THE HORMONAL CYCLE
Menstruation and ovulation result from of a very complex and delicatelybalanced chemical cycle that you go through every month or so.
We'll start with your hypothalamus, which is a gland in your brain.It sends chemical messengers all over your body to control many body functions,including eating, sleeping and menstruating. The hypothalamus is sensitiveto stress and other things going on in your life, so just as you mightbe unable to sleep if you are excited or upset, your menstrual cycle mayalso be influenced by your moods.
The hypothalamus keeps track of the hormone levels in your blood. Atday one of your menstrual cycle, the first day of your period, the levelsof estrogen, an important hormone for women, become very low. The hypothalamusresponds by telling the pituitary gland, which is also in the brain, tosend two kinds of hormones into the bloodstream. One hormone is calledLutenizing Hormone (LH) and the other is called Follicle Stimulating Hormone(FSH). These two hormones are made especially to encourage your ovariesto produce an egg.
When the LH and FSH arrive at the ovaries, they stimulate the growthof 10 to 20 follicles. A follicle is a little bundle of cells that containsan undeveloped egg. Follicle means "bag" or "sack"in Latin. You have around 400,000 follicles in your ovaries when you areborn, more than enough for a lifetime.
As the follicles grow, they start to produce the hormone estrogen. Theestrogen causes the lining of the uterus - the endometrium - to begin togrow again. Remember, this started on the first day of the cycle, whenthe bleeding began. The uterus lining is completely shed now, and needsto start growing again.
Around day 14, one of the follicles is bigger than all the others. Itholds the egg that will be released this month The estrogen levels arevery high at this point. The pituitary responds by decreasing the FSH ithas been releasing, but increasing the LH. This jump in the level of LHis what causes the giant follicle in the ovary, now called the Graafianfollicle, to burst open and explode right through the ovary wall, releasingthe egg. This is ovulation.
The ends of the Fallopian tubes are covered with little wavy fingerscalled fimbria. The fimbria pull the newly released egg up into the Fallopiantube, where it will spend 12 to 36 hours moving toward the uterus. It isduring this time that fertilization could occur if it meets a sperm. Onlyone ovary is stimulated per cycle, and one egg is released. Once in a whiletwo eggs are released, and this results in fraternal twins, twins who arenot identical because they came from two eggs and two sperm.
Back in the ovary, the exploded follicle still has work to do. It isnow called the corpus luteum, Latin for "yellow body" becauseit turns yellow after it releases the egg. It now becomes a temporary kindof gland. It continues to make estrogen, but it really pumps out the hormoneprogesterone ("the pregnancy hormone") The progesterone makesthe new uterine lining rich with nutrients needed to support an embryo.
Progesterone also signals the pituitary to stop sending LH, which iswhat keeps the corpus luteum going. So the corpus luteum only lasts abouttwelve days total, unless the egg is fertilized. As the corpus lutem beginsto die, it stops making progesterone and estrogen. Without these hormonesthe lining of the uterus begins to weaken, tiny blood vessels in it shutdown, and it begins to shed off. This shedding is menstruation. The uterussheds two thirds of its lining at each menstruation, leaving only the bottomlayer to grow again.
One the first day of menstruation, estrogen levels are very low, andthe hypothalamus signals the pituitary gland to send off LH and FSH, andit all begins again.