The thick muscles of the heart contract (tighten up) to pump blood out and then relax (loosen up) to let blood back in after it's circulated through the body.
(The thick muscles of the heart contract; tighten up; to pump blood out and then relax (loosen up) (it; 's circulated; ) (The thick muscles of the heart contract; to pump out; blood)
The thick muscles of the heart contract it The thick muscles of the heart contract
tighten up 's circulated to pump out
to pump blood out and then relax (loosen up
blood