Changes

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During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF_2α. Normally the concentration of PGF_2α in arterial blood is relatively low due to extensive metabolism via the lungs. However there is a large concentration gradient from the uterine vein to the ovarian artery resulting in higher concentrations of PGF_2α than in systemic circulation. If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF_2α remain below a threshold level allowing the corpus luteum to continue to secrete prostaglandin. There are two main factors involved in the regulation of uterine secretions of PGF_2α; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy.

During the latter stages of the luteal phase the corpus luteum begins to regress under the effect of PGF_2α. Normally the concentration of PGF_2α in arterial blood is relatively low due to extensive metabolism via the lungs. However there is a large concentration gradient from the uterine vein to the [[Ovary_-_Anatomy_%26_Physiology#Arterial_Supply|ovarian artery]] resulting in higher concentrations of PGF_2α than in systemic circulation. If a pregnancy is to remain viable then luteolysis needs to be avoided and this is achieved where concentrations of PGF_2α remain below a threshold level allowing the corpus luteum to continue to secrete prostaglandin. There are two main factors involved in the regulation of uterine secretions of PGF_2α; oxytocin secretions from the corpus luteum and molecules secreted by the developing embryo that facilitate the maternal recognition of pregnancy.

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Oxytocin secretion via the corpus luteum stimulates endometrial production of PGF_2α and by the end of the luteal phase the concentration of oxytocin and the number of oxytocin recptors within the endometrium allow the production of enough PGF_2α to breach the threshold level and cause luteolysis. During pregnancy the embryonically produced pregnancy recognition molecules inhibit the secretion of PGF_2α from the endometrium ensuring that luteolysis cannot occur.

Oxytocin secretion via the corpus luteum stimulates endometrial production of PGF_2α and by the end of the luteal phase the concentration of oxytocin and the number of oxytocin recptors within the endometrium allow the production of enough PGF_2α to breach the threshold level and cause luteolysis. During pregnancy the embryonically produced [[Maternal_Recognition_of_Pregnancy_-_Anatomy_%26_Physiology|pregnancy recognition]] molecules inhibit the secretion of PGF_2α from the endometrium ensuring that luteolysis cannot occur.

===Prostaglandin (PGE₂)===

===Prostaglandin (PGE₂)===