The testes are housed outside of the main part of the male's body, in a sac called the scrotum. This location is important because in order for the sperm to develop properly, they must be kept at a slightly lower temperature (95 to 97 degrees Fahrenheit, 35 to 36 degrees Celsius) than normal body temperature (98.6 F, 37 C).

The immature sperm travel from each testis to a coiled tube on the outer surface of each testis called the epididymis, where they mature in about 20 days. The sperm exit the body through the penis.

The penis is made of soft, spongy tissue. When engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect, which is important for the penis's main function -- to place the sperm inside the female.

As mentioned above, sperm are made in the testes. During sexual intercourse, smooth muscles contract and propel mature sperm from the end portions of the epididymis through a long tube (vas deferens or ductus deferens) inside the body, just beneath the bladder. From there, the sperm get mixed with nutrient-rich fluids from the seminal vesicles and a milky secretion from the prostate gland. This combination of sperm and fluids is called semen. The semen does three things:

• Provides a watery environment in which the sperm cells can swim while outside the body
• Provides nutrients for the sperm cells (fructose, amino acids, vitamin C)
• Protects the sperm cells by neutralizing acids in the female's sexual tract

Once the semen is made, it passes through another tube (urethra) within the penis, exiting the body through the opening of the penis.

One last male organ is a tiny, pea-sized set of glands inside the body at the base of the penis, called the bulbourethral or Cowper's glands. During sexual excitation, and just prior to the ejection of sperm (ejaculation), the Cowper's glands secrete a tiny amount of fluid that neutralizes any traces of acidic urine that may be leftover in the urethra. It is also believed that these secretions may lubricate the penis and female tract during sexual intercourse.

The two ovaries are the major female sex organs, the counterpart of the male testes. The ovaries make the eggs, or oocytes, which are the female gametes, and produce estrogen, the female sex hormone. Estrogen causes female secondary sexual characteristics such as pubic hair, breast development, widening of the pelvis and deposition of body fat in hips and thighs. The ovaries are located in the abdomen.

Eggs develop inside the ovary and are rreleased upon ovulation into a tube (the oviduct or Fallopian tube) lined with fingerlike projections. The egg travels through the Fallopian tube, where fertilization can take place, to a muscular chamber called the uterus.

The uterus is where the baby develops. It is made of smooth muscle and is normally about the size and shape of a small pear turned upside down. During pregnancy, it can stretch to about the size of a basketball to hold the developing baby. The base of the uterus (neck of the pear) is a muscular wall called the cervix. In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.

The vagina connects the uterus to the outside of the body, and its opening is covered by the labia. The vagina receives the male's penis during sexual intercourse and delivers the baby during childbirth. The vagina is normally narrow (except around the cervix), but can stretch during intercourse and childbirth.

Finally, two sets of glands, the greater vestibular gland (Bartholin's gland) and the lesser vestibular gland, are located on either side of the vagina and empty into the labial folds of skin. The secretions from these glands lubricate the labial folds during sexual excitation and intercourse.

Other Organs Involved in Sex
Although not located in the reproductive systems, two other organs are important for sexual function in both males and females:

• The hypothalamus in the brain - The hypothalamus has nerve cells that secrete a hormone called gonatotropin releasing hormone (GnRH) into the blood vessels leading to the anterior pituitary gland.
• The anterior pituitary gland just beneath the brain - GnRH causes the anterior pituitary cells to release two hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), into the general blood circulation. LH and FSH act on the testes/ovaries to stimulate the making and maturation of the sex cells and the production of sex hormones (testosterone, estrogen, progesterone).

The nerve cells time-release small, low-level spurts of GnRH every 90 minutes, which causes the anterior pituitary to secrete small pulses of LH and FSH. The sex hormones from the testes/ovaries give feedback to the hypothalamus and anterior pituitary to regulate the secretion of GnRH, LH and FSH -- this interplay is called the negative feedback control system. The chemical interplay between the hypothalamus, anterior pituitary gland and the testes/ovaries is important for sexual development, maintaining sexual function and sexual reproduction. An error in this chemical interplay can be a cause of infertility.