In the two photographs, a blue latex medium was injected into the utero-ovarian vein (UOV) and a red latex medium into the ovarian artery (OA).  The latex was allowed to polymerize and solidify.  The tissue was then dissolved with repeated treatments of saturated sodium hydroxide followed by washings with water until all of the tissue was removed (From Cody et al. 1999. Biol. Reprod. 60(Suppl 1): 90).  The dashed lines in the photo at left approximate the boundaries of the uterine horns (UH) and the ovary (O).  The uterus produces prostaglandin F2α that enters the venous drainage at high concentrations (vein-arrows).  In the photo below PGF2α diffuses from the utero-ovarian vein into the ovarian artery and is transported directly to the ovary (artery-arrows) where it causes luteolysis.

Schematic illustration of the countercurrent diffusion system in the cow, sow and ewe.  A portion of uterine PGF2α  diffuses directly from the utero-ovarian vein into the ovarian artery where it has a direct lytic effect on the corpus luteum.