Created by CK-12 Foundation/Adapted by Christine Miller

The geometric design on the ancient stone carving in Figure 18.6.1 represents a powerful symbol of female fertility: the vagina. The symbol is called yoni in Hindu, and it reflects the value placed by Hindu culture on the ability of females to give birth. The vagina is one of several organs in the female reproductive system.

The female reproductive system is made up of internal and external organs that function to produce haploid female gametes called ova (or oocytes), secrete female sex hormones (such as estrogen), and carry and give birth to a fetus. The internal female reproductive organs include the vagina, uterus, oviducts, and ovaries. The external organs — collectively called the vulva — include the clitoris and labia.

The vagina is an elastic, muscular canal leading from its opening in the vulva to the neck of the uterus, called the cervix. It is about 7.5 cm (about 3 in) long at the front, and about 9 cm (3.5 in) long at the back. The vagina accommodates the penis and is the site where sperm are usually ejaculated during sexual intercourse. In the context of pregnancy and natural (vaginal) childbirth, the vagina is referred to as the birth canal. In addition, it channels the flow of menstrual blood from the uterus.

Structure of the Vagina

Muscles and ligaments support the vagina within the pelvic cavity. The vagina itself is made up of several layers of fibrous and muscular tissues and lined with mucous membranes. Folds in the mucosa provide the vagina with extra surface area so it can stretch in both length and width during intercourse or childbirth. The elasticity of the vagina and the extra mucosa allow it to stretch to many times its normal diameter in order to deliver a baby.

Bacteria and pH in the Vagina

A healthy vagina is home to many symbiotic bacteria that help prevent pathogens (such as yeast) from colonizing the vagina. The pH in the vagina is normally between 3.8 and 4.5, and this acidity also helps keep pathogenic microorganisms from colonizing it. The vagina constantly sheds its epithelium, so it is considered self-cleaning. As a consequence, there is no need for douching to clean it. Physicians actually discourage the practice, as it may upset the normal bacterial and pH balance in the vagina, although washing the vulva with a mild soap is good practice.

The uterus (commonly called the womb) is a pear-shaped, muscular organ that is about 7.6 cm (about 3 in) long. It is located above the vagina and behind the bladder in the centre of the pelvis. The position of the uterus in the pelvis is stabilized by several ligaments and bands of supportive tissue. The uterus is where a fetus develops during gestation, and the organ provides mechanical protection and support for the developing offspring. Contractions of the muscular wall of the uterus are responsible for pushing the fetus out of the uterus during childbirth.

Parts of the Uterus

As shown in Figure 18.6.2, the lower end of the uterus forms the cervix, which is also called the neck of the uterus. The cervix is about 2.5 cm (almost 1 in) long and protrudes downward into the vagina. A small canal runs the length of the cervix, connecting the uterine cavity with the lumen of the vagina. This allows semen deposited in the vagina to enter the uterus, and a baby to pass from the uterus into the vagina during birth. Glands in the cervix secrete mucus that varies in water content and thickness, so it can function either as a barrier to keep microorganisms out of the uterus during pregnancy, or as a transport medium to help sperm enter the uterus around the time of ovulation. The rest of the uterus above the cervix is called the body of the uterus. The upper end of the uterus is connected with the two oviducts.

Tissues of the Uterus

As indicated in Figure 18.6.3, the uterus consists of three tissue layers, called the endometrium, myometrium, and perimetrium.

• The endometrium is the innermost tissue layer of the uterus. It consists of epithelial tissue, including mucous membranes. This layer thickens during each menstrual cycle and, unless an egg is fertilized, sloughs off during the following menstrual period. If an ovum is fertilized, the thickened endometrium is maintained by hormones and provides nourishment to the embryo. As gestation progresses, the endometrium develops into the maternal portion of the placenta.
• The placenta is a temporary organ that consists of a mass of maternal and fetal blood vessels through which the mother’s and fetus’s blood exchange substances.
• The myometrium is the middle layer of the uterus. It consists mostly of a thick layer of smooth muscle tissue. Powerful contractions of the smooth muscle allow the uterus to contract and expel an infant during childbirth.
• The perimetrium is the outermost layer of the uterus. It covers the outer surface of the uterus. This layer actually consists of two layers of epithelium that secrete fluid into the space between them. The fluid allows for small movements of the uterus within the pelvis, without causing it to rub against other organs.

The oviducts (often referred to as Fallopian tubes) are two thin tubes that lie between the ovaries and the uterus. The oviducts are not attached to the ovaries, but their broad upper ends — called infundibula — lie very close to the ovaries. The infundibula also have fringe-like extensions called fimbriae that move in a waving motion to help guide eggs from the ovaries into the oviducts. The lower ends of the oviducts are attached to the upper part of the body of the uterus on either side of the body. They open into the uterus.

The oviducts are made up of multiple tissue layers. The innermost layer consists of mucosal epithelium. The epithelium is covered with cilia, which can move in a sweeping motion to help ova move through the tube from the ovary to the uterus. In between the ciliated cells of the epithelium are cells that secrete a fluid called tubular fluid. This fluid contains nutrients for sperm, ova, and zygotes. The secretions in tubular fluid also remove certain molecules from the plasma membrane of sperm so they are better able to penetrate an egg. Other layers of the oviducts consist of connective tissue and smooth muscle. Contractions of the smooth muscle allow peristalsis to help move eggs through the tubes.

Like the testes in males, the ovaries in females are gonads that produce gametes and secrete sex hormones. The gametes produced by the ovaries are called ova, or oocytes. The main sex hormone secreted by the ovaries is estrogen. The position of the paired ovaries relative to the other reproductive system organs is shown in Figure 18.6.4. Each ovary lies along one side of the uterus and is about 4 cm (a little more than 1.5 in) long. Fibrous ligaments attach one end of each ovary to its nearby oviduct and the other and to its side of the uterus. These ligaments keep the ovaries in place within the pelvis.

Ovarian Follicles

The ovary consists of two main layers, called the ovarian medulla (the inner layer) and the ovarian cortex (the outer layer). The ovary also contains blood and lymphatic vessels. The ovarian cortex consists primarily of the functional units of the ovaries, which are called ovarian follicles. The follicles are nests of epithelial cells, within each of which is an ovum. The photomicrograph in Figure 18.6.5 shows an ovarian follicle and the developing ovum inside it. If an ovum and follicle complete maturation, the follicle ruptures and the ovum is released from the ovary. This event is called ovulation.

Ova in the Ovaries

Whereas the male testes produce sperm continuously after puberty, the female ovary already contains all the ova it will ever produce by the time a female is born. At birth, a baby girl’s ovaries contain at least a million eggs, each of which is contained within a follicle. Only about 500 of these eggs will eventually mature and be ovulated. This process starts at puberty and typically continues at monthly intervals until menopause occurs around age 52. The remaining eggs never mature, and their number declines as the woman ages. By menopause, a woman’s reserve of eggs is nearly depleted, and ovulation no longer occurs.

The vulva is a general term for all of the external female reproductive organs. The vulva includes the clitoris, labia, and external openings for the urethra and vagina.

Labia

The labia (singular, labium) refer to the “lips” of the vulva, which are folds of tissue that contain and protect the other, more delicate structures of the vulva (as shown in Figure 18.6.6). There are two pairs of labia: the outer and larger labia majora, and the inner and smaller labia minora. The labia minora contain numerous sebaceous glands. These glands release secretions that help lubricate the labia and vulvar area.

Clitoris

The clitoris, is located at the front of the vulva where the labia minora meet. The visible portion of the clitoris is called the glans clitoris. It is roughly the size and shape of a pea. It is highly sensitive, because it contains many nerve endings. A hood of tissue called the clitoral hood (shown in Figure 18.6.5 above), or prepuce, normally covers and protects the clitoris. The clitoris is the homologue to the male penis, and they both contain spongy tissue.  Stimulation of the glans clitoris during sexual activity generally results in sexual arousal in females, and may lead to orgasm. The glans clitoris is the only part of the overall clitoris visible externally, but this spongy tissue extends down either side of the openings to the urethra and vagina, as seen in Figure 18.6.7.

Other Vulvar Structures

The area between the two labia minora is called the vestibule of the vulva. Both the urethra and vagina have openings to the outside of the body in the vestibule. As you can see in Figure 18.6.7 above, the urethral opening (or meatus) is located just in front of, and is much smaller than, the vaginal opening. Both openings are protected by the labia. Two glands — called Bartholin’s glands — open on either side of the vaginal opening. These glands secrete mucus and a vaginal and vulvar lubricant.

The breasts are not directly involved in reproduction, but because they contain mammary glands, they can provide nourishment to an infant after birth. The breasts overlay major muscles in the chest from which they project outward in a conical shape. The two main types of tissues in the breast are adipose (fat) tissue and glandular tissue that produces milk. As shown in Figure 18.6.8, each mature breast contains many lobules, where milk is produced and stored during pregnancy. During breastfeeding (or lactation), the milk drains into ducts and sacs, which in turn converge at the nipple. Milk exits the breast through the nipple in response to the suckling action of an infant and is regulated by a positive feedback loop. The nipple is surrounded by a more darkly pigmented area called the areola. The areola contains glands that secrete an oily fluid, which lubricates and protects the nipple during breastfeeding.

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