FUNCTION OF THE FEMALE REPRODUCTIVE SYSTEM
At puberty (usually between ages 12 and 14, but earlier for some; 10 or 11 years of age is not uncommon), the ova begin to mature. During a period known as the follicular phase, an ovum enlarges as a type of cyst called a graafian follicle until it reaches the surface of the ovary, where transport occurs.
The ovum (or oocyte) is discharged into the peritoneal cavity. This periodic discharge of matured ovum is referred to as ovulation. The ovum usually finds its way into the fallopian tube, where it is carried to the uterus. If it meets a spermatozoon, the male reproductive cell, a union occurs and conception takes place. After the discharge of the ovum, the cells of the graafian follicle undergo a rapid change. Gradually, they become yellow (corpus luteum) and produce progesterone, a hormone that prepares the uterus for receivingthe fertilized ovum. Ovulation usually occurs 2 weeks prior to the next menstrual period.
The menstrual cycle is a complex process involving the reproduc-tive and endocrine systems. The ovaries produce steroid hormones, predominantly estrogens and progesterone. Several different es-trogens are produced by the ovarian follicle, which consists of thedeveloping ovum and its surrounding cells. The most potent of the ovarian estrogens is estradiol. Estrogens are responsible for developing and maintaining the female reproductive organs and the secondary sex characteristics associated with the adult female. Estrogens play an important role in breast development and in monthly cyclic changes in the uterus.
Progesterone is also important in regulating the changes that occur in the uterus during the menstrual cycle. It is secreted by the corpus luteum, which is the ovarian follicle after the ovum has been released. Progesterone is the most important hormone for conditioning the endometrium (the mucous membrane lining the uterus) in preparation for implantation of a fertilized ovum. If pregnancy occurs, the progesterone secretion becomes largely a function of the placenta and is essential for maintaining a normal pregnancy. In addition, progesterone, working with estrogen, pre-pares the breast for producing and secreting milk. Androgens are also produced by the ovaries, but only in small amounts. These hormones are involved in the early development of the follicle and also affect the female libido.
Two gonadotropic hormones are released by the pituitary gland: FSH and LH. Follicle-stimulating hormone (FSH) is primarily responsible for stimulating the ovaries to secrete estro-gen. Luteinizing hormone (LH) is primarily responsible for stimulating progesterone production. Feedback mechanisms, in part, regulate FSH and LH secretion. For example, elevated es-trogen levels in the blood inhibit FSH secretion but promote LH secretion, whereas elevated progesterone levels inhibit LH secre-tion. In addition, gonadotropin-releasing hormone (GnRH) from the hypothalamus affects the rate of FSH and LH release.
The secretion of ovarian hormones follows a cyclic pattern that results in changes in the uterine endometrium and in menstrua-tion (Fig. 46-3; Table 46-1). This cycle is typically 28 days inlength, but there are many normal variations (21 to 42 days). In the proliferative phase at the beginning of the cycle (just after men-struation), FSH output increases, stimulating estrogen secretion. This causes the endometrium to thicken and become more vascu-lar. In the secretory phase near the middle portion of the cycle (day 14 in a 28-day cycle), LH output increases, stimulating ovulation. Under the combined stimulus of estrogen and progesterone, the endometrium reaches the peak of its thickening and vasculariza-tion. The luteal phase begins after ovulation and is characterized by the secretion of progesterone from the corpus luteum.
If the ovum is fertilized, estrogen and progesterone levels re-main high and the complex hormonal changes of pregnancy fol-low. If the ovum has not been fertilized, FSH and LH output diminishes, estrogen and progesterone secretion falls, the ovum disintegrates, and the endometrium, which has become thick and congested, becomes hemorrhagic. The product consisting of old blood, mucus, and endometrial tissue is discharged through the cervix and into the vagina. After the menstrual flow stops, the cycle begins again; the endometrium proliferates and thickens from estrogenic stimulation, and ovulation recurs.
The menopausal period marks the end of a woman’s reproductive capacity. It usually occurs between the ages of 45 and 52 years but may occur as early as age 42 or as late as age 55; the median age is 51. Menopause is not a pathologic phenomenon but a normal part of aging and maturation. Menstruation ceases, and because the ovaries are no longer active, the reproductive organs become smaller. No more ova mature; therefore, no ovarian hormones are produced. (An artificial menopause may occur earlier if the ovaries are surgically removed or are destroyed by radiation or chemotherapy.) Besides changes in the reproductive system that reduce estrogen levels, multifaceted changes occur throughout the woman’s body. These changes include neuroendocrinologic, bio-chemical, and metabolic alterations related to normal maturation or aging (Table 46-2).
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