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(without pulmonary surfactant) to collapse and empty its air into larger alveolusPulmonary surfactant reduces surface tension of smaller alveolus more than that of larger alveolus; offsets the effect of the smaller radius in determining the inward-directed pressure with surfactant small alveolus does not collapseRespiratory cycle: maximum air lungs can hold is 5.7 L in males and 4.2 L in femalesDuring quiet breathing, lungs remain moderately inflated throughout respiratory cycle
Changes in lung volume can be determined bya spirometerTV: tidal volume (.5L)IRV: inspiratory reserve volume(3L)IC: Inspiratory capacity (3.5 L)ERV: Expiratory reserve volume(1L)RV: Residual volume (1.2 L)FRC: Functional residual capacity (2.2 L)VC: Vital capacity (4.5 L)TLC: total lung capacity (5.7 L)Alveolar ventilation = (tidal volume – dead space) * respiratory rateVolume of air exchanged between atmosphere and alveoli per minutePartial pressures: individual pressure exerted independently by a particular gas within a mixture of gasesGas exchange involves simple passive diffusion of O2 and CO2 down partial pressure gradientsPartial pressure = (total pressure) * (percent of gas inmixture)Partial pressure of atmospheric N2 = PN2 = 79%* 760 mm Hg = 600 mm HgPartial pressure ofatmospheric O2 = PO2 = 21%* 760 mm Hg = 160 mm HgHumidification (vapor saturation) ofinspired air “dilutes” the P of inspiredgases within the alveolusPwater= 47 mm Hg; PNitrogen = 563 mm Hg; Poxygen= 150 mm HgBecause humidification and smallturnover of alveolar air (at end ofinspiration, only ~13% of air inalveoli is fresh air) the alveolarPO2= 100 mm Hg
Alveolar PO2 remains around 100 mm Hg throughout respiratory cycleBlood returning to lungs from tissues still contains O2 (PO2 of systemic venous blood = 40 mm Hg; PCO2 of blood leaving lungs = 40 mm Hg)Gas exchangeNet diffusion of O2 occurs between alveoli and blood, then between blood and tissues as result ofO2 partial pressure gradientsNet diffusion of CO2 occurs between tissues and blood, then between blood and alveoli as result of CO2 partial pressure gradientsO2 and CO2 exchange across capillaries caused by partial pressure gradientsAlveolar PO2 is high and alveolar PCO2 is low – portion of alveolar air is exchanged for fresh atmospheric air with each breathBlood PO2 in lungs pulmonary vein systemic arterial circulation body tissue = ~100 mm Hg P; blood PO2 in pulmonary systemic tissues