Tuck is fighting a villain inside Jack's body, and his vehicle is low on power and oxygen reserves. Tuck asks Jack to produce more stomach acid so he can take care of the villain. Jack delivers the acid, and as Tuck claims, he digests the bad guy.
Tuck travels quickly through the bloodstream because Jack's heart rate has increased from being pursued by some villains. Tuck tries to stop his vehicle and avoid entering the heart through the tricuspid valve, which could be fatal to Jack.