blood gas transport

• GAS EXCHANGE

  • carries O2 from alveoli to systemic tissues and CO2 from systemic tissues to alveoli
  • occurs through diffusion and is dependent on:

  1. diffusion surface area (large, moist)
  2. diffusion distance for gases (short)
  3. concentration gradient between alveolar air and blood (differences in partial pressures)
  4. solubility of gases
  5. coordinated blood flow and airflow
• DALTONS LAW OF PARTIAL PRESSURE: the total pressure exerted by a mixture of gases in the sum of the pressures exerted independently by each gases in the mixture
• PARTIAL PRESSURE: the pressure exerted by each gas (directly proportional to its percentage in the total gas exchange)
• HENRY'S LAW = amount of gas that dissolves in water is determined by its solubility in water and its partial pressure in air

  • at equilibrium, the amount of dissolved gas in solution is proportional to the partial pressure of the gas
• HAEMOGLOBIN

  • 4 globular protein subunits (2 alpha + 2 beta)
  • each subunit:

  1. protein (globular)
  2. + non-protein group (haem)
  3. ahem: 4 Fe2+ in a porphyrin ring

  • Hb + O2 <--> HbO2
  • rapid and reversible
  • each Hb molecule (haem portion ) binds to 4 molecules of oxygen
• OXYGEN TRANSPORT

  • approx 97% of O2 transported in blood in combination with Hb
  • after binding with O2, Hb changes shape to facilitate further uptake (positive feedback)
  • Hb SATURATION = percentage of haem units in Hb molecule that contain bound oxygen
  • O2 binding affected by:

  1. PO2 of blood
  2. blood pH
  3. temperature
  4. state of O2 binding go the Hb molecule
• OTHER HAEMOGLOBIN

  • CARBOXYHAEMOGLOBIN

  1. CO binds tighter than O2 (200x tighter)
  2. dramatically reduces ability of O2 to bind to Hb

  • METHAEMOGLOBIN

  1. Fe2+ oxidised to Fe3+ by drugs
  2. unable to carry O2
  3. slowly converted back

  • FETAL HAEMOGLOBIN

  1. 2 alpha + 2 gamma
  2. higher affinity for O2
  3. important in transferring O2 across placenta
• CARBON DIOXIDE

  • generated by aerobic respiration in peripheral tissues
  • CO2 molecules enter the bloodstream + transported by:

  1. 70% converted to carbonic acid formation, H2CO3- and transported in plasma as bicarbonate ion HCO3-
  2. bound to haemoglobin: carbaminohaemoglobin (23%)
  3. dissolved in plasma (7%)
  4. chloride shift
• BUFFERING IN RED CELLS: HAEMOGLOBIN

  • every CO2 --> HCO3- yields a H+
  • pH would become very acidic
  • minimise size of pH changes by consuming or realising H+
  • best buffers in red cells: imidazole groups of histidine residues in haemoglobin
  • deoxygenated haemoglobin has the strongest affinity for H+