Physics of Respiration

Boyle’s Law - the universal law of gases

PV = nRT

P1V1 = P2 V2

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

159.2 20.95% of 760, of nitrogen PN2 is 600.6

79.02% 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% 0.056 x (760 -

47) = 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.