A penis hangs outside a guy’s body from the genital area and underneath the penis is a loose sack of skin that is called the scrotum. At the top of the penis (which is called the glans) is an opening called the urethra. Guys urinate (pee) through the urethra. The urethra is also the hole through which semen spurts out of the guy’s penis when he ejaculates during an orgasm. One of the biggest concerns of many young men, and many older men, is the size of their penis. There isn’t a right size for a penis at any particular age. Normally the penis is soft. But when a man gets sexually aroused blood flows into the penis at a faster rate than it flows out. This makes the penis larger and harder and it is called an erection. The average penis is between 5 and 6 inches long when erect. Not all erections are caused by sexual excitement. Guys, especially younger guys, can get erections at the weirdest times. These are called spontaneous erections and this can be embarrassing. However they go away fairly quickly if you ignore them. Many guys also get erections during their sleep and have a spontaneous orgasm in their sleep. This called a “wet dream” and doesn’t mean anything is wrong. Some men are circumcised (the foreskin which covers the glans is cut back after birth) and some are not. Guys who are not circumcised need to pull back the foreskin when they urinate or wash themselves.
The scrotum is the loose sack of skin in which the testicles (balls) are hidden. Depending on how cold or warm it is, whether the guy is sexually aroused or not the scrotum can shrink or expand in size. The scrotum is covered with public hair. The medical names for the inside parts are:
There are two testes (also called testicles or balls) that are inside the scrotum. Before puberty the testes are inside the body but “drop down” into the scrotum at puberty when a boy begins to physically become a man. The two testes grow in size and become able to produce sperm and male hormones like testosterone. The testes are healthiest when they are at a temperature slightly less than the inside of the guy’s body. They are very sensitive to injury. When a guy ejaculates, sperm that was made in the testes mixes with other fluids that are made inside the body and spurts out of the penis. Doctors recommend that guys do a Testicular Self Exam (TSE) once a month to make sure everything is okay. The links below tell you how to do this and what to do if something doesn’t seem quite right: http://kidshealth.org/teen/sexual_health/guys/tse.html
The epididymis is a narrow, tightly-coiled tube connecting the ducts from the bottom of each testicle to the vas deferens. The epididymis is sort of like a reservoir for sperm that is made in the testes.
The vas deferens transports sperm from the epididymis to be ready for ejaculation. During ejaculation the vas deferens contracts and pumps sperm towards the penis.
The seminal vesicles produce fluid that mixes with the semen before it is ejaculated.
The main function of the prostate is to store and release a milky white fluid that makes up about 25-30% of the semen which is combined with sperm and seminal vesicle fluid. The alkalinity of prostate fluid helps neutralize the acidity of the vagina, prolonging the life of sperm. The prostate contains some smooth muscles that help to pump the semen out of the penis during ejaculation There is more information helpful information about a man’s sexual body parts at the following link. Anatomy: Man's Best Friend - Male Sexual Anatomy