The participants will be required to attend the laboratory on two occasions for approximately 5 hours each visit, for arm A and B of the study. The two visits will be separated by 7 to 14 days. On each visit standard IV fluids will be administered The evening before each visit participants will be required to take 40mg of the water tablet called furosemide and refrain from drinking fluids. This is used in order to dehydrate the participant and reduce body mass by between 2 - 4%. The participant needs to be dehydrated in order to promote a thirst response so that the interventions can be studied in a cohort of participants representative of hypovolemic patients post surgery