Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
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Mechanics of Breathing
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Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
Institution
The University Of Hong Kong
The movement of the lungs and chest, breaking down volume pressure curves and describes the lung in diseased conditions (restrictive and obstructive disease) and the difference in their class
Phsyiological basis of heath and disease BBMS2006 (BBMS2006)
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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
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