The penis is the primary organ used for copulation (sexual intercourse) in males. It is flaccid for non-sexual actions, such as urination, and turgid and rod-like with sexual arousal. When erect, the stiffness of the organ allows it to penetrate the vagina and deposit semen into the female reproductive tract (Figure 1).

The attached portion of the penis at the perineum is called the Root, beginning directly below the bulbourethral glands. The elongated portion of the penis is known as the shaft. The shaft surrounds the urethra and comprises three column-like chambers of erectile tissue that span the length of the shaft. The two larger chambers, each called a corpus cavernosum (plural = corpora cavernosa), are located at the dorsolateral surface of the shaft and terminate at the distal shaft of the penis. Together, these make up the bulk of the penis.

The corpus spongiosum, which can be felt as a raised ridge on the erect penis, is a smaller chamber located at the inferior and middle of the penis. This surrounds the spongy (penile) urethra, and continues till the end of the penis within the glans.

The end of the penis, called the glans penis, has a high concentration of nerve endings, resulting in very sensitive skin that influences the likelihood of ejaculation. The skin from the shaft extends down over the glans and forms a collar called the prepuce (or foreskin). The foreskin also contains a dense concentration of nerve endings and lubricates and protects the sensitive skin of the glans penis.

Clinical Correlation 

Circumcision

A surgical procedure to remove the foreskin, prepuce, typically done within days of birth.  It is often performed for religious or social reasons. Circumcision is associated with lower risk of sexually transmitted infections, including HIV. It can also decrease the risk of penile cancer and cervical cancer in sexual partners.

Erection and Ejaculation

Both erection and ejaculation are under the control of the autonomic nervous system stimulated at sexual arousal.

Sexual excitement causes parasympathetic division activation, causing erection.  Penile erections are the result of congestion, or engorgement of the tissues because of more arterial blood flowing into the penis than is leaving in the veins. During sexual arousal, nitric oxide (NO) is released from nerve endings near the blood vessels within the corpora cavernosa and spongiosum. Release of NO activates a signaling pathway that results in relaxation of the smooth muscles that surround the penile arteries, causing them to dilate. This dilation increases the amount of blood that can enter the penis and induces the endothelial cells in the penile arterial walls to also secrete NO and perpetuate the vasodilation. The rapid increase in blood volume fills the erectile chambers, and the increased pressure of the filled chambers compresses the thin-walled penile venules, preventing venous drainage of the penis. This increased blood flow to the penis and reduced blood return from the penis causes an erection. Depending on the flaccid dimensions of a penis, it can increase in size slightly or greatly during erection, with the average length of an erect penis measuring approximately 15 cm.

With continuous stimulation, the sympathetic nervous system gets activated, leading to emission and movement of semen into the urethra, followed my smooth muscle contraction of the penis, forcing semen out of the urethra and ejaculation to occur.

Medication Intake 

Viagra is the trade name for a medication known as sildenafil. These pills work by augmenting the effect of nitric oxide (NO) in the body, including the penis, and are commonly prescribed for managing erectile dysfunction (ED). It’s important to note that Viagra does not boost sexual desire; instead, it assists in the natural physiological process of attaining and sustaining an erection when stimulated sexually.