1. Male sexual characteristics and the structure and function of reproductive organs

Male sexual characteristics and development

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The male reproductive organs are divided into internal and external parts. The internal reproductive organs include the testes, epididymis, vas deferens, ejaculatory ducts, seminal vesicles, prostate gland, bulbourethral glands, etc., while the external reproductive organs include the penis and scrotum.

1. Penis

The penis is a cylindrical organ that is normally flaccid and hangs down in front of the scrotum. It is covered by a layer of skin, the tip of which can be retracted and is called the foreskin. The anterior end of the corpus spongiosum expands to form the glans penis. There is a groove at the junction of the glans penis and the shaft called the coronal sulcus.

The urethra is a long, narrow tube, about 12 centimeters in length. The internal urethral opening is located in the bladder, and the external opening is on the glans penis. The vas deferens, ejaculatory ducts, seminal vesicles, prostate gland, and bulbourethral glands all connect to the urethra. The urethra is the passageway for urination and ejaculation.

When the corpora cavernosa and corpus spongiosum of the penis become engorged with blood, the penis enlarges and hardens; this phenomenon is called erection. The average adult penis is 7-10 centimeters long, and its length can increase during erection. The penis is also the male's sexual organ.

2. Scrotum

The scrotum is a sac of skin located behind the penis. The scrotal skin is thin, soft, and elastic, with many wrinkles, abundant sweat glands, and a small amount of pubic hair. The scrotum is sensitive to temperature changes. When heated, the scrotum relaxes; when cooled, the scrotum contracts, and the testicles rise.

3. Testes

The testicles are located in the scrotum, one on each side, and are oval in shape. The left testicle is slightly lower and smaller than the right. The surface of the testicle is covered by two smooth membranes with a small amount of fluid between them, allowing the testicle to slide naturally without injury during strenuous exercise. An important internal structure of the testicle is the seminiferous tubules, where sperm are produced. Within the ductus arteriosus between the seminiferous tubules are groups of interstitial cells, which produce male hormones (androgens, or testosterone). The testicles are the most important organs in the male reproductive system.

4. Epididymis

The epididymis is located behind the testis and consists of a coiled tube. One end connects to more than 20 efferent ductules of the testis, and the other end connects to the vas deferens. It is the pathway for sperm to travel from the testis to the vas deferens. Sperm produced in the testis need to remain in the epididymis for a period of time to fully mature. The epididymis also has the function of storing sperm and absorbing dead sperm.

5. Vas deferens

There are two vas deferens, one on each side, which are the conduits for transporting sperm. They are about the thickness of a matchstick and approximately 50 centimeters long. The fluid secreted within the vas deferens provides nutrients to the sperm and promotes their maturation. Contraction of the vas deferens allows sperm to be expelled. The vas deferens ascends through the pelvis into the lower abdomen and connects with the seminal vesicles. The excretory ducts of the seminal vesicles connect to the ejaculatory ducts, which open into the prostatic urethra. The section of the vas deferens located in the upper part of the scrotum is easily palpable due to the thin skin of the scrotum; vasectomy is performed at this location.

6. Prostate

The prostate is a solid gland, about the size of a chestnut, whose secretions are the main component of semen. The prostate is located around the base of the urethra, closely attached to the rectum. The urethra passes through the prostate and is connected to it by ducts.

7. Seminal vesicles

There are two seminal vesicles, slightly flattened like sacs, located behind the bladder and above the prostate. The seminal vesicles secrete an alkaline, gelatinous fluid containing protein, which is part of the semen and plays a role in nourishing sperm and promoting sperm motility.

8. Bulbourethral glands and paraurethral glands

The bulbourethral glands are located between the two layers of the triangular ligament and open into the bulbous urethra. The paraurethral glands are distributed in the anterior urethra. When sexual arousal occurs, they secrete clear mucus, which is discharged from the urethral opening and serves as a lubricant during intercourse.

Function

1. Testes

The testes, the primary male reproductive organ, have a dual function: producing sperm and secreting male hormones. Sperm production begins in puberty and ceases with age. In addition to sperm production, the testes also secrete male hormones from the interstitial cells of the seminiferous tubules, with testosterone being the most potent. The main functions of male hormones are: ① promoting sperm production and development; ② promoting the development of male accessory sex organs and maintaining their normal activity; ③ stimulating and maintaining male secondary sexual characteristics; ④ promoting protein synthesis, muscle development, and bone growth.

The spermatogenic and endocrine functions of the testes are not two isolated processes; they influence each other.

2. Epididymis

After sperm are produced in the testes, they do not move after leaving the seminiferous tubules and must undergo a maturation process in the epididymis. In addition, because the testes continuously produce sperm but do not ejaculate frequently, a considerable number of sperm remain in the epididymis, thus making it a "storage depot." Sperm are typically stored in the epididymis for 20 days, but can be stored for up to 40 days. However, sperm stored for too long have a significantly reduced fertilization capacity. If sperm are not ejaculated for an extended period, the epididymis can also digest and absorb some of them.

3. Vas deferens and ejaculatory duct

The vas deferens is the conduit for transporting sperm. Therefore, based on this physiological function, vasectomy is performed on men to prevent the vas deferens from carrying sperm, thereby achieving sterilization.

4. Seminal vesicles

The main physiological function of the seminal vesicle is to produce and secrete seminal fluid. Seminal fluid is the main component of semen, accounting for 50-80% of the volume of each ejaculation. Seminal fluid contains two important substances: one is fructose, which provides sperm with the necessary energy; the other is enzymes, which can keep the semen ejaculated into the female reproductive tract coagulated for a certain period of time, preventing it from flowing out of the vagina.

5. Penis and scrotum

The penis is the male sexual organ, which also serves the dual function of ejaculation and urination.

The primary functions of the scrotum are to protect the testicles and regulate temperature. This is because the scrotum's skin is rich in sweat glands, which help regulate temperature, keeping the testicles in a constant-temperature environment. The temperature required for sperm production in the human testicles is lower than body temperature. In hot summer weather, the scrotum's skin expands to increase its surface area, thereby dissipating heat and cooling the testicles, which is beneficial for sperm production and storage.

6. Prostate

In the male reproductive system, the prostate is the largest accessory gland. The prostate is a chestnut-sized, rice-gland-like, semi-fibrous muscular structure that completely surrounds the beginning of the urethra and then closely adheres to the rectum. The urethra passes through the prostate and is connected to it by ducts.

The prostate is surrounded by a thin but tough sac, separated by the rectovesical fascia.

The prostate gland is divided into five lobes: Anterior lobe: Located at the junction in front of the urethra, also called the isthmus, it contains no glandular tissue. Middle lobe: Located behind the urethra and in front of the two ejaculatory ducts. It also contains very little glandular tissue. When this lobe enlarges, it can act like a valve, blocking the internal urethral opening and causing significant obstruction to urination. The more the patient strains to urinate, the more tightly it blocks the urethral opening. Sometimes, even if the enlargement of this lobe is not severe, the urinary obstruction can be very serious. Posterior lobe: The part behind the ejaculatory ducts. Lateral lobes: During a digital rectal examination, there is a shallow longitudinal groove in the center; the two lateral lobes are located on either side of this groove.

The prostate gland secretes a milky fluid. During ejaculation, prostatic fluid, seminal vesicle fluid, sperm from the epididymis and vas deferens, along with secretions from the bulbourethral glands, are expelled through the urethra. Prostatic fluid accounts for 13-32% of the volume of ejaculate in a single ejaculation.

Prostatic fluid contains a particularly high concentration of acid phosphatase and fibrinolytic enzymes, which liquefy coagulated semen. Prostatic fluid is alkaline, which helps to neutralize the acidic secretions in the vagina, creating a more suitable environment for sperm survival and motility. It also contains a large amount of hyaluronidase, which facilitates sperm penetration of the cervical mucus plug and the ovum's gelatinous glands, thus promoting conception.

Due to its important location, the prostate gland is clinically significant. After the age of 45, some men experience atrophy, but more often it enlarges, affecting urination.

The growth and development of the prostate are closely related to that of the testes. In childhood, the prostate is very small, especially the glandular part, which is very underdeveloped. When puberty begins, around the age of 16, it grows rapidly along with the development of the testes, reaching its peak at an average age of 24, and then remains in a stable state, performing its normal functions.