Lecture 4

• Partial pressures of gases in the atmosphere are represented as 1 atm, 101.3 kPa, 1.013 bar, 760 mmHg, and 1033 cmH.
• The partial pressure of oxygen in the alveoli is lower than in room air because inspired air is humidified in the upper airway, oxygen is taken up while carbon dioxide is added, and the body consumes more oxygen molecules than it produces carbon dioxide molecules, typically 1.25 times.
• At 37°C, the saturated vapour pressure of water ≈ 6.3 kPa, meaning the partial pressure of water vapour is 6.3 kPa.
• Normal lung fields, atelectasis, and other conditions can also affect the diffusion pathway.
• Carbon monoxide diffusing capacity is calculated by subtracting the carbon monoxide uptake in the alveolar region from the carbon monoxide uptake in the pulmonary capillary region.
• Carbon monoxide diffusing capacity (transfer factor) is the measure of carbon monoxide uptake by the alveolar-capillary membrane.
• Carbon monoxide diffusing capacity is effectively zero in the case of very high affinity of CO for haemoglobin.
• Pneumonia, pulmonary oedema, and adult respiratory distress syndrome (ARDS) are diseases that affect the diffusion pathway.
• In fully humidified air at 37°C, the partial pressure of oxygen is (101.3 – 6.3) x 0.21 = 19.95 kPa.
• In theory, CO2 transfer is much more efficient than O2 transfer.
• The alveolar partial pressure of CO2 is typically 5 kPa.
• The majority of the pathway involves diffusion through fluids.
• Pulmonary capillary blood oxygen tension (PCO2) is 4.3 kPa in a healthy person during exercise.
• Blood enters capillary.
• Pulmonary capillary blood oxygen tension (PCO2) is 13.3 kPa in a healthy person at rest.
• Oxygen diffusing capacity is measured as oxygen uptake minus alveolar PCO2.
• Alveolar PCO2 is difficult to measure.
• Partial pressure of a gas in solution is inversely proportional to its solubility.
• Pulmonary capillary blood oxygen tension (PCO2) is 13.3 kPa in a patient with pulmonary fibrosis during exercise.
• Carbon dioxide is approximately 24 times more soluble in water than is oxygen.
• If one CO2 molecule was produced for every O2 molecule consumed, the alveolar partial pressure of oxygen would be 19.95 - 5 = 14.95 kPa.
• Rate of diffusion is proportional to the surface area x pressure gradient

  The surface area is dependent on the thickness and physiochemical properties
• The alveolar structure involves:

  1. Capillary
  2. Simple Squamous Epithelia Cells

  
• Pressure gradient is the difference in partial pressure of the respective gases in the alveolus and blood
• PV = nRT
• The partial pressure of a gas in a gas mixture = total pressure x fractional concentration of the gas, according to Dalton’s Law.