Measurement

Question 1

Measurements of body fluid volumes

Answer 1

• Inulin is used to measure ECF volume, mannitol may also be used
• Deuterium oxide or tritium are isotopes of water, and can be used to measure total body water. ECF volume can be subtracted from this to give ICF volume
• Plasma volume can be measured by radio-iodinated albumin
• Total red cell volume can be measured with chromium-labelled red cells

Question 2

Severinghaus carbon dioxide electrode

Answer 2

• Used to directly measure the partial pressure of dissolved cabon dioxide in a solution
• The device measures H+ concentration, which is proportional to the amount of dissolved CO2 present, as it exists in a chemical equilibrium
• CO2 + H2O <> H2CO3 <> H+ + HCO3-
• CO2 from blood diffuses across a teflon semi-permeable membrane into the sodium bicarbonate buffer solution. CO2 reacts with water to form carbonic acid, with some of this dissociating into H+ and HCO3- ions, in proportion to the amount of dissolved CO2 present. The concentration of H+ is then measured by an electrode which is separated from the solution by a hydrogen ion selective glass membrane. The bicarbonate solution is in direct contact with a reference electrode, The potential difference between the measuring and reference electrode is measured by a galvanometer and concerted to a partial pressure of CO2
• The response time is ~2 minutes

Question 3

Blood gas analysis

Answer 3

• Severinghaus electrode - PCO2
• Clark electrode - PO2
• Sanz electrode - pH
• Reference electrode
• May directly measure Hb concentration and ion concentrations
• Derived values include bicarbonate concentration, base excess and oxygen saturation