Lung Pressures and Compliance Flashcards
LO1290: To identify the distributions of ventilation and perfusion in the normal, upright lung and link them to effects on gas exchange.
During quiet breathing which muscle is doing most of the work?
The diaphragm does the majority of the work. In inspiration it flattens decreasing the pleural pressure (-5cmH2O to approx -8cmH2O) and therefore expanding the alveolar creating a pressure slightly below atmospheric (-1) which is sufficient to draw air in. Expiration is achieved as the diaphragm relaxed allowing the elastic recoil of the lungs, chest wall and abdomen.
By what mechanism can the lung volume be changed to allow ventilation?
The lung volume can be changed by the contraction and relaxation of the diaphragm resulting in the flattening and curving of the diaphragm (respectively) this lengthens and shortens the chest cavity. AND by the elevation and the depression of the ribs resulting in and increase and decrease (respectively) of the anteroposterior diameter of the chest cavity.
Which muscles elevate the chest cage and are therefore muscles of inspiration?
External intercostal muscles. The sternocleidomastoid can also be recruited to lift up the sternum, anterior serrati lift many of the ribs, and the scaleni lift the first two ribs.
Which muscles can be recruited to aid expiration?
Normal quiet breathing is achieved through elastic breathing, but when the work of breathing increases due to exercise for example or as a result of pathology the abdominal recti can pull down the lower ribs and other abdominal muscles can push the abdominal contents upwards further decreasing the thoracic volume. The internal intercostal muscles can also be used to reduce the thoracic volume.
Identify and label the muscles that are either primary muscles of respiration or can be recruited to aid inspiration or expiration.
External intercostals and diaphragm
Sternocleidomastoid, anterior serrati and scaleni lifting first two ribs
abdominal recti and internal intercostals
What are the forces acting on the lung?
The lungs are suspended from the mediastinum at their hilum. The negative pressure of the pleural fluid created by the constant removal of fluid by the lymphatic system creates suction between the visceral surface of the lung and parietal surface of the thoracic cavity. Increasing the volume of the thoracic cavity in inspiration makes the pleural pressure more negative, the force of which is transferred to the lung tissue and changes the alveolar pressure. The difference between the pleural pressure and the alveolar pressure is called the transpulmonary pressure. This is a measure of the elastic force in the lung that would tend to collapse the lung i.e. elastic recoil.
How does pleural pressure change during normal breathing?
At the beginning of inspiration when the long volume is 0 (or FRC) pleural pressure is at its highest, approximately -5cmH20 (below atmospheric pressure taken as 0). This pressure can hold the lungs open at their resting level. Expansion of the thoracic cage further decreases the pleural pressure to -7.5cmH2O. This is reversed in expiration.
Describe Alveolar pressure and how it changes during respiration.
Alveolar pressure is the pressure inside the alveolar. when the glottis is open and no air is flowing in or out the pressure is 0 or the same as the atmosphere. As the pleural pressure becomes more negative this acts on the lung wall pulling it outwards to try and counteract this and consequently pulling open the elastic walls of the alveoli. This decreases the alveolar pressure below that of the atmospher allowing the movement of air into the alveoli. The pressure only moves to about 1cmH2O below the atmosphere, but this is sufficient for movement of air. The movement of air in makes the alveolar pressure begin to rise again and as the chest wall begins to relax the alveolar space also contracts further rising the alveolar pressure and therefore moving it above atmospheric pressure (1cmH2O) creating a force for the movement of gases out of the alveolar and into the atmosphere which at that moment has a lower pressure than the alveolar. -1cmH2O allows movement of approx 500ml air into lungs in about 2 seconds and the same amout out in about 2-3 seconds.
What does transpulmonary pressure represent?
Transpulmonary pressure is the difference between the pleural pressure and the alveolar pressure. It represents the elastic forces of the lung that tend to pull in the direction of collapse. It is therefore a measure of recoil pressure.
What is the compliance of the lung?
How much the lung will expand for each increase in transpulmonary pressure (theoretical - as given time for equilibrium to be reached). Compliance in the normal adult lungs (combined) is about 200ml/cmH2O.
What factors determine the compliance of the lungs?
Compliance is determined by
1) the elastic forces of the lung tissue
* Elastin and collagen fibres in the lung parenchyma. Initially in deflated lungs these fibres are kinked and contracted, but as the lungs fill they stretch and exert more elastic force (reducing compliance?)*
2) elastic forces caused by the surface tension of the fluid that lines the inside walls of the alveoli
* The interface between fluid and air creates a surface tension that creates a collapsing pressure thereby decreasing the compliance (for each increase in the transpulmonary pressure the volume will increase by less). This is demonstrated in saline filled lungs which don’t have the fluid air interface and therefore have greater compliance. Further the absence of surfactant which reduces surface tension increases the compliance of the lungs.*
What does this diagram show?
This is a compliance curve demonstrating the increasing lung volume as the transpleural pressure (the difference between the alveolar pressure, -1-+1, and the pleural pressure represented by the increasing negativity of pleural pressure) increases. It demonstrates the difference in the curves between inspiration and expiration.
Describe the factors impacting on surface tension in the lung.
Surfactant in the lung produced by type II alveolar epithelial cells (type II pneomocytes). 10% of surface area of alveoli with granular containing lipid inclusions - most important phospholipid dipalmitoylphophatidylcholine, surfactant apoproteins, and calcium ions. The phospholipids dissolve non- uniformly across the surface disrupting the surface tension. this reduces the surface tension to a 12th or half that of pure water.
The pressure in an occluded alveoli is related to the surface tension in the following way:
Pressure = 2*Surface tension/Radius of alveoli.
The reduction in surface tension enables the lungs to be re-expanded after each expiration. Reduced radius of the alveoli also increases pressure.
Respiratory distress syndrome of the newborn - due to combination of surfactant not produced till 6th-7th month of gestation and alveoli size a quarter of adults. Surfactant given and positive pressure breathing may be required.
What is the compliance of the lungs and the thoracic cage combined?
Tested in the lungs of a totally relaxed or paralysed person - total compliance is about half the compliance of the lungs alone - 110ml/cmH2O. At extremes of expansion and compression can be even further reduced
What are the components of the work of breathing?
- Compliance or elastic work to overcome the chest elastic forces
- Tissue resistance work to overcome the viscosity of the lung and chest wall structures.
- Airway resistance work - overcome airway resistance to movement of air into the lungs.
