The second phase is the study itself which will take place whilst volunteers are having their bowel operation. They will attend theatre in the normal way, but once they have been anaesthetised (put to sleep), a special monitor called an oesophageal doppler probe will be placed into their oesophagus (food pipe) via the nose. This monitor is frequently used in bowel surgery to help assess how much intravenous fluid to administer to a patient by measuring the cardiac output (the amount of blood pumped out of the heart each minute). Using the cannula (drip) already inserted in the arm to allow administration of the anaesthetic, a special fluid, called an ultrasound contrast agent, will be injected into the drip, to allow a contrast enhanced ultrasound scan of the abdominal organs to be performed, to measure the blood flow to these organs. A small sample of blood will be taken from the earlobe to allow us to measure a chemical in the blood called lactate.