Overview of Respiration and Respiratory Mechanics

1. Overview of Respiration andRespiratory Mechanics Dr Shihab Khogali Ninewells Hospital & Medical School, University of Dundee

2. What is This Lecture About? • 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 respiration • Understand Ventilation - the first step of external respiration 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. ‘food’ + O2 ‘energy’ + CO2 Internal Respiration • Our body systems are made of cells • These cells need a constant supply of oxygen (O2) to produce energy and function • The carbon dioxide (CO2) produced by the cellular reactions must continuously be removed from our bodies • The internal respiration refers to the intracellular mechanisms which consumes O2 and produces CO2

5. Atmosphere External Respiration O2 CO2 • 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 body • External respiration is the topic for our four-linked physiology lectures • External respiration involves four steps Alveoli of lungs CO2 O2 Pulmonary circulation Systemic circulation CO2 O2 CO2 + HO2 + HTP Food +O2 Tissue cell

6. Atmosphere Steps of external respiration 1 Ventilation or gas exchange between the atmosphere and air sacs (alveoli) in the lungs O2 CO2 Alveoli of lungs 2 Exchange of O2 andCO2 between air in the alveoli and the blood CO2 O2 Pulmonary circulation 3 Transport of O2 andCO2 between the lungs and the tissues Systemic circulation CO2 O2 4 Exchange of O2 andCO2 between the blood and the tissues CO2 + HO2 + ATP Internal respiration Food +O2 Tissue cell Fig. 13-1, p. 452

7. 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 O2 and CO2 between the air in the alveoli and the blood in the pulmonary capillaries • Gas transport in the blood • The binding and transport of of O2 and CO2 in the circulating blood • Gas exchange at the tissue level • The exchange of O2 and CO2 between the blood in the systemic capillaries and the body cells

8. Atmosphere Three body systems are involved in external respiration O2 CO2 • The Respiratory System • The Cardiovascular System • The Haematology System Alveoli of lungs CO2 O2 Pulmonary circulation Systemic circulation CO2 O2 CO2 + HO2 + HTP Food +O2 Tissue cell

9. Ventilation The mechanical process of moving air between the atmosphere and alveolar sacs

10. as the volume of a gas increases the pressure exerted by the gas decreases Ventilation Boyle’s Law • 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? At any constant temperature the pressure exerted by a gas varies inversely with the volume of the gas

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.

13. Three Pressures are Important in Ventilation

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 5 • The external intercostal muscle contraction lifts the ribs and moves out the sternum. • The “bucket handle” mechanism.

16. Inspiration 760 • 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 (Boyle’s Law) • The air then enters the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressure Size of thorax on contraction of inspiratory muscles 759 754 Size of lungs as they are stretched to fill the expanded thorax

17. Expiration 760 • 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 (Boyle’s Law) • The air then leaves the lungs down its pressure gradient until the intra-alveolar pressure become equal to atmospheric pressure Size of thorax on relaxation of inspiratory muscles 761 756 Size of lungs as they recoil

18. Changes in intra-alveolar and intra-pleural pressures during the respiratory cycle Inspiration Expiration Intra-alveolar pressure Atmospheric pressure Intrapleural pressure Transmural pressure gradient across the lung wall Fig. 13-14, p. 462

19. Pneumothorax (air in the pleural space) abolishes the transmural pressure gradient

20. 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

21. 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

22. If we regard the alveoli as spherical bubles, then: (LaPlace’s Law) P = inward directed collapsing pressure T = Surface Tension r = radius of the buble Surfactant Reduces the Alveolar Surface Tension • 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 prevent this happening

23. Respiratory Distress Syndrome of the New Born • Developing fetal lungs are unable to synthesize surfactant until late in pregnancy • Premature babies may not haveenough pulmonary surfactant • This causesrespiratory 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.

24. 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

26. Fig. 13-11, p. 459

27. Lung Volumes and Capacities See Practical Class and Online Tutorial

28. Predicted normal values vary with age, height, gender,..

29. Lung Volumes and Capacities

30. Lung Volumes and Capacities

31. Spirometry for Dynamic Lung Volumes 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% = FEV1/FVC ratio

32. Normal Obstructive Lung Disease

35. F: Flow P: Pressure R: Resistance 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

36. Transairway Pressure = Airway Pressure – Pleural pressure 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) Ifthere 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 airwaysresulting in airway compression by the transairway pressure during active expiration. Dynamic Airway Compression During inspiration the airways are pulled open by the expanding thorax. Therefore in cases of increased airway resistanceexpiration tends to be more difficult.

37. 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

38. 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

39. 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