A method of warming a patient during a surgical procedure may involve catheterizing the patient's bladder with a catheter that permits simultaneous flows into and out of the bladder. Fluid may be warmed and pumped into the bladder through the catheter, via a supply path, and simultaneously drained out of the bladder, via a return path. In steady state, there may exist a continuous air-fluid interface along one or more sections of a drain line of the return path having a negative slope with respect to the earth's gravitational field.
