Mechanics of breathing Flashcards
What are the components of the mechanical respiratory system?
- Chest wall (including rib cage and diaphragm)
- Lungs (including alveoli and conducting airways)
- Pleural space
- Respiratory muscles
What additional muscles are involved in forced inspiration?
Scalenes (elevate ribs 1 + 2)
Sternocleidomastoids (raises sternum)
What muscles are involved in passive expiration?
None
What muscles are involved in active expiration?
Abdominal muscles (contract pushing diaphragm upwards) Internal intercostals (move ribs downwards) Pectoral girdle muscles
What muscles are involved in passive inspiration?
Intercostals (external + internal)
Diaphragm
What is “work of breathing”?
Energy expended during respiration made up of 3 components:
1) Compliance work - energy required to expand the lungs against the lung and chest elastic forces
2) Tissue resistance work - energy required to overcome the viscosity of the lungs and chest wall structures
3) Airway resistance work - energy required to overcome airway resistance to movement of air into the lungs
What is lung compliance?
The willingness of the lungs to expand
[volume change of the lung per unit of force applied]
What is surface tension?
At a surface, attraction between molecules of a liquid are much stronger than attraction between liquid and gas molecules (due to hydrogen bonds between water molecules)
Measured in terms of force per unit length (N.m-1)
What is Hooke’s Law?
The force needed to extend or compress a spring by some distance is proportional to that distance.
What is Laplace’s Law?
P = 2T/r (where P = pressure, T = surface tension, r = radius)
So if 2 bubbles have the same surface tension, the smaller bubble will have a higher pressure.
By what process does air travel from the atmosphere to the alveolar?
Bulk flow down pressure gradients (from positive atmospheric pressure to more negative peripheral airways created by lung expansion)
What nerve innervates the diaphragm?
Phrenic (C3-5)
What is the “bucket handle” effect?
Inspiration aided by external intercostal muscles moving the ribs upwards and outwards
Which phases of quiet breathing require energy?
Inhalation = active process Exhalation = passive (elastic recoil returns lungs to functional residual capacity)
What is the functional residual capacity?
Lung volume at which opposing forces of the chest wall and the contractile lung are in equilibrium
What anatomically permits the lungs to expand with the chest wall?
Pleura - parietal layer attached to chest wall and visceral layer attached to the lungs (with thin layer of pleural fluid between the two providing a seal between the layers)
Negative pressure in the pleural space becomes more negative as chest wall expands on inhalation meaning that the suction on the visceral layer and attached lung expands with the chest wall as the force of the lung’s elastic recoil is exceeded.
What lung volume is represented when the forces inflating and deflating the lungs are equal?
Functional residual capacity
[Occurs at end of exhalation during quiet breathing]
What prevents the lungs from collapsing away from the chest wall?
Negative pressure within the pleural space
What causes a pneumothorax?
Pleural seal is broken (e.g. commonly when lung surface or chest wall is punctured) meaning pressure in the pleural space will rise, removing the suction effect and causing the lung to collapse away from the chest wall due to unopposed force of the elastic recoil of the lungs
What signs on an X-Ray indicate a pneumothorax?
Deviated trachea
Displaced heart shadow
Black thorax (e.g. no evidence of bronchial tree)
Diaphragm depression
What is elastance?
A measure of the disposition of the lungs to return to resting position due to their intrinsic elasticity
What is the difference between compliance and elastance?
Compliance of the lungs refers to the willingness of the lungs to expand under pressure whilst elastance refers to the lungs ability to return to their resting position (due to elasticity)
How does emphysema impact compliance?
Increased compliance as there is less tissue to stretch due to tissue damage
What causes decreased compliance?
Fibrosis of the lungs
What is the role of surfactant in the lungs?
Reduces surface tension, stabilises alveoli and increases compliance (particularly at low lung volumes)
Reduces likelihood of tissue fluid transudation
Antioxidant effect
What is surfactant?
Secreted by epithelial cells (pneumocytes) lining the alveoli
Major ingredient of surfactant is phospholipid
What is interdependence and its importance?
Refers to the tight packing of alveoli limiting tendency to collapse
What is Poisuelle’s Law?
Determines resistance to flow in laminar flow
Resistance rises with the fourth power of the radius (e.g. if radius doubles, flow will increase by 2 to the power of 4 which is 16)
Which parts of the airways have the least and most resistance?
Least resistance (collectively) = smallest, individual bronchioles due to number of airways Most resistance = medium sized bronchi
What bedside test can be done to measure resistance?
Peak Expiratory Flow Rate
What factors determine airway resistance?
ANS (parasympathetic via CN X = constriction of airways, sympathetic stimulation by catecholamines on beta2 receptors relaxes smooth muscle thus dilating airways) Lung volume (increased lung volume = increased airway radius) Turbulent vs. laminar flow (large airways more prone to turbulent flow)
