Menstrual Cycle


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menstrual cycle

[′men·strə·wəl ′sī·kəl]
(physiology)
The periodic series of changes associated with menstruation and the intermenstrual cycle; menstrual bleeding indicates onset of the cycle.

Menstrual Cycle

 

a complex physiological process characterized by changes throughout the female body that recur every 21-30, but in most cases, every 28 days. The principal external manifestation of the menstrual cycle is bleeding from the genitalia (specifically, from the uterus), or menstruation. Menstruation begins during puberty, at age 11-15, and continues until the menopause at age 45-55. The onset of menstruation (menarche) varies with climate, living conditions, nutrition, and social and hygienic conditions. The cycle is not always established immediately. At first there may be delays, but after six to 12 months the cycle acquires a rhythm that recurs at regular intervals in each girl (woman). Menstruation does not occur before puberty, during pregnancy and breast feeding, and after the onset of the climacteric. Only human beings and anthropoid apes have menstrual cycles.

The menstrual cycle is regulated by highly differentiated nerve nuclei, the sex centers, which are located in the middle of the diencephalon in the hypothalamus. Complex substances called releasing hormones, or releasing factors, originate and accumulate in these centers. The hormones travel through the neural and vascular pathways to the hypophysis (pituitary), where they influence the secretion of gonadotropins, or gonadotropic hormones. The synthesis and secretion of each of the gonadotropic hormones by the hypophysis are controlled by a specific releasing factor. The gonadotropic hormones are carried in the bloodstream to every organ.

The most pronounced changes caused by the menstrual cycle occur in the uterus and ovaries. In the ovaries hormones affect the growth and maturation of the follicle containing the ovum. The mature follicle ruptures, and the ovum and the follicular fluid enter the abdominal cavity and, subsequently, the uterine tube. The process by which the follicle ruptures and the mature ovum emerges, ready for fertilization, is called ovulation, which in a 28-day cycle generally occurs between the 13th and 15th day, counting from the first day of menstruation. The corpus luteum is formed from the ruptured follicle. These morphological changes in the ovary are accompanied by the secretion of the steroid sex hormones—estrogens, which are secreted by the maturing follicle, and progesterone, which is released by the corpus luteum.

In the first phase of the menstrual cycle, the estrogens stimulate the regeneration and proliferation of the mucous membrane of the uterus (the endometrium) and the growth of the glands, which lengthen and become convoluted. The endometrium becomes four to five times thicker. The cervical glands secrete more mucus, and the cervical canal dilates, making it easier for spermatozoa to pass into the uterus. Epithelium is proliferated in the lactiferous ducts of the mammary glands.

In the second phase of the menstrual cycle, progesterone slows the metabolic rate. The proliferation of the endometrium in the body of the uterus ceases. The endometrium becomes looser and hydrated. The endometrial glands begin to secrete, creating favorable conditions for the fertilized ovum to attach itself to the endometrium and for the embryo to develop. The glands stop secreting mucus, and the cervical canal closes. In the mammary glands alveoli capable of producing and secreting milk develop from the epithelium that was proliferated in the terminal portions of the lactiferous ducts.

If pregnancy does not occur, the corpus luteum degenerates, the functional layer of the endometrium is sloughed off, and menstruation begins. The first day of menstruation is considered the first day of the new menstrual cycle. Menstrual bleeding lasts from three to six or seven days, depending on a woman’s physiological and physical characteristics. The blood loss varies from 40 to 150 g. Prior to menstruation some women have a somewhat rapid pulse and high arterial blood pressure. Some are irritable, tire easily, or feel sleepy. Owing to physiological changes in the functioning of the nervous system, wavelike changes occur in the circulation, thermoregulation, and metabolism. If the menstrual cycle is normal, these changes are within the limits of normal physiological vacillations and do not lower the woman’s capacity for work.

The main hygienic requirements during menstruation are cleanliness and sufficient physical and mental rest. Swimming in a river or ocean is not recommended, and showers are preferable to baths. Spicy food and alcohol should not be consumed because they cause blood to rush to the abdominal organs, thus increasing uterine bleeding. Sexual relations during menstruation are out of the question.

The menstrual cycle may be disrupted by certain diseases, by inadequate nutrition, or by psychological stress. Disturbances of the cycle are manifested by the absence of menstruation (amenorrhea), decreased menstrual flow (hypomenorrhea), extremely profuse flow (menorrhagia), brief or infrequent menstruation (oligomenorrhea), and painful menstruation (dysmenorrhea). Treatment aims at eliminating the main causes of the disruption of the cycle.

REFERENCES

Osnovy endokrinologicheskoi ginekologii. Moscow, 1966.
Kvater, E. I. GormonaVnaia diagnostika i terapiia v akusherstve i ginekologii, 3rd ed. Moscow, 1967.

L. S. PERSIANINOV

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