Lyric provided by www.seekalyric.com

Just the two of us,
In an esophagus,
It's time to get this mission a rollin'.

Don't wanna make a fuss,
But that was the pancreas,
Slow down before we reach the colon.

On the Hemoglobin Highway,
(Hemoglobin...)
In a mini submarine,
(Mini Submarine...)

If you're going my way,
You'll see what I mean,
Hang a left at the spleen...

http://phineasandferb.wikia.com/wiki/Hemoglobin_Highway