Physiology of Menstruation:
At the age of 12–14 years, the menstrual cycle begins. This marks the age of puberty. The onset of menstruation is known as menarche. During this stage, the uterus, vagina, and breasts enlarge with increased fat deposition, blood vessels, and connective tissue. Secondary sex characteristics such as body hair and voice changes also appear.
Menopause:
At around 40–45 years of age, the reproductive system ceases to function. The menstrual cycle stops, and the ovaries and breasts begin to shrink in size. This stage is called menopause.
Hormonal Regulation of Menstruation:
- Hormones regulate the menstrual cycle. The gonadotropins released from the hypothalamus control secretion of follicle stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland.
- FSH stimulates the development of ovarian follicles and the secretion of oestrogen.
- LH promotes further development of ruptured follicles and stimulates secretion of progesterone, relaxin, and inhibin.
Oestrogen:
Oestrogen is responsible for the development and maintenance of the female reproductive system and secondary sex characteristics such as fat distribution in the breasts, hips, and abdomen, hair growth, voice pitch, and broadening of the pelvis. It also controls electrolyte balance, assists in protein synthesis, and lowers blood cholesterol levels.
Progesterone, Relaxin, and Inhibin:
- Progesterone, secreted from the corpus luteum along with oestrogen, prepares the endometrium for implantation of the fertilized ovum and stimulates the mammary glands for milk secretion.
- Relaxin helps relax smooth muscles of the uterus and the pubic symphysis, enlarging the cervix to aid childbirth.
- Inhibin, secreted by granulosa cells and the corpus luteum, inhibits secretion of FSH and to some extent LH.
Phases of the Menstrual Cycle
The menstrual cycle occurs over approximately 28 days and is divided into three main phases:
- The Proliferative Phase
- The Secretory Phase
- The Menstrual Phase
(a) The Proliferative Phase:
During this phase, FSH from the pituitary gland stimulates the growth and development of the ovarian follicle containing the ovum. The follicle secretes oestrogen, which stimulates thickening of the endometrial lining by increasing the number of glands and blood capillaries. The matured follicle eventually ruptures, releasing the ovum—a process known as ovulation.
(b) The Secretory Phase:
After ovulation, the ruptured follicle transforms into the corpus luteum under the influence of LH, which secretes progesterone. Progesterone, along with oestrogen, enhances endometrial growth, making it thick, soft, and rich in blood vessels to support implantation of a fertilized ovum. The glands in the uterine walls secrete a nutrient-rich fluid, giving this phase its name.
(c) The Menstrual Phase:
If the ovum is not fertilized, progesterone levels fall as the corpus luteum degenerates. This leads to breakdown of the endometrial lining, rupture of blood capillaries, and discharge of the endometrial tissue, blood, and unfertilized ovum as the menstrual flow. When progesterone levels drop sufficiently, the pituitary gland resumes secretion of FSH, initiating the next cycle.
If fertilization occurs, the endometrium remains intact, and no menstrual flow takes place. The fertilized ovum implants in the endometrium, and chorionic gonadotropins maintain the corpus luteum for continuous progesterone secretion. The fertilized ovum is termed an embryo for the first 12 weeks and a foetus thereafter, developing within the uterus for about nine months.
Detailed Notes:
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