 2 CO 2 + HO 2 + ATP O2O2 CO 2 O2O2 1 Steps of external respiration Ventilation or gas exchange between the atmosphere and air sacs (alveoli) in the lungs Exchange of O 2 and CO 2 between air in the alveoli and the blood Transport of O 2 and CO 2 between the lungs and the tissues Exchange of O 2 and CO 2 between the blood and the tissues Internal respiration Fig. 13-1, p. 452
The Four Steps of External Respiration Ventilation The mechanical process of moving gas in and out of the lungs Gas exchange between alveoli and blood The exchange of O 2 and CO 2 between the air in the alveoli and the blood in the pulmonary capillaries Gas transport in the blood The binding and transport of of O 2 and CO 2 in the circulating blood Gas exchange at the tissue level The exchange of O 2 and CO 2 between the blood in the systemic capillaries and the body cells
The Respiratory System The Cardiovascular System The Haematology System Atmosphere Tissue cell Alveoli of lungs Pulmonary circulation Systemic circulation CO 2 O2O2 Food + O 2 CO 2 + HO 2 + HTP O2O2 CO 2 O2O2 Three body systems are involved in external respiration
The mechanical process of moving air between the atmosphere and alveolar sacs Ventilation
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 pressure During inspiration the thorax and lungs expand as a result of contraction of inspiratory muscles But: How the movement of the chest wall expand the lungs as there is no physical connection between the lungs and chest wall? as the volume of a gas increases the pressure exerted by the gas decreases Boyles Law At any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas Ventilation
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.
Three Pressures are Important in Ventilation
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 5 The external intercostal muscle contraction lifts the ribs and moves out the sternum. The bucket handle mechanism.
Inspiration is an active process brought about by contraction of inspiratory muscles The chest wall and lungs stretched The Increase in the size of the lungs make the intra- alveolar pressure to fall This is because air molecules become contained in a larger volume (Boyles Law) The air then enters the lungs down its pressure gradient until the intra- alveolar pressure become equal to atmospheric pressure Inspiration 759 Size of thorax on contraction of inspiratory muscles Size of lungs as they are stretched to fill the expanded thorax
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 properties The recoil of the lungs make the intra-alveolar pressure to rise This is because air molecules become contained in a smaller volume (Boyles Law) The air then leaves the lungs down its pressure gradient until the intra- alveolar pressure become equal to atmospheric pressure 761 Size of thorax on relaxation of inspiratory muscles Size of lungs as they recoil Expiration
InspirationExpiration Atmospheric pressure Intra-alveolar pressure Intrapleural pressure Transmural pressure gradient across the lung wall Fig , p. 462 Changes in intra-alveolar and intra-pleural pressures during the respiratory cycle
Pneumothorax (air in the pleural space) abolishes the transmural pressure gradient
What causes the lungs to recoil during expiration? (i.e. what gives the lungs their elastic behaviour) Elastic connective tissue in the lungs The whole structure bounces back into shape But even more important is the alveolar surface tension
What is alveolar surface tension? Attraction between water molecules at liquid air interface In the alveoli this produces a force which resists the stretching of the lungs If the alveoli were lined with water alone the surface tension would be too strong so the alveoli would collapse
According to the law of LaPlace: the smaller alveoli (with smaller radius - r ) have a higher tendency to collapse Pulmonary surfactant is a complex mixture of lipids and proteins secreted by type II alveoli It lowers alveolar surface tension by interspersing between the water molecules lining the alveoli Surfactant lowers the surface tension of smaller alveoli more than that of large alveoli This prevent the smaller alveoli from collapsing and emptying their air contents into the larger alveoli Surfactant Reduces the Alveolar Surface Tension If we regard the alveoli as spherical bubles, then: P = inward directed collapsing pressure T = Surface Tension r = radius of the buble (LaPlaces Law) Surfactant prevent this happening
Respiratory Distress Syndrome of the New Born Developing fetal lungs are unable to synthesize surfactant until late in pregnancy Premature babies may not have enough pulmonary surfactant This causes respiratory distress syndrome of the new born The baby makes very strenuous inspiratory efforts in an attempt to overcome the high surface tension and inflate the lungs.
Another factor which helps keep the alveoli open is: The Alveolar Interdependence If an alveolus start to collapse the surrounding alveoli are stretched and then recoil exerting expanding forces in the collapsing alveolus to open it
Fig , p. 459
See Practical Class and Online TutorialOnline Tutorial Lung Volumes and Capacities
Predicted normal values vary with age, height, gender,..
Lung Volumes and Capacities DescriptionAverage Value Tidal volume (TV) Volume of air entering or leaving lungs during a single breath 500 ml Inspiratory reserve volume (IRV) Extra volume of air that can be maximally inspired over and above the typical resting tidal volume 3000 ml Inspiratory capacity (IC) Maximum volume of air that can be inspired at the end of a normal quiet expiration (IC =IRV + TV) 3500 ml Expiratory 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 volume 1000 ml Residual volume (RV) Minimum volume of air remaining in the lungs even after a maximal expiration 1200 ml
Lung Volumes and Capacities DescriptionAverage Value Functional residual capacity (FRC) Volume of air in lungs at end of normal passive expiration (FRC = ERV + RV) 2200 ml Vital capacity (VC)Maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IRV + TV + ERV) 4500 ml Total lung capacity (TLC) Maximum volume of air that the lungs can hold (TLC = VC + RV) 5700 ml Forced expiratory volume in one second (FEV 1 ): Dynamic volume Volume of air that can be expired during the first second of expiration in an FVC (Forced Vital Capacity) determination FEV 1 % = FEV 1 /FVC ratio Normal >75%
Volume time curve - allow you to determine: FVC = Forced Vital Capacity (maximum volume that can be forcibly Expelled from the lungs following a maximum inspiration) FEV1 = Forced Expiratory volume in one second FEV1% = FEV 1 /FVC ratio Spirometry for Dynamic Lung Volumes
Normal Obstructive Lung Disease
Airway Resistance Resistance to flow in the airway normally is very low and therefore air moves with a small pressure gradient Primary determinant of airway resistance is the radius of the conducting airway Parasympathetic stimulation causes bronchoconstriction Sympathetic stimulation causes bronchodilatation Disease states (e.g. COPD or asthma) can cause significant resistance to airflow Expiration is more difficult than inspiration F: Flow P: Pressure R: Resistance
Dynamic Airway Compression If 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. 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 in airway pressure upstream. This helps open the airways (i.e. reduce the compressive transairway pressure) Transairway Pressure = Airway Pressure – Pleural pressure
Gives an estimate of peak flow rate The peak flow rate assess airway function The 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 meter The average of three attempts is usually taken The peak flow rate in normal adults vary with age and height You will practice taking the peak flow rate in the Clinical Skills Centre Peak Flow Meter
Compliance During inspiration the lungs are stretched –Compliance is measure of effort that has to go into stretching or distending the lungs –Volume change per unit of pressure change across the lungs –The less compliant the lungs are, the more work is required to produce a given degree of inflation –Decreased by factors such as pulmonary fibrosis
Work of Breathing Normally requires 3% of total energy expenditure for quiet breathing Lungs normally operate at about half full Work of breathing is increased in the following situations –When pulmonary compliance is decreased –When airway resistance is increased –When elastic recoil is decreased –When there is a need for increased ventilation