Lecture notes from Imperial College London, Medical Biosciences BSc, 2nd year, cardiovascular and thoracic biology (CTB) module.
Lecture 3 on ventilation and gas transport: fundamental processes required to achieve effective breathing and gas transport. These processes enable oxygen to get from ...
Ventilation & Gas transport
Volume, capacities and dead space
- lung volumes (don’t overlap):
=> Tidal volume (TV/ VT): air inspired + expired during regular breathing (not always at rest)
=> Inspiratory reserve volume (IRV): max air that can be inspired after a tidal respiration
=> Expiratory reserve volume (ERV): max air that can be expired after a tidal respiration
=> Residual volume (RV): air that can’t be emptied from the lungs (fixed)
vary according to height,
age, genetics, disease...
- total lung capacity (TLC) = RV + IRV + TV + ERV
- functional residual capacity (FRC) = RV + ERV
=> volume in the lungs following a tidal respiration (lung recoil + chest recoil at equilibrium)
- inspiratory capacity (IC) = TV + IRV
=> max volume lungs can draw in from the equilibrium FRC point
- vital capacity (VC) = TLC - RV = TV + IRV + ERV
=> volume between the max and min achievable volumes
- dead space (VD) = parts of the airways that don’t participate in gas exchange
=> anatomical dead space: conducting airways + upper respiratory tract
=> alveolar dead space: respiratory tissues unable to exchange gas (inadequate blood flow)
=> 0 in healthy ppl
=> physiological dead space = anatomical + alveolar
, Rq: a 10m snorkel would
create too much additional
dead space to allow gas
exchange (if narrow, too much
resistance)
Ventilation
- pulmonary ventilation (VE) = TV*Rf (breathing frequency) in mL/min: 500(mL)*15(breaths/ min)
=> amount of air moving into and out of the lungs per min
- alveolar ventilation (Valv) = (TV - VD)*Rf
=> amount of air per minute reaching the gas exchange surface
- each generation down the airway: 50% pressure + velocity (constant generational divergence)
- generate airflow by distorting the lungs equilibrium:
:
=> pressure outside lungs: positive-pressure breathing (on a ventilator)
=> pressure inside: negative-pressure breathing (normal): inspiration normalises from -8 to -5
=> diaphragm contracts downward + external intercostal muscles pull rib cage outwards &
upwards - intrathoracic pressure - lung stretch to fill space - suck air from outside
- breath out: chest wall force decreases + lung recoil inwards => lungs empty
(waste brought by capillaries + move
into alveolar sacs to be breathed out)
Cumulative
cross-sectional
area increases
- Poiseuille’s law: airway resistance = 8 η l / π (r)^4
with η = viscosity; l = length of tube; r = radius
- at first, resistance inversely proportional to r^4 BUT
after decreases according to constant generational
divergence in the airways
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