Subjects will arrive (fasted) at the lab at 08.00 on the morning of each study visit. A cannula will be inserted retrograde into a hand vein, after prior infiltration at insertion site with 1% lidocaine. The hand will be placed in a heated box, and remain there throughout the trial, to obtain arterialised venous blood samples. The cannula will be kept patent with a slow infusion of 0.9% saline, and samples will be taken via a three-way tap.