Respiratory Physiology

Physics of diffusion and gas pressures

molecular basis of gas diffusion

net diffusion of a gas in one direction depends on concentration gradient

gas pressures in a mixture of gases depends on partial pressures

Gas Solubility coefficient O2 0.024 CO2 0.57 N2 0.012

Henry’s law ¹

concentration of dissolved gas = pressure × SC

vapor pressure of water

47 mm Hg at 37°C

pressure gradient for diffusion ²

D µ (DP × A × S)/(d × (MW)^0.5)

D, diffusion rate; A, cross-sectional area; S, solubility

diffusion coefficient = S/(MW)^0.5

Gas Relative diffusion coefficient O2 1.0 CO2 20.3 N2 0.53

Boyle’s law ³

P₁V₁ = P₂V₂

Composition of atmospheric, humidified, alveolar, and expired air

Gas Atmospheric air Humidified air Alveolar air Expired air   partial pressure percent partial pressure percent partial pressure percent partial pressure percent N2 597.0 78.5 563.4 74.1 569.0 74.9 566.0 74.5 O2 159.0 20.9 149.3 19.6 104.0 13.7 120.0 15.8 CO2 0.3 0.04 0.3 0.04 40.0 5.26 27.0 3.55 H2O 3.7 0.49 47.0 6.18 47.0 6.18 47.0 6.18

Diffusion of gases through the respiratory membrane

factors affecting

thickness of membrane [0.63 µm on average]

surface area of membrane

diffusion coefficient

pressure difference

Transport of oxygen and carbon dioxide

role of hemoglobin in oxygen transport

carbon dioxide transport

in solution

as carbaminoHb

as bicarbonate ion

Control of respiration

brain stem respiratory centers and rhythmic breathing

components of nervous system control

generating inspiration/expiration rhythm

matching magnitude of ventilation with need

modification of respiratory function

voluntary for speech

involuntary for coughing, sneezing

respiratory control centers

respiratory center (in medulla)

dorsal respiratory group (DRG)

ventral respiratory group (VRG)

apneustic center (in pons)

pneumotaxic center (in pons)

regulation of magnitude of ventilation

role of increased arterial P_CO2

role of decreased arterial P_O2

peripheral chemoreceptors

carotid bodies and aortic bodies

role of increased arterial H⁺ concentration

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Links

• Referenced sites


• ¹ Full discussion of Henry’s law



• ² A technical sidebar with some biological problems concerning diffusion



• ³ Full discussion of Boyle’s law
  a Java applet illustrating Boyle’s law in action



• Other sites


• ChemTutor’s overview of gases and the gas laws