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Respiratory Physiology (elastic recoil & compliance
elastic recoil…
Respiratory Physiology
elastic recoil & compliance
elastic recoil = tendency of lung/alveolus to collapse
compliance = how much the lung/alveolus expands per unit increase in pressure
lung WANTS to follow its elastic recoil & collapse at rest, while the chest wall WANTS to bulge
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pressure needed to maintain shape of alveoli = 2 x surface tension/radius smaller : alveoli are more likely to collapse :red_flag: if small alveoli collapse, the pressure will be transferred to the large alveoli & then to the respiratory bronchioles #
determined by
surfactant
SURFACE TENSION B/W WATER MOLECULES IN THE ALVEOLAR WALLS CREATES ELASTIC RECOIL
phospholipid + protein -> has repelling forces that oppose the attractive forces b/w water molecules in the alveolar walls (separating the water molecules by reducing surface tension) THINK: detergent
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:red_flag: restrictive lung diseases = ability for lungs to expand (compliance) is LIMITED (e.g. due to obesity, kyphoscoliosis or fibrosis)
FEV1 & FVC are BOTH reduced PROPORTIONALLY because the total amount of air that can be inhaled/exhaled is reduced due to the limited ability of the lungs to expand, and the same applies for the air that can be exhaled in 1s
FEV1/FVC = ~ 80%
respiratory cycle
flow: U-shaped graph (no -ve flow in reality, just air entering or leaving the lungs)
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spirometry
FEV1 = most sensitive measurement because you can compare FEV1 values of individuals with different body sizes (as opposed to flow rates), which are indirectly measured by FEV1/FVC
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ventilation & perfusion
ventilation: gas exchange b/w alveoli & ext. env.
perfusion: supply of blood carrying gases in body to alveoli to facilitate gas exchange in lungs
pulmonary circulation: much lower pressure than systemic BP (if these were the same, it would cause pulmonary oedema :red_flag:)
hypoxia causes pulmonary vasoconstriction (to direct blood away from the diseased segment (decreasing perfusion) towards a normal segment of the lung where ventilation can occur
regional differences within the lung
- at the apex of the lung, the IPP is -8, compared to -5 at the base, so alveoli at the base undergo a greater change in volume during inspiration, so more VENTILATION occurs at the base of the lung
- this difference is even greater with PERFUSION, with some segments near the apex of the lung only undergoing perfusion during certain periods or not at all normally (BUT during exercise, there is capillary recruitment, so these alveoli near the apex provide a reserve)
- ventilation: perfusion ratio is greater at the apex of the lung
alveolar VENTILATION = 350mL x RR
DS VENTILATION = 150mL x RR
e.g. DVT :red_flag: emboli can enter & lodge in pulmonary aa., reducing perfusion to segments of the lung (physiological DS)
e.g. emphysema :red_flag: destroying alveolar walls where associated capillaries are found on the other side of the blood-gas barrier creates a physiological DS where segments of the lung are not being perfused #