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biology 0610printed 217 · PDF 217
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● However if the egg is not fertilised the spongy lining is not needed. It gradually
breaks down and is slowly lost through the vagina. This is called
menstruation.
● It usually lasts for about five days.
● The recurring events of the menstrual cycle are controlled by four hormones.
These include oestrogen, progesterone, FSH and LH.
● Oestrogen and progesterone are secreted by the ovaries. FSH and LH are
secreted by a small gland attached to the base of the brain called the
pituitary gland.
● During the first part of the cycle, following menstruation, both FSH and LH are
produced. FSH causes a follicle to develop in the ovary with a developing
female gamete inside it.
● The developing follicle secretes oestrogen in increasing amounts. Oestrogen
causes the lining of the uterus to become thicker and better supplied with
blood.
● When the follicle is fully developed there is a surge in production of LH. This
causes ovulation. This causes the oestrogen levels to fall.
● The follicle from which the egg was released develops into a structure called a
corpus luteum. This secretes progesterone so its concentration increases.
● It keeps the lining of the uterus thick and spongy, ready to receive a fertilized
egg.
● The increase in progesterone inhibits the pituitary gland which causes FSH
and LH production to decrease.
● If the egg is not fertilized the corpus luteum breaks down so the concentration
progesterone drops and menstruation occurs. Because progesterone is
now not inhibiting the secretion of FSH and LH, their levels start to increase
and the whole cycle runs again.
● However if the egg is fertilized progesterone levels stay high. This happens
because the corpus luteum remains in the ovary and continues to secrete