Antipyrine and creatinine are control references used to
characterize membrane permeability and perfusion-perfusion overlap.
Antipyrine diffuses rapidly, equilibrating between the two circuits,
while creatinine, a hydrophilic molecule, diffuses more slowly across the placenta. The transport of antipyrine is flow limited [70-72,76],
while creatinine transfer is limited by its hydrophilic property and
transferred through extra-cellular pathways [76-78]. Antipyrine
and creatinine are generally used to normalize data across multiple
experiments [28].