13 The Reproductive System
Learn how reproductive anatomy, gamete production, hormonal coordination, fertilization, and early development work together.
Functions and coordination
Reproductive organs produce gametes and hormones, enable fertilization, and, when pregnancy occurs, support development. Their activity is coordinated with the endocrine system. The circulatory system carries reproductive hormones and supports the and developing embryo. Reproductive anatomy varies among individuals; the structures described here represent typical patterns.
Reproductive anatomy
Testicular reproductive system
The testes produce sperm in tightly coiled seminiferous tubules and secrete testosterone. Sperm mature and are stored in the epididymides, then travel through the ductus deferens to the ejaculatory ducts and urethra. The scrotum helps keep the testes at a temperature suitable for sperm production.
The seminal vesicles, prostate, and bulbourethral glands add fluids that nourish, protect, and transport sperm. Together with sperm, these fluids form semen. The penis delivers semen into the reproductive tract. In people with this anatomy, the urethra carries both urine and semen, though not at the same time.
Ovarian reproductive system
The ovaries produce oocytes and hormones, including estrogens and progesterone. At ovulation, an oocyte enters a uterine tube, where fertilization usually occurs. The uterus has a muscular wall and an inner lining called the endometrium, where a may implant and develop.
The cervix is the lower opening of the uterus into the vagina. The vagina receives a penis and semen during intercourse and serves as the birth canal. The vulva refers to the external genital structures.
Gamete production
Gametes are cells containing chromosomes, half the chromosome number of most body cells. They form through , which reduces chromosome number and creates genetic variation.
Sperm production
occurs in the seminiferous tubules and generally begins at puberty. Each primary spermatocyte undergoing can produce four sperm. The sperm then mature in the epididymis.
Oocyte development
begins before birth: primary oocytes form and pause during . After puberty, typically one oocyte resumes development during a cycle. At ovulation, it is released as a secondary oocyte with II paused. If a sperm enters, II is completed. produces one large functional gamete and small polar bodies.
Hormonal coordination
The coordinates reproductive function. The hypothalamus releases , which signals the anterior pituitary to release follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These hormones travel through the blood to the gonads.
Hormonal regulation in the testes
In the testes, LH stimulates Leydig cells to produce testosterone. FSH and testosterone support sperm production through Sertoli cells. Testosterone and inhibin help regulate the axis through feedback to the brain and pituitary.
Hormonal regulation in the ovaries
In the ovaries, FSH supports follicle development, and developing follicles produce estrogen. Estrogen helps rebuild the endometrium after menstruation. A sustained rise in estrogen triggers an LH surge, which causes ovulation.
After ovulation, the emptied follicle becomes the , which secretes progesterone to maintain the uterine lining. If pregnancy does not occur, the breaks down, hormone levels fall, and the lining is shed during menstruation. If pregnancy begins, human chorionic gonadotropin (hCG) from developing embryonic tissue supports the early in pregnancy.
Fertilization and early development
After ejaculation, sperm travel from the vagina through the cervix and uterus toward a uterine tube. In the reproductive tract, sperm undergo , changes that enable fertilization. A sperm penetrates the oocyte’s outer layers and fuses with its membrane. The oocyte then undergoes changes that prevent additional sperm from entering.
The genetic material from the two gametes combines to form a diploid with chromosomes.
Early development
As the travels toward the uterus, it divides repeatedly in a process called . produces a solid ball of cells called the morula, followed by a fluid-filled . The implants in the endometrium.
During , cells form three foundational layers: ectoderm, mesoderm, and endoderm. Body tissues and organs develop from these layers. establishes the early organ systems. In standard developmental terminology, the developing human is called an embryo through the eighth week after fertilization and a fetus from the ninth week until birth.
The and system-wide connections
The is a temporary organ formed from fetal and uterine tissues. It exchanges oxygen, nutrients, and wastes between maternal and fetal blood supplies without normally mixing them directly. It also produces hormones that help maintain pregnancy.
Pregnancy illustrates how reproduction depends on interactions among the reproductive, endocrine, and circulatory systems. It also changes the demands on maternal respiratory, urinary, and other systems. Overall, reproductive organs make gametes and hormones while the hypothalamus and pituitary coordinate their activity; produces sperm and oocytes, and fertilization restores the diploid chromosome number.