Penis

The penis is the external male genital organ involved in both the urinary and reproductive systems. It is located in the perineal region, extending from the root, which is anchored to the pelvic bones, to the external portion visible outside the body. The penis is positioned above the scrotum, which houses the testicles. It consists of three main parts: the root, which anchors the penis to the pelvic region; the shaft, or body, which extends outward; and the glans, the rounded tip of the penis. The urethra runs through the penis, allowing for the passage of both urine and semen.

Structure and Anatomy

The penis is a complex structure composed of erectile tissues, connective tissues, muscles, nerves, and blood vessels. It plays an essential role in both the urinary and reproductive systems. Below is a detailed description of its anatomical components.

External Structure

The penis can be divided into three main parts: the root, the shaft (or body), and the glans.

• Root of the PenisThe root of the penis is the portion that is anchored to the pelvic bones and lies deep within the perineal region. It consists of:
  • Crura of the Corpora Cavernosa: These are the two diverging extensions of the corpora cavernosa that attach to the ischial and pubic rami of the pelvis. The crura form the lateral parts of the root and are anchored to the pelvic bones.
  • Bulb of the Penis: The bulb is the proximal, enlarged portion of the corpus spongiosum, located in the midline between the two crura. It is attached to the perineal membrane and surrounds the spongy urethra.
• Shaft (Body) of the PenisThe shaft, or body of the penis, is the elongated, cylindrical portion that extends outward from the root. It is the visible part of the penis and consists of the following structures:
  • Corpora Cavernosa: The corpora cavernosa are two cylindrical bodies of erectile tissue that run parallel along the length of the penis. They are positioned on the dorsal (top) side of the shaft and are responsible for the rigidity of the penis during erection. Each corpus cavernosum is surrounded by a tough fibrous sheath called the tunica albuginea, which helps maintain the structure and rigidity of the erectile tissue.
  • Corpus Spongiosum: The corpus spongiosum is a single cylindrical body of erectile tissue that runs along the ventral (bottom) side of the shaft, surrounding the spongy urethra. The corpus spongiosum is more flexible than the corpora cavernosa, and it expands at the distal end to form the glans penis.
  • Skin and Fascia: The shaft of the penis is covered by skin and a thin layer of subcutaneous tissue. Beneath the skin, the penis is encased in two layers of fascia: the superficial (Dartos) fascia and the deep (Buck’s) fascia. These layers provide support and allow the skin to move freely over the underlying erectile tissues.
• Glans PenisThe glans penis is the conical, bulbous tip of the penis, formed by the distal expansion of the corpus spongiosum. It is covered by a thinner, more delicate layer of skin compared to the shaft. The glans has several important anatomical features:
  • Corona of the Glans: The corona is the raised edge that encircles the base of the glans, marking the transition between the glans and the shaft. Behind the corona is a groove called the coronal sulcus.
  • Urethral Meatus: The external urethral meatus is the opening at the tip of the glans through which urine and semen exit the body.
  • Foreskin (Prepuce): In uncircumcised males, the glans is covered by the foreskin, a retractable fold of skin that protects the glans. The foreskin can be retracted to expose the glans.

Internal Structure

The penis contains three cylindrical masses of erectile tissue: two corpora cavernosa and one corpus spongiosum. These erectile tissues are responsible for the ability of the penis to become erect during sexual arousal.

• Corpora CavernosaThe corpora cavernosa are paired structures that run along the length of the penis, extending from the crura at the root to the distal shaft. They are primarily responsible for penile rigidity during erection. The structure of the corpora cavernosa includes:
  • Trabeculae: The corpora cavernosa are made up of a network of trabeculae, which are smooth muscle fibers and connective tissue that create a mesh-like framework. The spaces between the trabeculae are called cavernous spaces (lacunae), which fill with blood during erection.
  • Tunica Albuginea: Surrounding each corpus cavernosum is a thick, fibrous layer called the tunica albuginea. This layer provides structural support and helps maintain the rigidity of the penis during erection by trapping blood within the cavernous spaces.
• Corpus SpongiosumThe corpus spongiosum is a single column of erectile tissue located on the ventral side of the penis. It surrounds the spongy (penile) urethra, protecting it during erection. Unlike the corpora cavernosa, the corpus spongiosum remains softer during erection, allowing the urethra to remain open for the passage of semen and urine. The corpus spongiosum expands distally to form the glans penis.
• Spongy UrethraThe spongy urethra (penile urethra) runs through the center of the corpus spongiosum. It carries both urine from the bladder and semen from the reproductive organs to the external urethral meatus at the tip of the glans. The urethra begins at the bulb of the penis and continues through the entire length of the shaft, opening at the urethral meatus in the glans.

Vascular Supply

The penis has a rich blood supply, primarily from the internal pudendal artery, a branch of the internal iliac artery. Blood flow to the penis is essential for erectile function and overall penile health.

• Arterial Supply
  • Deep Arteries of the Penis: The deep arteries of the penis (also called the cavernosal arteries) run through the center of each corpus cavernosum. They are responsible for delivering blood to the erectile tissue, enabling the corpora cavernosa to engorge with blood during erection.
  • Dorsal Arteries of the Penis: The dorsal arteries run along the dorsal (top) side of the penis and supply blood to the skin and other superficial structures of the penis.
  • Artery of the Bulb of the Penis: This artery supplies blood to the bulb of the penis and the corpus spongiosum.
• Venous DrainageThe venous drainage of the penis occurs through the deep dorsal vein and other smaller veins that carry blood away from the erectile tissues after an erection.Deep Dorsal Vein: The deep dorsal vein drains blood from the corpora cavernosa and the glans penis, returning deoxygenated blood to the systemic circulation.

Nerve Supply

The penis is richly innervated by both autonomic and somatic nerves, which are responsible for regulating sexual function and providing sensory feedback.

• Autonomic Nerves
• • Parasympathetic Nerves: Parasympathetic fibers, which originate from the pelvic plexus, are responsible for initiating erection by causing vasodilation of the blood vessels supplying the erectile tissues. This vasodilation increases blood flow to the penis during sexual arousal.
  • Sympathetic Nerves: Sympathetic fibers, also from the pelvic plexus, are involved in controlling ejaculation and the resolution of erection.
• Somatic Nerves:Dorsal Nerve of the Penis: A branch of the pudendal nerve, the dorsal nerve of the penis provides sensory innervation to the skin, glans, and other parts of the penis. This nerve is responsible for the sensations of touch, pressure, and temperature.

Fascia and Skin

The penis is covered by multiple layers of fascia and skin, which protect the underlying structures and allow for flexibility and mobility.

• Superficial Fascia (Dartos Fascia)The superficial fascia, also known as Dartos fascia, is a thin layer of connective tissue that lies just beneath the skin of the penis. It contains smooth muscle fibers and allows for the movement of the skin over the underlying structures.
• Deep Fascia (Buck’s Fascia)The deep fascia, or Buck’s fascia, is a thicker layer of connective tissue that lies beneath the superficial fascia. It surrounds the corpora cavernosa and the corpus spongiosum, providing structural support and helping to contain the erectile tissues during erection.
• SkinThe skin of the penis is thin and loosely attached to the underlying structures, allowing it to stretch during erection. In uncircumcised males, the foreskin (prepuce) covers the glans when the penis is flaccid. The foreskin can be retracted to expose the glans during sexual activity or cleaning.

Development and Embryology

The penis develops from the genital tubercle during fetal development under the influence of male sex hormones (androgens). The corpora cavernosa, corpus spongiosum, urethra, and glans all arise from the differentiation of tissues in the embryonic genital region.

Relationship to Surrounding Structures

• Scrotum: The scrotum, which contains the testicles, is located just beneath the penis. It is attached to the perineum and lies directly below the root of the penis.
• Perineal Region: The root of the penis is located in the perineal region, which provides structural support and contains the muscles that aid in sexual and urinary functions.

Function

The penis serves several critical functions, primarily related to the male reproductive and urinary systems. These functions include facilitating sexual intercourse, ejaculation, urination, and sensory feedback. Below is a detailed description of the key functions of the penis.

Reproductive Function: Facilitating Sexual Intercourse

The penis is essential for male reproductive activity, particularly during sexual intercourse, where it functions as the organ of penetration. Several sub-functions contribute to this role:

• Erection:
  • During sexual arousal, blood flows into the corpora cavernosa and corpus spongiosum, causing the penis to become firm and erect. This process allows the penis to penetrate the vagina for reproduction.
  • The corpora cavernosa provide the rigidity needed for penetration, while the corpus spongiosum remains softer to protect the urethra during erection.
  • Parasympathetic nervous system fibers trigger the relaxation of smooth muscle in the trabeculae of the erectile tissues, allowing blood to flow into the cavernous spaces. This process is facilitated by the deep arteries of the penis.
• Penetration:Once the penis is erect, it can enter the female vagina during intercourse, allowing for the transfer of sperm. The stiffness and length provided by the corpora cavernosa help facilitate effective penetration.
• Sexual Sensation:
  • The penis is richly innervated with sensory nerves, particularly in the glans penis. The dorsal nerve of the penis, a branch of the pudendal nerve, provides sensory feedback, enhancing pleasure during intercourse.
  • Mechanoreceptors in the glans and shaft of the penis detect pressure, touch, and movement, contributing to the sensation of sexual pleasure.

Ejaculation: Expelling Semen

The penis is responsible for ejaculation, the expulsion of semen from the male reproductive system through the urethra. This process is necessary for the delivery of sperm to the female reproductive system for fertilization.

• Emission:
  • The first phase of ejaculation is emission, where semen is moved from the seminal vesicles, prostate, and vas deferens into the urethra. This phase is controlled by the sympathetic nervous system.
  • The smooth muscle in the walls of the reproductive organs contracts, pushing seminal fluid into the prostatic urethra.
• Ejaculation:
• • The second phase involves the forceful expulsion of semen through the spongy urethra and out of the penis. This is accomplished through rhythmic contractions of the bulbospongiosus muscle, which surrounds the bulb of the penis.
  • The bulbospongiosus muscle contracts to compress the bulb of the penis, expelling semen through the urethra with sufficient force.
  • Reflex arcs from the somatic nervous system help trigger these muscle contractions, while sympathetic fibers control the closure of the bladder neck to prevent the backflow of semen into the bladder (retrograde ejaculation).
• Role of the Urethra:The spongy urethra, which runs through the corpus spongiosum, serves as the conduit for semen during ejaculation. The expansion of the corpus spongiosum helps keep the urethra open, allowing semen to be expelled without obstruction.

Urination: Expelling Urine

The penis also plays an essential role in the urinary system, functioning as the conduit for urine to leave the body.

• Passage of Urine:
  • Urine from the bladder travels through the urethra, which runs the length of the penis. The urethra carries urine from the bladder neck to the external urethral meatus, located at the tip of the glans penis.
  • The spongy urethra, which is surrounded by the corpus spongiosum, allows urine to flow through the penis. The urethra remains open during urination, as the corpus spongiosum prevents it from being compressed.
• Control of Urine Flow:
• • Urine flow is controlled by the external urethral sphincter and the bulbospongiosus muscle. After urination, the bulbospongiosus muscle contracts to expel any remaining urine from the urethra, ensuring that the urethra is fully emptied.
  • The foreskin, in uncircumcised males, can be retracted during urination to prevent urine from accumulating under the foreskin.

Sensory Function: Sexual Arousal and Pleasure

The penis is a highly sensitive organ, with numerous sensory receptors that contribute to sexual pleasure and arousal.

• Sensory Receptors:The penis, particularly the glans, contains a high density of sensory nerve endings, including mechanoreceptors and free nerve endings, which detect changes in pressure, touch, and temperature. These receptors are stimulated during sexual activity, leading to sensations of pleasure.
• Erogenous Zone:The glans penis, along with the corona and frenulum, are particularly sensitive areas that contribute to sexual stimulation. The frenulum, located on the underside of the glans, is especially responsive to touch and pressure, adding to the overall sensation during sexual activity.
• Role of the Dorsal Nerve of the Penis:The dorsal nerve of the penis, a branch of the pudendal nerve, provides sensory innervation to the skin and glans. It transmits signals of pleasure from the penis to the brain during sexual activity, contributing to the experience of sexual arousal and orgasm.

Role in Maintaining Urethral Patency During Erection

The penis must maintain the patency (openness) of the urethra during erection to allow for the passage of semen and urine at appropriate times.

Protection of the Urethra:

• The corpus spongiosum, which surrounds the urethra, prevents the urethra from being compressed during erection. This allows the urethra to remain open for ejaculation while also protecting it from the high pressures generated during penile rigidity.
• By maintaining an open urethra, the penis ensures that semen can be expelled during ejaculation, even when the penis is fully erect.

Structural Support and Stability

The root of the penis, composed of the crura of the corpora cavernosa and the bulb of the penis, provides structural stability during both flaccid and erect states.

• Anchoring to the Pelvic Bones:The crura, which attach to the ischial rami of the pelvis, stabilize the penis during sexual activity. This firm attachment allows the external portion of the penis to remain aligned with the body, preventing excessive movement during erection and penetration.
• Bulb Stability:The bulb of the penis, which is part of the corpus spongiosum, is attached to the perineal membrane. This attachment provides support to the root of the penis, helping maintain its position during sexual activity.

Thermoregulation and Protection of the Glans

In uncircumcised males, the foreskin plays a role in protecting and regulating the temperature of the glans penis.

• Protection:The foreskin covers the glans when the penis is flaccid, protecting it from external environmental factors, such as friction and contaminants. It helps keep the glans moist by creating a protective barrier, reducing the risk of dryness or irritation.
• Thermoregulation:The Dartos muscle in the superficial fascia of the penis, particularly in the foreskin, helps regulate the tension and temperature of the skin. In cold conditions, the foreskin can contract to preserve warmth, while in warm conditions, it relaxes to increase ventilation.

Clinical Significance

The penis plays a central role in both the urinary and reproductive systems, and its health is crucial for overall male well-being. Various medical conditions and injuries can affect its function, leading to significant clinical implications:

• Erectile Dysfunction (ED): One of the most common conditions affecting the penis, ED is the inability to achieve or maintain an erection sufficient for sexual intercourse. It can result from vascular, neurological, psychological, or hormonal issues and has a significant impact on a man’s quality of life.
• Peyronie’s Disease: This condition involves the development of fibrous scar tissue (plaque) inside the tunica albuginea, leading to abnormal curvature of the penis during erection. It can cause pain, erectile dysfunction, and difficulties with sexual intercourse.
• Penile Cancer: Though rare, penile cancer can affect the skin and tissues of the penis, particularly the glans and foreskin. Early detection and treatment are crucial for favorable outcomes.
• Phimosis and Paraphimosis: Phimosis is the inability to retract the foreskin over the glans, leading to potential hygiene issues and infection (balanitis). Paraphimosis occurs when the foreskin is retracted and cannot return to its normal position, potentially restricting blood flow to the glans and requiring urgent treatment.
• Trauma and Fracture: Physical trauma, including penile fractures, can occur due to injury during sexual activity or accidents. This requires immediate medical attention to prevent long-term damage to erectile function.