LEARNING OBJECTIVE: Recall the parts of the male reproductive system and their function(s).
The organs of the male and female reproductive systems are concerned with the process of reproducing offspring, and each organ is adapted to perform specialized tasks. The primary male sex organs of the reproductive system are the testes. The other structures of the male reproductive system are termed accessory reproductive organs. The accessory organs include both internal and external reproductive organs. See figure 1-60 for an illustration of the male reproductive system.
Figure 1-60.—The male reproductive system.
TESTES INTERNAL ACCESSORY ORGANS
The testes, as stated earlier, are the primary male reproductive organs. They produce sperm cells (spermatozoa) and male hormones, both necessary for reproduction.
The testes are oval glands suspended inside a sac (the scrotum) by a spermatic cord. The spermatic cords are formed by the vas deferens, arteries, veins, lymphatics, and nerves, all bound together by connective tissue.
Each testis is encapsulated by a tough, white, fibrous tissue called the tunica albuginea. The interior of the testis is divided into 250 lobules (small lobes). Each lobule contains 1 to 4 highly coiled, convoluted tubules called seminiferous tubules. These tubules unite to form a complex network of channels called the rete testis. The rete testis give rise to several ducts that join a tube called the epididymis (fig. 1-60).
The testes perform two functions: to produce sperm cells and to secrete male sex hormones. The process by which sperm cells are produced is called spermatogenesis. Spermatogenesis occurs in the seminiferous tubules of the testes. Once the sperm cells are formed, they collect in the lumen of each seminiferous tubule. When the sperm cells are ready, they pass through the rete testis to the epididymis, where they remain for a time to mature. The production of sperm cells occurs continually throughout the reproductive life of a male.
The male hormone testosterone is produced in the testes. This hormone is initially responsible for the formation of the male reproductive organs. During puberty, testosterone stimulates the enlargement of the testes and various other accessory reproductive organs. It also causes the development of the male secondary sexual characteristics. Refer to the section titled “The Endocrine System” for more detailed discussion on male secondary sexual characteristics.
Other actions of testosterone include increasing the production of red blood cells. As a result, the average number of red blood cells in blood is usually greater in males than in females.
The internal accessory organs of the male reproductive system include the epididymis, vas deferens, ejaculatory ducts, seminal vesicle, urethra, prostate gland, bulbourethral glands, and semen (fig. 1-60).
Each epididymis is a tightly coiled, thread-like tube that is approximately 6 meters long. This tube is connected to the ducts within the testis. The epididymis covers the top of the testis, runs down the testis' posterior surface, and then courses upward to form the vas deferens.
The epididymis secretes the hormone glycogen, which helps sustain the lives of stored sperm cells and promotes their maturation. When immature sperm cells enter the epididymis, they are not mobile. However, as the sperm cells travel through the epididymis, they mature and become mobile. Once the sperm cells are mature, they leave the epididymis and enter the vas deferens.
The vas deferens is a small tube that connects the epididymis and ejaculatory duct. It ascends as part of the spermatic cord through the inguinal canal of the lower abdominal wall into the pelvic cavity, and transmits the sperm to the ejaculatory ducts.
The vas deferens and the seminal vesicles converge, just before the entrance of the prostate gland, to form the ejaculatory ducts (fig. 1-60). The ejaculatory ducts open into the prostatic urethra. Its function is to convey sperm cells to the urethra.
The seminal vesicles are two pouches that are attached to the vas deferens near the base of the urinary bladder. The lining of the inner walls of the seminal vesicles secrete a slightly alkaline fluid. This fluid is thought to help regulate the pH of the tubular contents as sperm cells are conveyed to the outside. The secretion produced by the seminal vesicles also contains a variety of nutrients, such as fructose (simple sugar), that provides the sperm cells an energy source.
At the time of ejaculation, the contents of the seminal vesicles are emptied into the ejaculatory ducts. This action greatly increases the volume of fluid that is discharged by the vas deferens.
The urethra is an important organ of both the urinary and reproductive systems. The role of the urethra, in the reproductive system, is to transport sperm through the penis to outside the body. See “The Urinary System” section for information on the structure of the urethra.
The prostate gland, made of smooth muscle and glandular tissue, surrounds the first part of the urethra. It resembles a chestnut in shape and size, and secretes an alkaline fluid to keep the sperm mobile, protecting it from the acid secretions of the female vagina. This substance is discharged into the urethra as part of the ejaculate, or semen, during the sexual act.
Bulbourethral glands, also known as Cowper's glands, are two pea-sized bodies located below the prostate gland and lateral to the membranous urethra. These glands are enclosed by fibers of the external urethral sphincter. They release a mucous-like fluid in response to sexual stimulation and provide lubrication to the end of the penis in preparation for sexual intercourse.
Semen is composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands. It is discharged as the ejaculate during sexual intercourse. There are millions of sperm cells in the semen of each ejaculation, but only one is needed to fertilize the ovum. It is generally considered that fertilization of the ovum occurs while it is still in the fallopian tubes. Therefore, it is apparent that sperm cells can move actively in the seminal fluid deposited in the vagina and through the layers of the secretion lining the uterus and fallopian tubes.
EXTERNAL ACCESSORY ORGANS
The external accessory organs of the male reproductive system include the scrotum and penis (fig. 1-60).
The scrotum is a cutaneous pouch containing the testes and part of the spermatic cord. Immediately beneath the skin is a thin layer of muscular fibers (the cremaster), which is controlled by temperature and contracts or relaxes to lower or raise the testes in relation to the body. This muscular activity of the scrotum is necessary to regulate the temperature of the testes, which is important in the maturation of sperm cells.
The penis is a cylindrical organ that conveys urine and semen through the urethra to the outside. The penis is composed of three columns of spongy cavernous tissue, bound together by connective tissue and loosely covered by a layer of skin. Two of the columns, the corpora cavernosa, lie superiorly side by side; the third column, the corpus spongiosum, lies below the other two columns. The urethra is located in the corpus spongiosum. The dilated distal end of the corpus spongiosum is known as the glans penis (fig. 1-60). The urethra terminates at the glans penis.
The cavernous tissue becomes greatly distended with blood during sexual excitement, causing an erection of the penis. The loose skin of the penis folds back on itself at the distal end (forming the prepuce, or foreskin) and covers the glans. The prepuce is sometimes removed by a surgical procedure called a circumcision.