Principles of Gas Exchange

what is gas exchange

• O₂ uptake into and CO₂ elimination from blood

• occurs by simple diffusion

rate of diffusion

• rate of diffusion α A x ΔP

• A=surface area

• ΔP= pressure gradient

factors affecting diffusion

• surface area

• pressure gradient

• thickness of membrane

• physicochemical properties

membrane of alveolus

pressure gradient

• difference in partial pressure of respective gases in alveolus and the blood

• partial pressure→ pressure it would exert if it was the only gas in the container

universal gas equation

pV=nRT

dalton’s law of partial pressure

• partial pressure of gas in gas mixture= total pressure x fractional concentration of gas

partial pressure of oxygen in alveoli

• lower than atmospheric pressure:

  • inspired air humidified in upper airway

  • in alveoli→ oxygen taken up, CO₂ added

  • body consumes more O₂ molecules than it produces CO₂ molecules (1.25x more O₂)→ due to respiratory quotient

effect of humidification

• at 37C saturated vapour pressure ≈ 6.3kPa

• in fully humidified air at 37C partial pressure of oxygen= (101.3-6.3) x 0.21=19.95kPa

effect of CO₂

• typical value of alveolar partial pressure of CO₂ is 5kPa

  • if one CO₂ was produced for every O₂ consumed= ppO₂ would be (19.95-5)=14.95kPa

• however, 1.25x more O₂ than CO₂ so ppO₂ in alveoli→ 13.7 kPa

how does carbon dioxide and oxygen move into/out of blood

• simple diffusion through fluids

what primarily affects partial pressure of gases in fluids

• solubility of gas in fluid

  • less soluble→ more remains out of solution and is free to pass out of solution to create partial pressure

carbon dioxide solubility compared to O₂

CO₂ is approx. 24 times more soluble in water than oxygen

theoretical rate of diffusion of O₂ and CO₂

• more soluble and therefore higher diffusing capacity

• in theory, CO₂ transfer much more efficient than O₂ transfer

effect of higher diffusing capacity of CO₂

alveoli/ capillary gap equilibrates quicker

partial pressures of O₂ and CO₂ of healthy person at rest

partial pressures of O₂ and CO₂ of healthy person during exercise

partial pressures of O₂ and CO₂ of patient with pulmonary fibrosis during exercise

• results in type 1 respiratory failure (hypoxia)

clinical measurement of diffusing capcity

• mean pulmonary capillary PO₂ is difficult to measure, so this is generally not used

carbon monoxide diffusing capacity (transfer factor)

• mean pulmonary capillary pCO effectively zero because of high affinity of CO for haemoglobin

alveolar fibrosis

• thickening of alveolar wall

• higher diffusion distance→ less O₂ in lungs

pneumonia

• alveolar consolidation

• gas cannot get into membranes

pulmonary edema

• frothy secretions

• often due to left-sided heart failure

• pink frothy sputum

interstitial edema

• froth in between alveolus and capillary

• sepsis, surgery reaction, trauma

emphysema

• alveolar-capillary destruction

• caused by smoking

atelectasis

• alveolar collapse

• often caused by anaesthesia

CRX of pneumonia

• infection in lung

CRX of pulmonary oedema

adult respiratory distress syndrome

• response to disease

• lungs become stiff