A human heart, donated for use of its valves in a transplant, was accidentally left on board a Southwest Airlines flight, forcing a Dallas-bound plane to make a midair U-turn back to Seattle, officials said Thursday.

The organ was loaded onto Flight 3606 in Sacramento on Sunday and set to be picked up by a courier at Seattle–Tacoma International Airport shortly after landing at 2:15 p.m. PT (5:15 p.m. ET) — but was accidentally left on board, officials said.

Flight 3606 took off from Seattle at 3:02 p.m. PT with the heart still on board and was bound for Dallas before the pilot of the Boeing 737-700 made a U-turn over eastern Idaho, about 69 minutes into the flight, according to FlightAware tracking records.

It landed back in Seattle at 5:56 p.m. PT — more than 3 1/2 hours after the jet initially landed in the Emerald City, according to FlightAware.

A human heart, set for transplant, typically has a tight window of viability — between four and six hours — between the time it is goes from the donor into the chest of a recipient.

But in this case, the heart was harvested only for its valves, and thus had 48 hours of viability until reaching a lab, said Deanna Santana, spokeswoman for Sierra Donor Services, which organized the donation and delivery to the Sacramento airport.

"This delay did not affect the tissue transplant. It got to the lab in plenty of time to be used," Santana told NBC News. "There was no patient in a hospital waiting for a valve."

Hearts that are harvested only for their valves are typically transported in cargo without a human carrying it, Santana said. She declined to reveal the name of the Seattle-area hospital or courier service that were responsible for picking up the organ at that city's airport.

Even after the flight landed back at Seattle, and the heart was taken off the plane, it was not a quick turnaround for the passengers who had been on board since mid-afternoon.

Due to a mechanical issue unrelated to the delayed heart delivery, Southwest ordered a new plane and crew for the flight. So, passengers did not arrive at Dallas until 4:22 a.m. CT on Monday, more than seven hours after they were supposed to get there.

New Orleans Dr. Andrew Gottschalk, an orthopedist, who was on board Flight 3606, told NBC News that looking back he questions whether Southwest needed to turn the flight around to deliver the heart.

He said Southwest should have known that the heart wasn't being used for a full transplant — and thus didn't urgently need to be returned to Seattle.

"Passengers were happy to be inconvenienced, even horrifically inconvenienced, to save someone's life," Gottschalk said. "But that wasn't the case here."

Southwest declined to comment on specifics of the matter, beyond a statement acknowledging the turnaround.

"During Flight #3606 with scheduled service from Seattle to Dallas this past Sunday evening, we learned of a life-critical cargo shipment onboard the aircraft that was intended to stay in Seattle for delivery to a local hospital," the airline said in a statement on Monday.

"Therefore, we made the decision to return to Seattle to ensure the shipment was delivered to its destination within the window of time allotted by our cargo customer."