Menstruation NEET Notes | EduRev

Biology Class 12

NEET : Menstruation NEET Notes | EduRev

The document Menstruation NEET Notes | EduRev is a part of the NEET Course Biology Class 12.
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Duration – 28 days Ideally (Range - 22 to 32 days)

This is exhibited by the primate group of animals. In this cycle, the female body prepares itself for a possible pregnancy. If the pregnancy does not occur then the body aborts all preparation done and restarts the preparation for pregnancy again in a monthly cyclic manner.

Flow Chart of the Menstrual CycleMenstrual cycle has three main phases:

  • Bleeding phase or menstruation phase.
  • Proliferative/preovulatory/follicular phase or oestrogenic phase.
  • Secrectory/post ovulatory/luteal phase or progesteronic phase.
    Schematic Diagram of Menstrual Cycle
Bleeding Phase

The cycle starts with the bleeding phase in its first four to five days. During this bleeding, the part of the layer of the endometrium gets shed off. The total loss of blood per day is about 20 ml, so an average of 40 to 80 ml blood/cycle is lost. This blood can not clot.

Preovulatory/Proliferative Phase
  • After the first four of five days this phase begins. During this phase, due to the release of some GnRH, the Pituitary secretes some FSH to stimulate the ovarian follicle. The ovarian follicle now begins to develop.
    Level of Hormones during Menstruation
  • Developing follicle now starts secreting an increasing amount of oestrogen. The rising level of oestrogen causes the endometrium to proliferate and thicken
  • It also causes an increase in the vascularity and glandularity of the endometrium. The rising level of oestrogen also activates the hypothalamus. 
  • Due to this, the hypothalamus releases more GnRH. This GnRH induces the pituitary to release more of FSH. The rising FSH levels now cause :
    (i) Further growth and development ovarian follicle to form Graafian follicle
    (ii) Even further release of oestrogen from the theca interna of this developing follicle.
  • As the oestrogen level goes on rising, by the end of 10 day the extreme levels of oestrogen (which have by then caused maturation of Graafian follicle and growth of endometrium) now give positive feedback of high concentration of oestrogen causing a rise in LH secretion but due to release of inhibin by graffian follicle, FHS falls secretopm falls, therefore, the LH secretion from the pituitary goes on rising. 
  • This abrupt rise (on 11th to 13th day) in LH concentration in blood is called an LH surge. This LH now causes the Graafian follicle to rupture after partial completion of II meiotic division in oocyte and thus the secondary oocyte released. The release of egg (secondary oocyte) which occurs around 14 days is called ovulation.
Post Ovulatory/Secretory Phase
  • After ovulation, the ruptured Graafian follicle transforms into corpus luteum. The granulosa and theca cells of the ruptured Graafian follicle (which is now called corpus luteum) is found only in mammals and contain yellow lutein or carotene pigment.
  • In case of absence of pregnancy, this corpus luteum will get degenerated after 14 days of its formation. The degenerated corpus luteum is called corpus albicans (white body).

Functions of Corteus Luteam

  • Stimulated by the rising levels of LH, the corpus luteum secretes progesterone hormone.
  • The progesterone facilitates the preparation of the endometrium for receiving the embryo and its implantation. Progesterone inhibits the contractions of the uterus so that the pregnancy could be maintained. 
  • Progesterone also inhibits the development of the next new ovarian follicle. If pregnancy occurs then the corpus luteum persists and secretes progesterone. 
  • Progesterone is important to maintain the pregnancy and it is thus called the pregnancy hormone
  • By the fourth month of pregnancy, the placenta has developed completely. 
  • This placenta now takes over the job of further progesterone secretion. 
  • Ovary also secretes some amount of relaxin at the time of parturition
  • If pregnancy does not occur after ovulation, then as the progesterone level rise, its rising levels inhibit the release of GnRH from the hypothalamus. Due to this FSH, LH secretion by the pituitary falls and thereby progesterone secretion by the corpus luteum (which was due to the influence of LH) also now falls. 
  • As the progesterone level drops, the corpus luteum begins to degenerate and transform in corpus Albicans (which can not secrete progesterone). Due to the lack of progesterone.

1) The overgrown endometrium now begins to break and separate from the inner uterine wall causing bleeding.

2) The uterine contraction (which was till now inhibited due to the presence of progesterone) now start.

Thus the separated endometrium along with blood is now being passed out via the vaginal route. This is again the beginning of the next menstrual or bleeding phase. The period between ovulation and the next menstrual bleeding (post-ovulatory period) is always constant (i.e. 14 days). However, the ovulation date may vary (causing a change in the pre-ovulatory period). After ovulation, the ovum is viable only for two days, while sperms introduced into the vagina can survive for a maximum of four days. On the basis of the above data, the safe period method for family planning is calculated. Normally it is considered to be day 1 to day 8 and then from day 20 to day 28.

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