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Chapter 1: Hormones and the Female Reproductive System
To understand menopause and the changes it brings, it is important to have a basic understanding of the body's hormonal, or endocrine, system and the female reproductive system. This information will help you understand more about how your body works, how it changes at menopause, what you can do to minimize the effects, and how various medications work in your body.
Hormones are chemical substances produced in the body by organs called endocrine glands. When a hormone is released by an endocrine gland into your bloodstream, it produces an effect in a specific part of the body (sometimes called a "target" organ). Hormones act as "chemical messengers" that help control and coordinate various body functions, such as metabolism (the way the body turns food into energy), growth and development, sex and reproduction, composition of the blood, reaction to emergencies, and the release of hormones themselves. If the body does not produce the needed amount of each hormone, serious health problems can occur.
The major endocrine glands include the adrenal glands, the pituitary gland, the thyroid gland, the parathyroid glands, and the ovaries.
As the body develops from infancy to adulthood, complex physical changes take place; hormones play a major role in regulating these changes. Puberty, the period of sexual development, begins between ages 10 and 15. At the start of puberty, the hypothalamus (the part of the brain that controls many basic bodily functions, including the endocrine system) secretes more gonadotropin-releasing hormone (GnRH). This hormone stimulates the pituitary gland to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which are the gonadotropic hormones. These hormones in turn act on the ovaries.
Under the influence of FSH and LH, the ovaries develop and begin to release large amounts of sex hormones into the bloodstream. The female sex hormones are progesterone and estrogen. The adrenal glands also secrete some sex hormones, especially androgens (male sex hormones). The ovaries also secrete some male sex hormones, including testosterone.
The sex hormones control the rapid changes that occur during puberty. For example, they help trigger the increases in height and weight that occur and, at the end of puberty, they help to stop this growth.
The female reproductive system depends primarily on two hormones: estrogen and progesterone. Both are produced in the ovaries. The main role of estrogen and progesterone is to help regulate the menstrual cycle and prepare the uterus for pregnancy. Like all hormones, estrogen and progesterone circulate throughout the body and interact with other hormones and with the body's metabolism. Estrogen affects many tissues throughout the body, including the urethra, bladder, vagina, breasts, bones, skin, liver, arteries, and brain.
When a girl begins puberty, her ovaries start to release increasing amounts of estrogen into her bloodstream. Estrogen stimulates the development of the breasts and causes the genital organs to enlarge and mature. Estrogen also stimulates the endometrium (the lining of the uterus) to thicken.
Estrogen plays an important role in the menstrual cycle and affects almost every part of a woman's body. It influences height and weight, skin tone, muscular strength, digestion, heart rate, and circulation. The ovaries produce varying levels of estrogen throughout a woman's life span. Fat tissues also produce some estrogen.
Estrogen provides several benefits:
- Estrogen helps prevent heart disease by increasing the level of "good" cholesterol in the blood and decreasing the level of "bad" cholesterol, and by keeping coronary arteries open and resistant to plaque (patches of fatty tissue on the inside of arteries) (see Chapter 4).
- Estrogen helps keep breast tissue firm.
- Estrogen helps maintain calcium in the bones, which helps make them dense, strong, and resistant to breaking.
- Estrogen helps keep the vaginal lining moist, keeping it resistant to infection and more comfortable during sexual activity.
Progesterone is produced mainly by the ovaries during a woman's reproductive years and by the placenta during pregnancy. Small amounts are also produced by the adrenal glands.
Progesterone helps prepare the uterus for pregnancy. About midway through a woman's monthly menstrual cycle, one of the two ovaries releases an egg. This process, called ovulation, results in high levels of progesterone being released into the blood for 10 to 12 days. Progesterone stimulates growth of the endometrium (the lining of the uterus) so that the egg, if it has been fertilized, can implant (attach to the uterine wall). If pregnancy does not occur, the ovary stops producing high levels of progesterone. The uterine lining then breaks down and passes out of the body as menstrual fluid.
The increase in progesterone levels after ovulation also causes other cyclic changes. The woman's body temperature increases slightly and her breasts may feel fuller or become sensitive.
Menarche, or the onset of menstruation, is the result of a series of hormonal changes that take place over the course of several years inside a young girl's body. Before age 8, only small amounts of estrogen are released into the bloodstream. When a girl reaches puberty, her pituitary gland begins to produce the hormones that are vital to sexual development. One of them is FSH (follicle-stimulating hormone), which will play an important part in the reproductive cycle. Another hormone, growth hormone, accelerates body growth, causing the typical "growth spurt" of the teenage years.
As puberty continues, the pituitary gland produces more FSH, causing the ovaries to produce estrogen. The higher level of estrogen in the bloodstream causes the development of female sex characteristics in classic sequence: first, the breasts develop and pubic hair begins to grow; then, underarm hair appears and menstruation begins; and finally, the hips begin to broaden, producing the characteristic female shape.
Estrogen also produces other changes in the body. By closing the ends of the bones, it stops skeletal growth. Generally, girls stop growing in height one or two years after menstruation begins. Estrogen also causes fat to be deposited in the labia, making them thicker. Pubic hair becomes denser and coarser in texture. The clitoris enlarges and becomes more sensitive. The vagina increases in length, and growth occurs in the uterus, fallopian tubes, and ovaries. Finally, as the body, brain, and endocrine glands mature, menstruation begins.
The first menstrual period usually occurs between ages 11 to 14, but this varies widely. It may be influenced by diet, exercise, genetics, and other factors. Ovulation is rare during the first year of menstrual cycles, which are often irregular, and sometimes heavy.
The female reproductive system (see illustration) consists of the ovaries, fallopian tubes, uterus, and vagina. At birth, a female infant has about 2 million eggs (or ova) in her ovaries. By puberty, the ovaries contain just 300,000 eggs, each surrounded by a casing of cells called a follicle.
During a woman's reproductive years, about 450 eggs reach maturity and travel through the fallopian tubes, where they can be fertilized by sperm. The rest of the eggs slowly disintegrate, and by menopause, only about 3,000 remain.
As mentioned above, menstrual cycles (see illustration) depend on the interrelationships and functioning of the various endocrine glands. Menstruation is initially triggered by hormones released from the hypothalamus.