Ventilitory Apparatus – balloon in a box model

• elastance: opposition to deformation, ability to spring back (decreased by emphysema or old age)
• E = D P/D V
• compliance: distensibility (decreased by fibrosis; lower at high volume; high at low volume)
• C = D V/D P
• resistance: depends on radius (r⁴), length, flow pattern, velocity, viscosity
• R = D P/Flow = l/r⁴
• transpulmonary pressure (P_l) – pressure difference across wall of lungs
• as you get older, you lose stiffness and elasticity

• tissue forces – elasticity
• collagen: not elastic alone, but network is elastic; at high volumes, impedes expansion
• elastin – like nylon stocking
• surface forces
• alveoli are lined with fluid, filled with air Þ surface tension tends to collapse
• must overcome surface tension to inflate
• surface tension is counteracted by surfactant (dipalmatoyl lecithin - phospholipid)
• (1) reduces effort needed to expand lungs
• (2) prevents lungs from collapsing on expiration
• (3) stabilizes alveoli deflating at different rates
• (4) surface tension is very low – as area ß , surface tension also ß
• 
• interdependence: alveoli tend to prevent unequal expansion or collapse
• alveolar stability – if one alveolus wants to collapse, adjacent alveoli keep it from doing so
• law of LaPlace P = 2T/r so two alveoli in parallel: larger one would tend to expand and smaller would tend to contract (T = tension, r = radius, P = pressure needed to keep alveolus open)
• surfactant has more effect on smaller alveoli (small alveoli have less surface tension); this counteracts law of LaPlace to help equalize pressure
• 

• chest wall and lung are mechanically connected in series
• FRC: functional residual capacity (volume where chest wall and lung are equally opposed)
• volume after normal exhalation
• RV: residual volume - minimal vol (achieved by balance of chest wall and expiratory muscles)
• TLC: total lung capacity - max vol (balance of lungs and chest wall against inspiratory muscles)
• vital capacity: TLC - RV