Gas Exchange

• Alveolar-capillary exchange


  Mechanism: Simple diffusion
• Processes responsible for movement of O2 & CO2

  • Convection - active, requires energy, acts over long distances (airways)
  • Diffusion - passive, no energy required, acts over short distances (alveoli)
• Gas exchange

  The movement of O2 & CO2 through the capillaries (lungs & tissues) depends on gas diffusion

  
• Factors that influence rate of gas diffusion across the respiratory membrane

  • The Pressure gradient for the gas
  • The Diffusion coefficient of the gas
  • The Tissue properties (surface area & thickness of membrane)

  
• Partial pressure


  The pressure a gas would exert if it alone occupied the total volume available to the mixture of gases
• Dalton's law: the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by the individual gases
• Partial pressure gradients

  The pH2O in the alveoli equals the saturated water vapour pressure at 37°C (47mmHg) irrespective of the total alveolar pressure
• Alveolar air differs from the atmospheric air: it has higher PH2O, lower O2 because of constant removal of O2 in the pulmonary blood, and higher CO2 (addition of CO2)

  
• Driving force for diffusion

  Partial pressure difference (ΔP) between alveoli and pulmonary capillary blood for each gas

  
• ΔPO2= (PAO2 – Ppulm cap O2) = 60 mmHg, ΔP CO2 = (Ppulm cap CO2 – PACO2) = 5 mmHg
• Diffusion coefficient


  A measure of the ease with which the gas can diffuse through the body fluids
• Diffusion coefficient for O2 = 1.0, CO2 = 20.3, N = 0.53

  
• The six components of the alveolar-capillary barrier

  • air/fluid interface - a fluid layer containing surfactant
  • the alveolar epithelium
  • a fluid-filled interstitial space
  • the capillary endothelium with basement membrane
  • the plasma
  • the erythrocyte membrane
• Tissue properties at the site of exchange

  Surface area & diffusion distance
• Large alveolar area (~70 m2) available for diffusion (in contact with pulmonary capillary blood), Small diffusion distance

  
• Diffusing capacity of the lung (DL)


  A measurement of the lung's ability to transfer gases, the amount of a gas that is exchanged in a minute between alveolar air and capillary blood per unit partial pressure difference

  
• RBC spend ~ 0.75 s in pulmonary capillaries at rest (equilibration achieved within 0.25 sec), the last 0.50 sec of the transit time provide a safety margin
• Ventilation and perfusion

  Must be tightly regulated for efficient gas exchange
• Alveolar Ventilation (VA)


  The amount of gas reaching the alveoli

  
• Perfusion (Q)

  The blood flow reaching the alveoli
• Ventilation/Perfusion Ratio (VA/Q)

  The most efficient gas exchange occurs when VA/Q is ~ equal to 1 (0.8)
• Normal ventilation to perfusion ratio: for the lungs under normal conditions, the mean value for VA/Q ~ 0.8

  
• VA/Q imbalances

  The most common cause of hypoxia
• At the apex of the lung VA/Q ~ 3.0 (contributes to a physiological dead space), V/Q ~ 0.6 at the bases (contributes to the physiological shunt)

  
• High VA/Q ratio

  Caused either by excessive ventilation or inadequate blood flow to an area of a lung (alveoli are ventilated but not properly perfused)

  
• Low VA/Q ratio

  Caused by inadequate ventilation or excessive blood flow to an area of a lung, can lead to arterial hypoxia
• Oxygen in blood


  1.5% dissolved in plasma, 98.5% carried by hemoglobin
• Oxygen Content (CaO2)


  The total amount of O2 being carried by the blood (sum of O2 bound to Hb and O2 dissolved)
• Normal arterial O2 content (CaO2) ~200 ml O2/L blood (at PaO2 = 95 mmHg), normal venous O2 content (CvO2) ~150 ml O2/L blood (at PvO2= 40 mmHg)
• ~ 50 ml of O2 are released from every 1 L of blood in the systemic capillaries to supply tissue metabolism
• Hemoglobin O2 saturation (SaO2)

  The percentage of Hb that is combined with O2
• Hemoglobin-oxygen affinity

  Represented by the P50, the PaO2 required to achieve 50% hemoglobin saturation (~ 27 mmHg)
• Influences on the dissociation curve (changes of Hb affinity to O2)

  Increased pCO2, decreased pH, increased temperature, increased 2,3-bisphosphoglycerate (BPG)
• Bohr effect

  Oxygen affinity decreases in the presence of increased pCO2 (H+)
• Oxygen delivery


  The amount of O2 released from Hb to the tissues, normally ~ 50 ml of O2 are unloaded to the tissues by each liter of blood flow (at rest)
• The percentage of blood that gives up its O2 as it passes through the tissues = utilization coefficient => ~ 25%
• Factors affecting O2 content

  • Hemoglobin concentration
  • Effects of CO poisoning (HbCO 50%) & anemia (Hb 75 g/L)
• Dissociation curve


  • Shifts right under certain conditions (factors aid in releasing O2 from Hb)
• Factors that aid in releasing O2 from Hb


  • Ï pCO2
  • Ï[ H+] = low pH
  • Ï temperature
  • Ï 2,3-bisphosphoglycerate(BPG)
• These parameters are all high in systemic capillaries where oxygen unloading is a goal