The menstrual cycle lasts about 28 days, although there is some variation.
It involves interactions between the pituitary gland in the base of the brain, the follicles in the ovary, and the lining of the uterus.
The cycle is normally counted from the beginning of menstruation - due to the lining of the uterus peeling away, with associated loss of blood.
After this stage, FSH (follicle stimulating hormone) together with LH (luteinising hormone), both released from the pituitary, stimulate a follicle in the ovary, causing it to develop so that the ovum (egg) within it matures. This also causes the ovary to release the hormone oestrogen.
The combination of FSH and LH, and oestrogen, has a positive feedback effect, causing the release of more and more oestrogen, FSH and LH.
Oestrogen causes the muscle and lining layers of the uterus to grow thicker, in preparation for the possible embryo.
The surge of LH on or about day 14 causes ovulation (release of an egg from the follicle). The ovum enters the oviduct (Fallopian tube) and travels towards the uterus.
Along the way, it may or may not meet up with a sperm cell and become fertilised.
The remaining section of the follicle develops into the corpus luteum (“yellow body”), which secretes the hormone progesterone for several days.
Progesterone has the effect of maintaining the lining of the uterus, and developing more blood vessels. This would be necessary for the interchange of materials with an embryo developing from a fertilised egg, if present.
The combination of oestrogen and progesterone and has a negative feedback effect on the pituitary, stopping the release of FSH and LH.
If the ovum is not fertilised
If the ovum is fertilised
Eventual deterioration of the corpus luteum causes progesterone production to stop.
Falling levels of progesterone and oestrogen cause the uterine lining to shrink and lose blood – menstruation.
Increased FSH and LH production causes the cycle to repeat.
The implanted embryo (ball of cells) produces HCG (human chorionic gonadotrophin) which passes into the mother's blood stream and maintains the corpus luteum so that it continues to produce progesterone, causing steady conditions during pregnancy.
Whereabouts in the body are the following hormones produced?
> corpus luteum within ovary
> pituitary (gland)
> pituitary (gland)
What are the effects of the following hormones ?
> causes the lining layer of the uterus to grow thicker
> maintains the lining of the uterus and provides constant conditions during pregnancy
> starts development of egg before release (stimulates follicle!)
> causes release of ovum (egg) - ovulation