Countdown clocks began ticking Saturday for the shuttle Columbia's blastoff Wednesday on an unprecedented medical research flight featuring seven astronauts, 30 doomed rats and 2,700 tiny jellyfish.

Columbia's four-man, three-woman crew was scheduled to take off at 6 a.m. MDT Wednesday to kick off a planned nine-day Spacelab mission to find out how weightlessness affects human physiology, research considered crucial for future long-duration flights to Mars.Engineers began the ship's intricate countdown to launch at 5 p.m. Saturday, earlier than usual because of extra work required to load the rats and jellyfish aboard the day before liftoff.

In one of several "firsts," civilian cardiologist Andrew "Drew" Gaffney will make the climb to space with a catheter snaked into his chest through veins in his arm to find out how the heart responds to the rigors of launch and the onset of weightlessness.

It is the first in a battery of medical tests expected to provide the best look yet at how the human body reacts to the absence of gravity and how it adapts after the return to Earth.

"The human body is showing that it's able to adapt pretty well to space flight," said astronaut James Bagian, a physician. "In fact, many of the things we're looking at . . . are when we come back, the rapid transition to a weighted environment like we see here on Earth.

Joining the seven human members of the crew will be 30 rats and 2,700 tiny jellyfish. Scientists hope to gain insights into how weightlessness affects the human inner ear by studying how it affects similar "gravity receptors" used by the jellyfish to orient themselves in the sea.

The rats will not undergo any tests during the mission but they will be decapitated after the flight so tissue samples can be studied to find out if rodents can be used to mimic the responses of human systems.