Mechanics of breathing
Monday, 20 September 2021 5:00 PM
Respiratory cycle
- Inhalation
o Respiratory muscles contract
o Thoracic Lung volume increase
o Decrease in intrapleural pressure -4 to -7
o Transpulmonary pressure increase - +4 to +7
§ Increase in outward pressure to distend the lung
o Decrease in gas pressure in alveoli below atmospheric pressure
o Drives air flow into lungs
- Expiration
o Inspiratory muscles relax
o Lung volume decreases driven by lung recoil
o Intrapleural pressure restored from -7 to -4
o Transpulmonary pressure reduces
o Alveolar gas pressure increases higher than atmospheric pressure
o Air flows out of lung
Elastic structure of lungs
- Deformation of structure must be caused by an external force
o Lung’s inflation caused by pressure that distends – enlarges - the lungs due to
contraction of respiratory muscles
o Energy consuming process to distend the lungs
§ muscular contraction that lowers pressure
- When external force is removed, the structure recoils to its resting position
o When respiratory muscles relax, decrease external force
- The distending pressure is equal to the recoil pressure
- Chest wall / lungs possess elastic property
o Chest wall has an outward recoil tendency
o Lungs have inward recoil tendency
o Intrapleural pressure is negative when all respiratory muscles are relaxed (end-
expiration) - -4mmhg
§ Important to counterbalance the recoil pressure of the chest and the lungs
- Pneumothorax
o Right chest enlarges and right lung collapses
o Due to pressure of air in right pleural cavity
§ Can be caused by air leakage from punctured lung
o Air leakage in pleural cavity causes a loss of the negative intrapleural pressure so that it
becomes 0 – same as Patm
o So, chest recoil outwardly and lung recoil inwardly
Factors affecting changes in lung volume
- Volume pressure curve
, o Air leakage in pleural cavity causes a loss of the negative intrapleural pressure so that it
becomes 0 – same as Patm
o So, chest recoil outwardly and lung recoil inwardly
Factors affecting changes in lung volume
- Volume pressure curve
o Volume – % vital capacity
o At 0 recoil pressure (absence of any external force)
§ lung is at its natural position – collapsed
§ small amount of gas left in lungs – residual volume
• lung volume you cannot exhale out after maximal expiration
§ small increase in recoil pressure
o At 50% VC (half of maximal lung capacity)
§ Recoil pressure of lungs increases
o At TLC
§ 100% VC – largest size of lung
§ Recoil pressure becomes higher
o Direction of lung recoil pressure always inwards
o Increase size, increase recoil pressure
o Non-linear slope
§ steep at 40-50% VC
§ gentle near TLC
- Volume pressure curve of saline filled lung
o Pressure required to inflate air filled lung is higher than pressure to inflate saline filled
lung
o Difference in pressure
o Surface tension at air-liquid interface
§ Intermolecular force between water molecules
§ Resolved direction of surface tension
• pointing towards inside of alveoli
• Due to circular structure of alveoli
Origin of lung recoil pressure
- Tissue elasticity
o Due to presence of elastin, collagen
o Responsible for 1/3 of recoil pressure
o Obeys Hooke’s law
§ volume change is proportional to pressure applied
- Surface tension
o The force exerted by water molecules on the surface of the lung tissue
§ Since air is moist
§ Polarity of water creates an inward force on surfaces which can act by decreasing
surface area by pulling the tissues in
§ Elasticity of alveoli means they do not resist surface tension thus helping to deflate
the alveoli during expiration
o Presence of air-liquid interface
o Responsible for 2/3 of recoil pressure
o Obeys Laplace’s law
§ Pressure defined by 2 times surface tension and inversely related to radius
o Without something to stop this, the airways would collapse after expiration and
inspiration would be difficult
Monday, 20 September 2021 5:00 PM
Respiratory cycle
- Inhalation
o Respiratory muscles contract
o Thoracic Lung volume increase
o Decrease in intrapleural pressure -4 to -7
o Transpulmonary pressure increase - +4 to +7
§ Increase in outward pressure to distend the lung
o Decrease in gas pressure in alveoli below atmospheric pressure
o Drives air flow into lungs
- Expiration
o Inspiratory muscles relax
o Lung volume decreases driven by lung recoil
o Intrapleural pressure restored from -7 to -4
o Transpulmonary pressure reduces
o Alveolar gas pressure increases higher than atmospheric pressure
o Air flows out of lung
Elastic structure of lungs
- Deformation of structure must be caused by an external force
o Lung’s inflation caused by pressure that distends – enlarges - the lungs due to
contraction of respiratory muscles
o Energy consuming process to distend the lungs
§ muscular contraction that lowers pressure
- When external force is removed, the structure recoils to its resting position
o When respiratory muscles relax, decrease external force
- The distending pressure is equal to the recoil pressure
- Chest wall / lungs possess elastic property
o Chest wall has an outward recoil tendency
o Lungs have inward recoil tendency
o Intrapleural pressure is negative when all respiratory muscles are relaxed (end-
expiration) - -4mmhg
§ Important to counterbalance the recoil pressure of the chest and the lungs
- Pneumothorax
o Right chest enlarges and right lung collapses
o Due to pressure of air in right pleural cavity
§ Can be caused by air leakage from punctured lung
o Air leakage in pleural cavity causes a loss of the negative intrapleural pressure so that it
becomes 0 – same as Patm
o So, chest recoil outwardly and lung recoil inwardly
Factors affecting changes in lung volume
- Volume pressure curve
, o Air leakage in pleural cavity causes a loss of the negative intrapleural pressure so that it
becomes 0 – same as Patm
o So, chest recoil outwardly and lung recoil inwardly
Factors affecting changes in lung volume
- Volume pressure curve
o Volume – % vital capacity
o At 0 recoil pressure (absence of any external force)
§ lung is at its natural position – collapsed
§ small amount of gas left in lungs – residual volume
• lung volume you cannot exhale out after maximal expiration
§ small increase in recoil pressure
o At 50% VC (half of maximal lung capacity)
§ Recoil pressure of lungs increases
o At TLC
§ 100% VC – largest size of lung
§ Recoil pressure becomes higher
o Direction of lung recoil pressure always inwards
o Increase size, increase recoil pressure
o Non-linear slope
§ steep at 40-50% VC
§ gentle near TLC
- Volume pressure curve of saline filled lung
o Pressure required to inflate air filled lung is higher than pressure to inflate saline filled
lung
o Difference in pressure
o Surface tension at air-liquid interface
§ Intermolecular force between water molecules
§ Resolved direction of surface tension
• pointing towards inside of alveoli
• Due to circular structure of alveoli
Origin of lung recoil pressure
- Tissue elasticity
o Due to presence of elastin, collagen
o Responsible for 1/3 of recoil pressure
o Obeys Hooke’s law
§ volume change is proportional to pressure applied
- Surface tension
o The force exerted by water molecules on the surface of the lung tissue
§ Since air is moist
§ Polarity of water creates an inward force on surfaces which can act by decreasing
surface area by pulling the tissues in
§ Elasticity of alveoli means they do not resist surface tension thus helping to deflate
the alveoli during expiration
o Presence of air-liquid interface
o Responsible for 2/3 of recoil pressure
o Obeys Laplace’s law
§ Pressure defined by 2 times surface tension and inversely related to radius
o Without something to stop this, the airways would collapse after expiration and
inspiration would be difficult
