The human female reproductive system is a group of organs working together to enable reproduction. It is responsible for producing female egg cells (ova) and transporting them to a site for fertilization by sperm. The system also produces hormones that influence other aspects of health.
Ovaries and Egg Maturation
The ovaries are a pair of small, oval-shaped glands located on either side of the uterus. Their primary responsibilities are producing egg cells (ova) and secreting hormones for fertility and sexual maturation. A female is born with a finite number of immature eggs, housed inside the ovaries within small sacs called follicles. These eggs remain in a non-growing state until puberty.
Each month, the reproductive cycle prompts several ovarian follicles to develop. Only one follicle becomes dominant and brings a single egg to full maturity in a process known as oogenesis. When the egg is mature, the follicle and ovary wall rupture, releasing the egg in a process called ovulation.
The ovaries also produce the primary female sex hormones: estrogen and progesterone. These hormones are involved in the monthly maturation and release of an egg and prepare the reproductive system for a potential pregnancy. The cyclical production of these hormones governs the reproductive process by signaling other organs to perform their functions.
Fallopian Tubes and Uterus
Following ovulation, the released egg begins its journey through the fallopian tubes, which are two narrow passages connecting the ovaries to the uterus. Near each ovary, fringe-like projections called fimbriae help capture the egg and guide it into the tube. The inner lining of the fallopian tubes is covered in tiny, hair-like cilia that propel the egg toward the uterus over several hours or days.
Fertilization, the fusion of an egg and a sperm, occurs in the fallopian tubes. If the egg is fertilized, it continues to the uterus, where it can implant and begin development. If fertilization does not happen, the unfertilized egg passes through the uterus and exits the body.
The uterus is a hollow, pear-shaped organ with muscular walls designed to house a developing fetus. It has three layers. The innermost layer is the endometrium, a lining that thickens monthly to prepare for a fertilized egg. The middle layer, the myometrium, is a muscle that expands to accommodate a growing baby and contracts during childbirth. The perimetrium is the thin, outer layer.
Cervix and Vagina
The cervix is the lower, narrow part of the uterus that opens into the vagina. It acts as a channel for sperm to enter the uterus and for menstrual fluid to exit. The cervix also produces mucus, and its consistency changes throughout the monthly cycle. It can become thin to help sperm travel toward the egg or thick to form a barrier.
During childbirth, the cervix dilates or widens, allowing the baby to pass from the uterus into the birth canal. This function is a part of the labor process.
The vagina is a muscular canal connecting the cervix to the outside of the body. It serves three main purposes: receiving sperm during sexual intercourse, providing a passageway for menstrual blood, and functioning as the birth canal during delivery.
Hormonal Control of the Reproductive Cycle
The reproductive cycle is directed by an interplay of hormones from the brain and ovaries. This system functions as a feedback loop where hormones signal organs to act and receive signals back. The cycle is driven by Follicle-Stimulating Hormone (FSH) and Luteinizing Hormone (LH) from the brain’s pituitary gland, and estrogen and progesterone from the ovaries.
The cycle begins with the follicular phase, when the brain releases FSH. This hormone stimulates follicles in the ovaries to grow and produce estrogen. Rising estrogen levels cause the uterine lining, the endometrium, to thicken in preparation for a potential pregnancy.
High estrogen levels trigger the brain to release a surge of LH. This LH surge signals ovulation, causing the most mature follicle to rupture and release its egg. Following ovulation, the cycle enters the luteal phase, where the ruptured follicle transforms into the corpus luteum and begins producing high levels of progesterone.
Progesterone maintains the thickened uterine lining, creating a stable environment for a fertilized egg. This hormone also signals the brain to reduce FSH and LH production, preventing other follicles from developing. If pregnancy does not occur, the corpus luteum breaks down, causing progesterone and estrogen levels to fall. This drop triggers the shedding of the uterine lining (menstruation) and signals the brain to begin the cycle again.