Men's Health Education: In-depth Analysis of the Physiological Structure and Function of the Penis and Scrotum

Reproductive Organs

Pension

The penis is the male sexual organ, with the anterior urethra passing through it. It has functions of urination and ejaculation. Morphologically, the penis has two states: erect (hard) and flaccid (soft). Penis size varies considerably from person to person. In my country, the average length of an adult male penis is 7–11 cm in the flaccid state and 14–18 cm in the erect state. Penis size is not directly related to a person's height or weight; a slender person does not necessarily have a small penis, and a large person does not necessarily have a large penis. Penis size, like height, is determined by genetics. Studies have also found that: penis size is unrelated to male sexual function; penis size is unrelated to a woman's ability to achieve orgasm; and the degree of stimulation of the female clitoris does not depend on penis size.

The penis is composed of two corpora cavernosa and one corpus spongiosum. The corpora cavernosa are relatively large, located dorsally, tapering at the posterior end to form the penile crura, which are fixed to the pubic bone, forming the root of the penis. The corpus spongiosum is smaller, located ventrally, with its anterior end enlarging to form the glans penis. The urethral opening is located below the midline of the glans penis. The penile skin is thin, without pubic hair, and the subcutaneous tissue is loose, suitable for the stretching of erectile tissue. The penis and corpora cavernosa are enclosed by a strong tunica albuginea, and the three corpora cavernosa are further bound together by the penile fascia, forming a round body.

The corpora cavernosa are the erectile tissue. They consist of numerous sheet-like and columnar trabeculae and the spaces between them. The trabeculae are composed of connective tissue, elastic fibers, and smooth muscle, interwoven into a network containing tortuous spiral arteries with varying wall thicknesses; the thicker sections protrude into the lumen and run longitudinally along the long axis of the vessels.

Penile erection is a highly complex process of sexual response control, governed by the spinal cord, brain, and sex hormones. The physiological basis of penile erection lies in the engorgement of the corpora cavernosa under the control of the erection center. Penile erection can be triggered by psychogenic stimuli, such as visual, olfactory, auditory, tactile, and sexual fantasies;

it can also be triggered by reflexive stimuli, such as exsensory stimulation of the genital area and endsensory stimulation of the bladder and rectum, triggering an arousal reflex and leading to erection. When male sexual arousal begins, the erection center generates nerve impulses due to excitation. These impulses travel along the erection nerves to the genitals, causing the penile arteries to dilate, the "body of erection" within the penile veins to contract, and the communicating branches between arteries and veins to close, allowing blood to flow into the corpora cavernosa, making the penis thicker. Simultaneously, due to the restriction of the penile fascia, the veins beneath the fascia are compressed and closed, further strengthening the erect penis.

The skin of the penis is thinner and softer than the skin of other parts of the body, allowing for a certain degree of mobility. At the glans penis, the skin folds into a double layer, forming the foreskin. The cavity formed between the foreskin and the glans is called the foreskin cavity. The foreskin cavity should be cleaned frequently; otherwise, smegma (prepuce) easily accumulates inside. Long-term irritation of the glans penis, especially the coronal sulcus, by smegma can easily cause inflammation, commonly known as coronal sulcus inflammation, a very common clinical condition. This condition is one of the causes of penile cancer. Beneath the glans penis, the fold connecting the foreskin to the urethral opening is called the frenulum. The frenulum and the skin near the middle of the foreskin are also major erogenous zones for men. As men age, the foreskin gradually retracts towards the back of the glans, gradually exposing the glans.

The penis is exceptionally rich in blood vessels. The penis is primarily innervated by the 2nd to 4th sacral nerves. Motor nerves reach the penis via the pudendal nerve and the pelvic nerve plexus. The parasympathetic nervous system originates from the pelvic nerves and is the primary nerve for erection; hence, it is also called the erectile nerve. Penile sensation is mainly innervated by the dorsal penile nerve, transmitted to the sacral spinal cord via pudendal nerve sensory fibers.

Scrotum

The scrotum is located at the base of the penis, resembling a thin-walled, resilient leather pouch. It lies between the base of the penis and the perineum, hanging when a person stands. It connects to the pubic bone, penile skin, inner thigh skin, and perineal skin. The scrotum contains the testes, epididymis, and the initial segment of the ureter, separated by the scrotal septum.

The skin of the scrotum is soft, thin, smooth, and elastic, rich in sweat glands and sebaceous glands, with noticeable pigmentation and a small amount of pubic hair. Beneath the skin is a single layer of smooth muscle membrane containing dense connective tissue and elastic fibers. The anterior and posterior walls of the scrotum have fibers extending deep along the midline, connecting to form a septum that divides the scrotum into two small pockets, containing the testes and epididymis on either side. The scrotum also contains the spermatic cord, a circular cord-like structure composed of the vas deferens, testicular artery, pampiniform plexus, small blood vessels, nerve plexus, lymphatic vessels, and capsule.

The scrotal skin is very sensitive to changes in external temperature. Lacking subcutaneous fat, the scrotum has abundant sweat glands, which aid in heat dissipation. When the external temperature is significantly lower than body temperature, the smooth muscles of the scrotum and the cremaster muscle contract, raising the position of the testes. The scrotal skin then tightens into dense folds to prevent heat loss and retracts to the perineum, thus helping to retain heat. Conversely, when the external temperature rises, the smooth muscles and cremaster muscle relax, the testes descend away from the body, and the scrotal skin relaxes, increasing the surface area for heat dissipation, thus facilitating local heat dissipation. Furthermore, the arteries in the spermatic cord are wrapped around bundles of parallel veins. Blood flows in opposite directions through these two vascular systems, separated only by thin vessel walls, forming a countercurrent exchange system. Venous blood continuously carries away heat from the arterial blood in the abdominal cavity, making the temperature of the testicular arterial blood 2-5°C lower than that of the abdominal aorta. The scrotum regulates the temperature of the scrotum and testes through these mechanisms. This ability of the scrotum is crucial for human reproduction, as temperature significantly affects spermatogenesis. A temperature approximately 2°C lower than the body's internal temperature is the optimal temperature for spermatogenesis. Excessive temperature can hinder or even completely halt spermatogenesis.

The skin color of the scrotum is often darker due to the influence of sex hormones. The concentration of sex hormones in the sex organs and other areas is high, and long-term hormonal stimulation results in significant melanin deposition, darkening the skin, especially noticeable as wrinkles thicken.