Physiology - Lung Mechanics Flashcards
What does ventilation do?
Delivers the oxygen and removes the carbon dioxide that is exchanged across the alveolar capillary membrane.
In order for ventilation to occur, what needs to be overcome?
Elastic recoil in lung (and chest wall) to change volume Airways resistance (friction) to get airflow = Work of Breathing
What is the Work of Breathing
Need to overcome the elastic recoil in the lungs and the airways resistance
State Boyle’s Law.
At constant temperature, the volume of a gas varies inversely with absolute pressure.
P1V1 = P2V2
What is the primary principle of ventilation?
Air moves from an area of high pressure to an area of lower pressure.
Describe what happens in inspiration.
Diaphragm and external intercostal muscles contract
Increase in thoracic volume
Lungs expand, increase volume (decrease pressure)
Air moves into lungs
Describe what happens in expiration.
Inspiratory muscles relax Diaphragm moves upwards Decrease in thoracic volume and hence lung volume Increase alveolar pressure Air moves out of the lungs
Why is the intrapleural pressure sub atmospheric?
Lung has tendency to recoil inwards while the chest wall tends to expand outwards - they pull away from each other.
Explain what drives expiration.
Expiration driven by elastic recoil of resp system
What happens in forced expiration?
Use accessory muscles to augment expiration (generate more pressure)
State what is shown in an Xray showing pneumothorax.
Usually caused by trauma
Trachea gets displaced
Intrapleural space is disrupted and lung is unventilated
What does intubation do?
Creates a pressure difference. Increase the outside pressure causing air to move into the lungs
Name the two factors involved in the work of breathing.
Elastic Recoil/Compliance
Airways Resistance
What is lung compliance?
Measure of the elastic properties (stiffness) of the lung
What causes compliance to vary?
Lung volume - as the lung is stretched, it gets harder to stretch
Disease - fibrosis/loss of elastic tissue
Age - lungs are more compliant
Alveolar surface tension
What graph do we look at for compliance?
Change in volume/change in pressure graph
What happens to the compliance in a lung with fibrosis?
Lung is non-compliant, very stiff.
Thus for a given change in pressure, the change in volume is less.
What happens to the compliance in a lung with emphysema?
Destruction of the elastic tissue makes the lung more compliant.
Thus for a given change in pressure, the change in volume is more than normal.
Draw the compliance curve for a normal, fibrosed and emphysematic lung
xxx
What is surface tension and what does it contribute to?
Fluid lining the alveoli exerts surface tension which causes them to contract/resist expansion.
Need to overcome this force to expand the lung.
Component of compliance
What can reduce surface tension?
Surfactant
What is surfactant?
Phospholipid produced by alveolar type II cells
How does surfactant lower the surface tension?
Reduces attractive forces of hydrogen bonding by becoming interspersed between the water molecules.
Surface tension is therefore reduced and lungs become more compliant.
What occurs as a result of a lack of surfactant?
Stiff lungs e.g. in neonates or Adult Respiratory Distress Syndrome
Describe airflow in normal quiet breathing.
Airflow is mainly laminar flow.
Why are bronchoconstriction and bronchodilation important elements in airway resistance?
Poiseuille’s Law shows that resistance is inversely proportional to the radius.
1/2 r = 16 x R
Where is the main area of airway resistance?
Bronchi
Where does most of the resistance to airflow arise?
Upper airway and the first 6 generations of the lower airway
What factors determine airway resistance?
Bronchoconstriction (e.g. vagal parasympathetic fibres, local chemical mediators - histamine, leukotrienes)
Bronchodilation (e.g. activation of B2 receptors via adrenalin/salbutamol)
Dynamic compression of airways (breathe out = squash the airways)
Explain what dynamic compression of airways is.
Some airways are very collapsible.
Increased intrathoracic pressure can sometimes lead to airways collapse which increases airway resistance and limits airflow.
Limits airflow in normal subjects during a forced expiration.
Breathe out and squash the airways
In what conditions can dynamic compression of the airways occur?
Loss of elastic tissue (emphysema) - lung will collapse more readily
Old lungs - increase in airway resistance
