Class 12th Biology 2017 Set1 Outside Delhi Board Paper Solution
Explain the following phases in the menstrual cycle of a human female :
(i) Menstrual phase
(ii) Follicular phase
(iii) Luteal phase
(i) Menstrual phase:
In human females, menstruation is repeated at an average interval of about 28/29 days and the cycle of events starting from one menstruation till the next one is called the menstrual cycle. This cycle starts with the menstrual phase, when menstrual flow occurs and it lasts for 3-5 days. The menstrual flow results due to breakdown of endometrial lining of the uterus and its blood vessels which forms liquid that comes out through vagina. During this phase the levels of estrogen and proges- terone are low.
(ii) Follicular phase:
The menstrual phase is followed by the follicular phase. During this phase the primary follicles in the ovary grow to become a fully mature graffian follicle and simultaneously the endometrium of uterus regenerates through proliferation. The secretion of gonadotropins increase gradually during the follicule phase and stimulates follicular development as well as secretion of estrogens by the growing follicles.
(iii) Luteal phase:
Luteal phase during which the remaining parts of the graffian follicle transform as the corpus luteum. The corpus luteum secretes large amounts of progesterone which is essential for maintenance of the endometrium.
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