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Your heart never rests, and during hard exercise it can push your entire blood supply around in just seconds.
At rest, a full circuit of blood through the body, from the heart out through the arteries, into capillaries feeding every tissue, and back through the veins, takes roughly a minute. The heart pumps continuously, pushing blood into the aorta and through smaller vessels until it reaches individual cells, before collecting waste and returning for another pass through the lungs and heart.
An hour is far too slow, that pace would leave tissues starved of oxygen for long stretches, while a mere fifteen minutes undersells how efficiently a resting heart moves blood; the one-minute figure matches typical resting heart rate and blood volume.
During intense exercise, circulation time can shrink dramatically because the heart beats faster and pumps more per beat, so your entire blood supply can complete a loop in well under the resting time, which is part of why muscles get fresh oxygen so quickly during a sprint.
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