Ever wonder how the universe could allow the existence of someone as annoying as your bratty little brother or sister? The answer lies in reproduction. If people - like your parents (ew!) - didn't reproduce, families would die out and the human race would cease to exist.

All living things reproduce. Reproduction - the process by which organisms make more organisms like themselves - is one of the things that sets living things apart from nonliving matter. But even though the reproductive system is essential to keeping a species alive, unlike other body systems it's not essential to keeping an individual alive.

In the human reproductive process, two kinds of sex cells, or gametes (pronounced: gah-meetz), are involved. The male gamete, or sperm, and the female gamete, the egg or ovum, meet in the female's reproductive system to create a new individual. Both the male and female reproductive systems are essential for reproduction.

Humans, like other organisms, pass certain characteristics of themselves to the next generation through their genes, the special carriers of human traits. The genes parents pass along to their children are what make children similar to others in their family, but they are also what make each child unique. These genes come from the father's sperm and the mother's egg, which are produced by the male and female reproductive systems.

What Is the Male Reproductive System?
Most species have two sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure, but both are specifically designed to produce, nourish, and transport either the egg or sperm.

Unlike the female, whose sex organs are located entirely within the pelvis, the male has reproductive organs, or genitals (pronounced: jeh-nuh-tulz), that are both inside and outside the pelvis. The male genitals include:

• the testicles
• the duct system, which is made up of the epididymis and the vas deferens
• the accessory glands, which include the seminal vesicles and prostate gland
• the penis

In a guy who's reached sexual maturity, the two testicles (pronounced: tes-tih-kulz), or testes (pronounced: tes-teez), produce and store millions of tiny sperm cells. The testicles are oval-shaped and grow to be about 2 inches (5 centimeters) in length and 1 inch (3 centimeters) in diameter. The testicles are also part of the endocrine system because they produce hormones, including testosterone (pronounced: teh-stass-tuh-rone). Testosterone is a major part of puberty in guys, and as a guy makes his way through puberty, his testicles produce more and more of it. Testosterone is the hormone that causes guys to develop deeper voices, bigger muscles, and body and facial hair, and it also stimulates the production of sperm.

Alongside the testicles are the epididymis (pronounced: eh-puh-dih-duh-mus) and the vas deferens (pronounced: vass de-fuh-runz), which make up the duct system of the male reproductive organs. The vas deferens is a muscular tube that passes upward alongside the testicles and transports the sperm-containing fluid called semen (pronounced: see-mun). The epididymis is a set of coiled tubes (one for each testicle) that connects to the vas deferens.

The epididymis and the testicles hang in a pouch-like structure outside the pelvis called the scrotum. This bag of skin helps to regulate the temperature of testicles, which need to be kept cooler than body temperature to produce sperm. The scrotum changes size to maintain the right temperature. When the body is cold, the scrotum shrinks and becomes tighter to hold in body heat. When it's warm, the scrotum becomes larger and more floppy to get rid of extra heat. This happens without a guy ever having to think about it. The brain and the nervous system give the scrotum the cue to change size.

The accessory glands, including the seminal vesicles and the prostate gland, provide fluids that lubricate the duct system and nourish the sperm. The seminal vesicles (pronounced: seh-muh-nul veh-sih-kulz) are sac-like structures attached to the vas deferens to the side of the bladder. The prostate gland, which produces some of the parts of semen, surrounds the ejaculatory ducts at the base of the urethra (pronounced: yoo-ree-thruh), just below the bladder. The urethra is the channel that carries the semen to the outside of the body through the penis. The urethra is also part of the urinary system because it is also the channel through which urine passes as it leaves the bladder and exits the body.

The penis is actually made up of two parts: the shaft and the glans (pronounced: glanz). The shaft is the main part of the penis and the glans is the tip (sometimes called the head). At the end of the glans is a small slit or opening, which is where semen and urine exit the body through the urethra. The inside of the penis is made of a spongy tissue that can expand and contract.

All boys are born with a foreskin, a fold of skin at the end of the penis covering the glans. Some boys have a circumcision (pronounced: sur-kum-sih-zhun), which means that a doctor or clergy member cuts away the foreskin. Circumcision is usually performed during a baby boy's first few days of life. Although circumcision is not medically necessary, parents who choose to have their children circumcised often do so based on religious beliefs, concerns about hygiene, or cultural or social reasons. Boys who have circumcised penises and those who don't are no different: All penises work and feel the same, regardless of whether the foreskin has been removed.

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