A catheter for medical or veterinary use is provided which is especially adapted for gentle and pristine introduction into a body cavity or vasculature. The catheter includes an outer and inner tube with a cylindrical membrane connecting distal ends of the two tubes. A fluid may be introduced into an annular space between the tubes to inflate the membrane, which may have a variable thickness to inflate into various configurations within the body cavity. The catheter may be advantageously provided with a guide wire for directing the catheter during insertion into the body cavity by diverting the distal end of the outer tube. The invention hereof also includes a method of using the catheter including the steps of inserting, directing and locating the catheter, extending inner tube beyond the outer tube, isolating the body cavity or vessel and then withdrawing the catheter from the body cavity or vessel.