The key component of the male reproductive system consists of a network of tubes that begins in the testes and eventually leads to the urethra, which then exits the body. Along the way are several accessory organs that contribute to the formation of the semen. Let's begin our discussion with the testes, the male gonads. Although they develop in the abdominal cavity, the testes are housed externally in the scrotum. This is essential for their function since sperm formation requires a temperature slightly lower than normal body temperature.

Each testis is composed of a highly coiled seminiferous tubule. It is within these tubules that sperm production occurs. The tissue that surrounds the seminiferous tubules is the interstitial tissue and contains the cells that produce testosterone. These cells are called the Leydig cells. The seminiferous tubules come together and converge to exit the testes and unite to form the epididymis.

Although the sperm forms in the seminiferous tubules of the testes, final maturation takes place in the epididymis. For example, sperm taken directly from the seminiferous tubules are not motile (they can't swim) and would not be able to move through the female system. Only after spending about 10-14 days in the epididymis do, they become motile. The sperm remains in the epididymis until ejaculation. The epididymis is continuous with the vas deferens. This duct exits the scrotum and passes into the abdominal cavity around the bladder and into the prostate gland to the ejaculatory duct and eventually the penis. 

The seminal vesicles are the first of the accessory organs that contribute to the semen. They produce approximately 60% of the seminal fluid. The seminal vesicles provide several important components to the sperm. These include:

1) Fructose and citric acid to provide nutrients to fuel the sperm;

2) Fibrinogen that contributes to a weak, transient coagulation reaction once the semen enters the female tract which helps the semen adhere to the vaginal walls near the cervix as well as protecting the sperm from destruction by the acid environment of the vagina; and

3) Prostaglandins which help decrease the viscosity of the cervical mucus allowing the sperm to enter the uterus more easily. Prostaglandins also stimulate uterine contractions, helping the sperm move up the female tract.

The ejaculatory duct then enters the second of the accessory organs, the prostate gland, where it unites with the urethra. The prostate accounts for approximately 30% of seminal fluid. The prostatic secretion has a high pH, which helps neutralize the acidic environment of both the male urethra and the female tract.

After the urethra exits the prostate, it receives secretions from the last of the accessory organs, the bulbourethral glands which contribute about 5% of the seminal volume. Secretions of the bulbourethral glands contain mucus to lubricate the urethra and buffers to neutralize acids in the urethra and vagina. The final 5% of the seminal volume is the sperm that comes from the epididymis.