The Endocrine System
Thymus Gland (Upper Torso)
The ovaries, a pair of tiny glands in the female pelvic cavity, are the most important organs of the female reproductive system. Their importance is derived from their role in producing both the female sex hormones that control reproduction and the female gametes that are fertilized to form embryos. Each ovary is a small glandular organ about the shape and size of an almond. The ovaries are located on opposite sides of the uterus in the pelvic cavity and are attached to the uterus by the ovarian ligament. The open ends of the fallopian tubes rest just beyond the lateral surface of the ovaries to transport ova, or egg cells, to the uterus. The ovaries play two central roles in the female reproductive system by acting as both glands and gonads. Acting as glands, the ovaries produce several female sex hormones including estrogens and progesterone. Estrogen controls the development of the mammary glands and uterus during puberty and stimulates the development of the uterine lining during the menstrual cycle. Progesterone acts on the uterus during pregnancy to allow the embryo to implant and develop in the womb.
The testes commonly known as the testicles, are a pair of ovoid glandular organs that are central to the function of the male reproductive system. The testes are responsible for the production of sperm cells and the male sex hormone testosterone. The testes produce as many as 12 trillion sperm in a male's lifetime, about 400 million of which are released in a single ejaculation. The testes are connected to the vital organs of the ventral body cavity via the spermatic cords. Nerves, blood vessels, and lymphatic vessels travel through the spermatic cords to support the testes. The vas deferens also passes through the spermatic cord carrying sperm out of the testes toward the prostate and urethra. The cremaster muscle wraps around the exterior of the spermatic cord to lift the testes closer to the body or permit them to descend. The testes are wrapped by the tunica vaginalis, an extension of the peritoneum of the abdomen, and the tunica albuginea, a tough, protective sheath of dense irregular connective tissue. Each testis is divided by invaginations of the tunica albuginea that divide it into several hundred small segments called lobules. Each lobule contains several tightly coiled tubes called seminiferous tubules. Each sperm produced by the testes takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat that keeps the testes and sperm at the correct temperature. It may be surprising that the testes should lie in such a vulnerable place outside the body, but it is too hot for them inside. Spermatogenesis requires a temperature that is three to five degrees Fahrenheit below body temperature. If it becomes too cool on the outside, the cremaster muscle will contract to bring the testes closer the body for warmth.