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16 Mechanics of Ventilation


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- diaphragm
- forced or labored uses
1) external intercostals ( raise anterior chest wall)
2) sternocleidomastoid ( lifts sternum and upper ribcage up and out)
3) scalenes
- normal: diaphragm expands by pushing lower ribs out
- contraction expands thoracic cavity by pushing lower ribs out
- passive from relaxation of inspiratory muscles and compression of thoracic volume
- forced: contraction of abs and external intercostals
Gas Flow
- pressure gradients
- inspiration: contraction of muscles and increased thoracic volume (incrased volume,decrease pressure)
Intrapleural pressure
- potential space between pleura (parietal and visceral)
- less than atmospheric at end of expiration (-5 cmH20)
Alveolar Pressure
- PA
- pressure in alveoli
- varies with respiratory cycle
Atmospheric Pressure
- PB
- not change so value considered 0 (reference for teh rest)
Transmural Pressure
- pressure difference across vessel wall (between inside and outside)
Transpulmonary Pressure
- differnce between alveolar and intrapleural pressure
- Pa - PIP
intrapleural pressure at FRC
- lungs recoil inward
- chest wall (at low lung volumes recoil outward
- at resting expiration in balance with lungs with FRC
- PIP -5 cm H2O
- PA = PB
- no air flow
During respiratory cycle
- PIP becomes more (-) on inspiration
- change in volume/ change in pressure
- L/cmH2O
- change in volume on inspiration not linear
- hysteresis: difference between inspiratory and expiratory curves
- due to airway resistance and surface tension
- slope of inspiratory curve --> changes in compliance with increasing volume
surface tension
- unbalanced molecular cohesive forces
- bubble: surface forces contract sphere so smallest volume occupied
Law of Laplace
- decreasing radius increases surface tension on spere and collapse it
- alveoli basically sphere shaped
- connected by airways (like grapes)
- la place would collapse the small alveolus into larger ones
- difference between inspiratory and expiratory curves
- air liquid interface in alveoli
- air filled lungs: more difficult to expand than saline filled lungs
- air filled lungs less compliant at very low and high volumes
- mixture of lipids and proteins
- alveolar type 2 pneumocytes
- constitutive: low production and secretion before birth (35 weeks)
- increased with lung hyperinflation, exercise, beta agonists
- increases lung compliance
- minimizes fluid accumulation in lung
thoracic compliance
- = to total compliance
- ΔV/ΔP = determined by compliance of chest wall and lung combined
- 1/CT = 1/CL + 1/CCW)
- lung always recoils inward
- Cell Wall can go in or out
equilibrium point
- cell wall recoil outward and lung recoil inward =
- lung volume : FRC
- airway pressure = 0
Compliance Curves
- PV curves
- lower area of curve shows work done to get a breath
- work due to airway resistance and lung compliance
Distribution of ventilation
- PIP more negative at apex than at bases
- alveoli in apex larger than at base, flatter and less compliant as well
- base: steeper portion of P/V curve so ventilate better

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