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Fertilisation, Female Reproductive System - Menstrual Cycle, Hormones and…
Fertilisation
During sexual intercourse, about 300 million sperm enter the vagina.
Soon afterward, millions of them will either flow out of the vagina, or die in its acidic environment.
However, many survive because of the protective elements provided in the fluid surrounding them.
Next, the sperm must pass through the cervix, an opening into the uterus. (usually it remains tightly closed, but here the cervix is open for a few days while the woman ovulates).
The sperm swim through the cervical mucus, which has thinned to a more watery consistency for easier passage.
Once inside the cervix, the sperm continue swimming toward the uterus, though millions will die trying to make it through the mucus. (some sperm remain behind, caught in the folds of the cervix, but they may later continue the journey as a backup to the first group).
Inside the uterus, muscular uterine contractions assist the sperm on their journey toward the egg (however, resident cells from the woman's immune system, mistaking the sperm for foreign invaders, destroy thousands more).
Next, half the sperm head for the empty fallopian tube, while the other half swim toward the tube containing the unfertilized egg.
Now, only a few thousand remain. Inside the fallopian tube, tiny cilia push the egg toward the uterus.
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Female Reproductive System - Menstrual Cycle, Hormones and Regulation
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The ovaries connect to the Uterus via the Fallopian Tube essentially, but it's now known as a uterine tube.
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So as the follicle matures, they will produce more estrogen which, at low concentrations, will inhibit the secretion of luteinizing hormone and also the increase in estrogenic concentrations will cause a decrease in follicle stimulating hormone secretion.
Estrogen is an important hormone, especially for females because it is basically males equivalent of testosterone.
Estrogen peripheral effects include stimulating bone and muscle growth, it stimulates endometrial growth, maintains female secondary characteristics and maintains a female the glands, the breasts amongst many other things.
Up to ten days of the menstrual cycle, there is a steady low concentration of luteinizing hormone due to the inhibitory effects of estrogen and we also see a steady drop of follicle stimulating hormone due to the increase in estrogen levels as well, which has a negative feedback.
After 10 days, estrogen levels will continue to rise as the follicles mature in the ovaries and after 10 days, as estrogen levels rise, it will have a positive feedback and it will stimulate the release of luteinizing hormone.
So, at low concentrations, estrogen will inhibit luteinizing hormone secretion, however, at high concentrations, estrogen will stimulate luteinizing hormone secretion.
Now, things are changing the increase in gonadotropin releasing hormone and oestrogen will stimulate then, luteinizing hormone secretion.
The massive luteinizing hormone concentration will trigger ovulation of the most mature follicle in the ovary.
The ovulation of the follicle will release what we know as the female egg, the oocyte.
After ovulation, the luteinizing hormone levels will drop back down and Gonadotropin releasing hormone will slowly drop as well, follicle stimulating hormone also had a small spike as a side effect of the surge of luteinizing hormone release.
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And the last 14 days is known as the luteal phase because we have the formation of the corpus luteum.
The corpus luteum will essentially slowly degrade however, it also has a purpose; it secretes hormones (it secretes three hormones; estrogen again, inhibin and progesterone).
So, what we see in the concentration levels of these ovarian hormones, oestrogen will increase until ovulation and then drop slightly, inhibin was not present until after ovulation, then it begins to increase thanks to the corpus luteum, progesterone levels were low until after ovulation during the luteal phase.
So, at 21 days of the female reproductive cycle, progesterone is increasing, inhibin is increasing and estrogen is still detectable and all these hormones are all produced thanks to the corpus luteum.
Inhibin has a negative feedback and it will essentially inhibit the secretion of follicle stimulating hormone. Because we are at the luteal phase, we don't need any more follicles to mature just yet. So that is why inhibin is preventing follicle stimulating hormone release to prevent follicle maturation.
As the secondary corpus luteum develops, inhibiting secretion will suppress follicle stimulating hormone release.
Progesterone is the most important hormone in the luteal phase and has many functions, one of which is having a negative effect on the hypothalamus, inhibiting the secretion of gonadotropin releasing hormone.
Now, after ovulation during the luteal phase, the progesterone levels will increase slowly and the oestrogen levels will decrease slowly, kind of, and this will suppress gonadotropin releasing hormone release.
And so, progesterone will inhibit gonadotropin releasing hormone release which will also affect the release of luteinizing hormone and follicular stimulating hormone.
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Ovulation
The reproductive cycle begins with ovulation, the release of a ripe egg, or ovum, from the ovary.
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