Physics of Respiration Boyle’s Law - the universal law of gases PV = nRT P 1 V 1 = P 2 V 2

Physics of Respiration Dalton’s Law In a mixture of gases the total pressure is the sum of the pressure each gas would exert it were alone. In dry atmospheric air the barometric pressure is 760 mm Hg the PO2 is % of 760, of nitrogen PN2 is % of 760 and of CO2 is 0.2 mm Hg 0.03% of 760.

Physics of Respiration Atmospheric air is never completely dry, and its water vapor exerts a partial pressure PH2O corresponding to the water vapor content in the air. The water content in the air of course is dependent on air temperature The partial pressure of the other gases is than reduced in exact proportion

Physics of Respiration If the 760 mm Hg contains 6.2% of water vapor (the quantity it contains at body temperature) PH2O OF 46.9 The total pressure of the remaining gases is 713 mm Hg and their individual partial pressures are in proportion of their relative concentrations to make up 713 mm Hg

Physics of Respiration In the alveoli some oxygen diffuses into the pulmonary capillaries which reduces the PO2 in alveolar gas.. In alveolar gas the F of O2 is normally 14.3% or.143 (760-47)= 103 mm Hg. The deficit is made up by CO2 that enters from the blood FACO2 = 5.6% x ( ) = 40 mm Hg. OVERHEAD

Physics of Respiration Henry’s Law When a mixture of gases is in contact with a liquid each gas will dissolve in the liquid in proportion to its partial pressure

Ventilation Movement of air bulk flow is proportional to difference in pressure Flow = (P atmospheric - P alveolar) Inspiration = decrease alveolar pressure below atmospheric Expiration = increase alveolar pressure above atmospheric

Ventilation Inspiration -Active process expands the thoracic cavity by expanding rib cage (contracting external intercostal muscles) and lowering the diaphragm ( dome shape at rest, contraction causes it to flatten and hence position). This causes the visceral pleura and causes the lungs to expand-

Ventilation (insp cont.) causing the alveolar pressure to drop to -2 mm Hg air flows in until pressure equalizes- quiet respiration called costal breathing labored inspiration called diaphragmatic breathing involves accessory muscles which help increase the size of the thoracic cavity further (sternocleidomastoid-elevates sternum, scalenes- elevate superior two ribs, pectoralis minor- elevates third through fifth rib

Ventilation Expiration- passive process inspiratory muscles relax, As the internal intercostal relax ribs move downward and as the diaphragm relaxes it resumes its dome shape and pushes upwards. The elastic properties of the lungs help them recoil. As a result lung volume decreases the alveolar pressure increases to + 2 mm Hg and air flows out until pressure equilibrate.

Ventilation (expir. cont.) In labored expiration the process becomes active, abdominal muscles contract moving ribs downward which compress abdominal viscera which push the diaphragm upwards. Contraction of intercostals depress rib cage.