Detailed Explanation of the Anatomy and Functions of the Male Reproductive System
The male reproductive system includes the following organs: scrotum, testes, epididymis, vas deferens, seminal vesicles, ejaculatory ducts, prostate, and penis. The testes are enclosed within the scrotum; the scrotum and penis are the parts that protrude from the body and are called the external genitalia. The remaining organs are located in the lower abdomen and are called the internal genitalia. A brief introduction to each is given below.
1. The scrotum is located behind the penis and in front of the anus. It is a sac made of skin. The skin is brown, thin, and soft, with many wrinkles. The subcutaneous tissue contains a large number of smooth muscle fibers called the dartos fascia. The dartos fascia forms the scrotal septum along the midline, separating the testes and epididymis on both sides. The dartos fascia contracts when cold and expands when warm, thereby regulating the temperature inside the scrotum, which is conducive to sperm production and survival. When the muscles contract, the scrotum naturally contracts and shrinks, and the wrinkles deepen. The scrotal cavity is divided into two parts, left and right, and is lined with a smooth membrane that surrounds the testes and epididymis.
2. Testes: The testes are located in the scrotum, one on each side, and are oval in shape. In newborns, the testes are about the size of a peanut; in toddlers, they grow to the size of a sparrow's egg; and in adulthood, they are about the size of a pigeon's egg. Generally, the left testis is slightly larger and lower than the right. The surface of the testes is covered by a dense connective tissue membrane called the tunica albuginea, which slides naturally within the scrotum, thus preventing injury during strenuous exercise. At the posterior border of the testis, the tunica albuginea thickens and protrudes into the testicular parenchyma, forming radial septa that divide the testicular parenchyma into many cone-shaped lobules. Each lobule contains 2-3 seminiferous tubules, the epithelium of which is the site of sperm production. The connective tissue between the seminiferous tubules contains interstitial cells that secrete male hormones. At the tips of the testicular lobules, the seminiferous tubules converge into straight tubules, which then interweave to form a network, finally giving off more than ten efferent ductules at the posterior border of the testis, which enter the epididymis. As can be seen from the above, the testicles are the most important organ in the male reproductive system.
3. Epididymis: The epididymis is closely attached to the upper end and posterior border of the testis and can be divided into three parts: head, body, and tail. The head is formed by the coiling of efferent ductules, the ends of which connect to an epididymal duct. The epididymal duct is about 4-5 meters long, coiled to form the body and tail. The end of the duct abruptly turns upwards and continues directly into the vas deferens. In addition to storing sperm, the epididymal duct also secretes epididymal fluid, which contains certain hormones, enzymes, and specific nutrients that help sperm maturation.
4. The vas deferens and epididymis are connected by a common tube called the vas deferens. There is one on each side, with a well-developed muscular wall that feels like a tight, round cord when touched in vivo. The vas deferens is about 40 cm long and has a relatively long course, starting from the scrotum and extending to the subcutaneous tissue, then passing through the inguinal canal into the abdominal and pelvic cavities. Behind the base of the bladder and inside the seminal vesicles, it dilates to form the ampulla of the vas deferens, which tapers at its end and merges with the excretory ducts of the seminal vesicles to form the ejaculatory duct. It then passes through the prostate gland and leads into the urethra. Why doesn't the vas deferens directly enter the urethra nearby, but instead takes such a circuitous route? This is because during fetal development, the testes are not originally in the scrotum, but rather on the back of the abdominal cavity, near the kidneys. At this stage, the vas deferens enters the urethra from top to bottom. However, during growth, as the testes gradually descend, they pass through the abdominal wall and enter the scrotum, thus creating the circuitous path of the vas deferens. In some individuals, the testicles do not descend into the scrotum after birth, but remain on their descent path; this condition is clinically known as cryptorchidism.
5. Ejaculatory duct: The ejaculatory duct is about 2 cm long, penetrates the prostate parenchyma, and opens into the prostatic urethra.
6. Spermatic cord: The spermatic cord is a pair of flattened, round cords that extend from the upper end of the testis to the internal opening of the inguinal canal. It is mainly composed of the vas deferens, testicular artery, pampiniform plexus, nerve plexus, and lymphatic vessels, and is covered by three layers of fascia.
7. Seminal Vesicles: There are two seminal vesicles, slightly flattened from front to back like sacs, located at the bottom of the bladder, in front of the rectum. The main function of the seminal vesicles is to secrete a gelatinous fluid called seminal vesicle fluid, which is a component of semen and promotes sperm motility.
8. The prostate gland is a chestnut-shaped secretory gland located around the base of the urethra, slightly anterior to the seminal vesicles, and connected to the urethra by ducts. It secretes a milky fluid called prostatic fluid. During ejaculation, prostatic fluid, seminal vesicle fluid, sperm from the epididymis and vas deferens, and secretions from the bulbourethral glands (two small glands located on either side of the upper urethra, whose ducts also connect to the urethra) are expelled from the body through the urethra together; this is semen. Therefore, semen is composed of secretions from the seminal vesicles, prostate gland, and bulbourethral glands, plus sperm. Prostatic fluid promotes sperm motility and provides a suitable environment and nutrients for sperm.
9. The penis is a cylindrical organ that is normally flaccid and hangs in front of the scrotum. From an external perspective, the penis can be divided into three parts: the glans, the body, and the root. The glans is the swollen anterior part of the penis, with the urethral orifice at its tip; the slightly tapering part behind the glans is called the neck. The root is hidden deep in the skin, fixed to the inferior pubic and ischial ramusees. The part between the root and neck is the body. In fact, the penis consists of two corpora cavernosa and one corpus spongiosum, all covered by fascia and skin. The two corpora cavernosa are closely joined and located side-by-side on the dorsal side of the penis, their anterior ends embedded in the depression behind the glans, and their posterior ends separating to form the root. The corpus spongiosum is located on the ventral side of the corpora cavernosa, with the urethra running through its entire length; its anterior end is the glans, and its posterior end expands to form the bulbourethral region, fixed to the urogenital diaphragm. The corpora cavernosa are erectile tissues, covered by a thick tunica albuginea, and internally composed of connective tissue and smooth muscle forming a spongy framework, with cavities communicating with blood vessels. When the cavity fills with blood, the penis becomes thicker and harder, achieving an erection. The penile skin is thin and soft, with loose subcutaneous tissue that is easily stretched; however, the skin of the glans penis lacks subcutaneous tissue and is immobile. The skin of the penile shaft extends freely forward to the penile neck, forming a ring-shaped fold that surrounds the glans penis, called the foreskin. On the ventral midline of the glans penis, the fold of skin connecting the foreskin to the external urethral orifice is called the frenulum. Care should be taken not to damage this frenulum during circumcision.
10. Male urethra: The male urethra is both a urination passage and a ejaculation duct. It begins at the internal urethral orifice and ends at the external urethral orifice at the tip of the glans penis. In adults, it is about 18 cm long and can be divided into three parts: the prostatic part (the part that passes through the prostate), the membranous part (the part that passes through the urogenital diaphragm, about 1.2 cm long), and the spongy part (the part that passes through the corpus spongiosum). Clinically, the prostatic part and the membranous part are collectively referred to as the posterior urethra, and the spongy part is referred to as the anterior urethra.
The male urethra has three narrowings and two bends. The three narrowings are the internal urethral orifice, the membranous portion, and the external urethral orifice. The two bends are located below the pubic symphysis (corresponding to the initial segments of the membranous and spongy portions, concave upward) and anteroinferior to the pubic symphysis (corresponding to the space between the penile root and body, concave downward). The latter bend disappears when the penis is lifted upward. Therefore, in clinical practice, when performing catheterization or urethral dilation, the penis is first lifted to eliminate this bend, facilitating catheter insertion.
11. Male accessory glands
(1) Seminal vesicle: a flat, oval-shaped sac-like organ located behind the base of the bladder and on the outer side of the ampulla of the vas deferens. Its excretory duct merges with the end of the vas deferens to form the ejaculatory duct, and its secretions contribute to the formation of semen.
(2) Prostate: The main gland that secretes semen, it is chestnut-shaped and located between the base of the bladder and the urogenital diaphragm, with the prostatic urethra passing through it. The interstitium of the prostate contains a large amount of smooth muscle and is relatively firm. The ducts of the gland eventually merge into 20-30, opening into the prostatic urethra. The prostate is smaller in children. It grows rapidly after sexual maturity. In old age, the glandular tissue degenerates, and connective tissue proliferates, resulting in prostatic hyperplasia.
(3) Bulbourethral glands: These are a pair of pea-shaped glands embedded in the urogenital diaphragm. Their ducts open at the beginning of the corpus spongiosum of the urethra, and their secretions can lubricate the urethra during ejaculation.