Scientific Male Health Care: In-depth Analysis of Physiological Structure and the Mechanism of Healthy Erection

Anatomical Structure and Erection of the Penis

The penis is the primary sexual organ and also functions as a urinary organ. The urethra, used for both urination and ejaculation, is enclosed by the corpus spongiosum. The distal end of the corpus spongiosum expands to form the tactilely sensitive glans penis. The urethral opening is located at the tip of the glans penis, through which both urine and ejaculate are expelled. During ejaculation, the muscles of the corpus spongiosum surrounding the bulbous urethra contract strongly, rhythmically ejecting semen. Penile erection results from the engorgement of the corpora cavernosa. The mechanism of penile erection involves multiple factors, including the intact function of the cerebral cortex, psychological factors, spinal cord, local stimulation, neural pathways, and vascular system, the normal state of the penile erectile tissue, and endocrine hormone levels. Problems in any of these areas can affect normal erectile function.

The penis is a cylindrical erectile organ, suspended and attached to the urogenital diaphragm. The two crura of the corpora cavernosa attach to the anterior surface of the pubic arch, while the dorsal side of the penis is composed of the triangular fascia sheath and the suspensory ligament of the penis, formed by the linea alba. The penis has three longitudinal corpora cavernosa forming the corpus spongiosum and two corpora cavernosa, all encased in the tough Buck's fascia. Between the corpora cavernosa and the skin lies a very loose layer of fatty connective tissue containing superficial penile blood vessels. The fascia between the penile skin and the corpora cavernosa continues to connect with Colle's fascia of the perineum and Scarpa's fascia of the lower abdominal wall. There are two larger corpora cavernosa on either side of the penis, and a smaller corpus spongiosum in the ventral middle. The corpus spongiosum contains the urethra. The corpora cavernosa are encased in a tough elastic sheath and are tightly attached to the distal end and penile suspensory fossa of the penis. The proximal end of the corpus spongiosum branches into two penile crura, attaching to the inferior pubic ramus, inferior ischial ramus, and the sublayer of the urogenital diaphragm, forming the fixed penile root. The penile root is deeply embedded between the urogenital diaphragm and the superficial perineal fascia in the perineum. The corpus spongiosum enlarges proximally to form the bulbous urethra. In addition, it is attached to the lower layer of the urogenital diaphragm, covered by the urethral spongiform muscle, and its distal end expands to form the glans penis, covering the distal end of the corpora cavernosa. The external urethral orifice is located anteriorly and inferiorly. In adults, the penis can double in length when erect. The ischiocavernosus muscle connects to the base of the penis, helping to maintain the erect position. The corpora cavernosa are mainly composed of blood vessels, nerves, smooth muscle cells, and interstitial tissue. Smooth muscle cells, collagen fibers, and elastic fibers interweave to form a network containing abundant blood vessels. The inner surface of the network is lined with flattened epithelium, forming blood sinuses that are interconnected and directly communicate with arteries and veins. Relaxation of the smooth muscle cells within the corpora cavernosa expands the blood sinus space, causing blood to stagnate and increasing the pressure within the sinuses. This contributes to improving the rigidity of the erection and the duration of erection.

The penis has a rich blood supply, divided into superficial and deep groups:

① The superficial group of arteries consists of the dorsal penile artery, a branch of the internal pudendal artery from the hypogastric artery. It runs beneath Buck's fascia, on either side of the deep dorsal vein. Its branches penetrate the tunica albuginea and enter the corpora cavernosa, where they, along with branches of the deep penile arteries that traverse the corpora cavernosa, disperse within the trabeculae of the penis. In the flaccid state, these trabecular arteries are tortuous, called spiral arteries, and elongate during erection. The terminal portion of the dorsal penile artery forms communicating branches at the coronal sulcus.

② The deep group of arteries branches from the internal pudendal artery. One branch enters the crura of the penis, called the deep penile artery, supplying the corpora cavernosa and forming the main blood supply. The internal pudendal artery also branches laterally into the corpus spongiosum, called the urethral arteries, supplying the entire length of the corpus spongiosum. In fact, the entire penis is supplied by the internal pudendal artery, while only a portion of the penile skin and subcutaneous tissue receives blood from branches of the superficial and deep external pudendal arteries, which branch off from the femoral artery.

The penile veins are divided into superficial and deep groups:

① The superficial group originates from the penile skin and flows into the superficial dorsal vein of the penis, returning to the great saphenous vein via the superficial external genital vein.

② The deep group originates from the erectile tissue of the penis and drains into the deep dorsal vein of the penis. This vein runs beneath Buck's fascia, enters the urogenital diaphragm, and merges with the prostatic venous plexus, participating in the formation of the Santorini venous plexus before the prostate gland. It then flows through the inferior vesicle vein and returns to the internal iliac vein.

The nerves of the penis are controlled by both the craniospinal nervous system and the autonomic nervous system. The dorsal penile nerve belongs to the craniospinal nerves and is the sensory nerve of the penis. The pudendal nerve originates from the 2nd, 3rd, and 4th sacral spinal cord segments and runs on both sides of the dorsal penile artery, supplying the glans penis and penile skin. The autonomic nervous system supplies the erectile tissues of the penis. Parasympathetic nerves, originating from the hypogastric plexus, accompany the arteries to the corpora cavernosa and corpus spongiosum. Excitation of the parasympathetic nerves causes vasodilation and penile erection; therefore, they are also called erectile nerves. The nerves that control vasoconstriction are the sympathetic nerves. Additionally, branches of the ilioinguinal nerve supply the penile skin at the base of the penis. Therefore, during circumcision, local anesthesia should include the epidermis and the dorsal nerve of the penis. The penis, especially the glans, has abundant sensory nerve endings, which are crucial for sexual arousal. During intercourse, the mechanical friction caused by the penis's thrusting within the vagina stimulates the penile sensory nerve endings, which are transmitted to the primary sexual centers in the spinal cord via the dorsal nerve of the penis.

Regarding the relationship between penile size and sexual function, there are often unscientific understandings. Penile length varies significantly from person to person. However, because the percentage of expansion volume in a smaller penis can reach 100%, while the volume change in a larger penis is much smaller, some compensation can be obtained. The upper two-thirds of a woman's vagina is not significantly related to the production of orgasm, so men with slightly smaller penises do not need to worry about it unnecessarily.