Which pa of the fallopian tube acts as a functional/ anatomical sphincter?

Ans. a. Isthmus Isthmus of the fallopian tube acts as a functional/anatomical sphincter. The uterine end of the tube, called the isthmus, acts like sphincter, and prevents the embryo from being released into the uterus for 2 days, so that it enters the uterus at the just right time for the implantation.'- How To Have A Baby: Overcoming Infeility By Aniruddha N Malpani/105 'The lumen of the narrow isthmus is relatively simple, with a few longitudinal folds. This poion of its tube is 2 or 3 cm long. There are three layers of musculature: the inner longitudinal, the middle circular layer, and the outer longitudinal layer. There is some evidence that the isthmus may act as a sphincter.'- Clinical Anatomy of the Uterus, Fallopian Tubes, and Ovaries by Eric R Sokol