Male Reproductive Structures: Penis and Scrotum

1. Introduction

The function of the male, or testicular, reproductive system is to produce sperm and transfer them to the female reproductive tract. The paired testes are a crucial component in this process, as they produce both sperm and androgens, the hormones that support male reproductive physiology. For people with a penis, several accessory organs and ducts aid the process of sperm maturation and transport the sperm and other seminal components to the penis, which may deliver sperm to the female reproductive tract.

Male reproductive anatomy includes both external (scrotum and penis) and internal (testes, seminal vesicles, prostate gland, and bulbourethral glands) structures. These structures as well as their location and function are summarized in the table below. The remainder of this lesson will explore the external structures in more detail, and a future lesson will explore the internal structures.

2. The Penis

The penis is the male organ of copulation (sexual intercourse). It is flaccid for non-sexual actions, such as urination, and turgid and rod-like upon sexual arousal. When erect, the stiffness of the organ allows it to penetrate the vagina and deposit semen (ejaculatory fluid, which includes sperm) into the female reproductive tract.

The root of the penis, or radix penis, is the most proximal part of the human penis and is internal, located within the perineum (in males, this is the area between the anus and scrotum). The shaft (or body) of the penis surrounds the urethra. The shaft is composed of three column-like chambers of erectile tissue that span the length of the shaft. Each of the two larger lateral chambers is called a corpus cavernosum (plural = corpora cavernosa). 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 that surrounds the spongy, or penile, urethra. The corpora cavernosum and spongiosum are composed of erectile tissue that fill with blood during an erection.

The neck separates the shaft from the end of the penis, called the glans penis, which has a high concentration of nerve endings; this results in very sensitive skin that influences the likelihood of ejaculation. The proximal part of the glans penis is a projecting border referred to as the corona. The skin from the shaft extends down over the glans and forms a collar called the prepuce (or foreskin in males). The foreskin also contains a dense concentration of nerve endings, and both lubricate and protect the sensitive skin of the glans penis. At the tip of the glans penis is the external urethral meatus, which is the urethral opening.

did you know

A surgical procedure called circumcision, often performed for religious or social reasons, removes the prepuce, typically within days of birth.

Both sexual arousal and REM sleep (during which dreaming occurs) can induce an erection. Penile erections are the result of vasocongestion, 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 blood vessels within the corpora cavernosa and spongiosum. The release of NO activates a signaling pathway that results in the 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. The result of this increased blood flow to the penis and reduced blood return from the penis is erection. Depending on the flaccid dimensions of a penis, it can increase in size slightly or greatly during erection.

EXAMPLE

Male Reproductive System Disorder: Erectile Dysfunction

Erectile dysfunction (ED) is a condition in which a person has difficulty either initiating or maintaining an erection. The combined prevalence of minimal, moderate, and complete ED is approximately 40% in males at age 40 and reaches nearly 70% by 70 years of age. In addition to aging, ED is associated with diabetes, vascular disease, psychiatric disorders, prostate disorders, the use of some drugs such as certain antidepressants, and problems with the testes resulting in low testosterone concentrations. These physical and emotional conditions can lead to interruptions in the vasodilation pathway and result in an inability to achieve an erection.

Recall that the release of NO induces relaxation of the smooth muscles that surround the penile arteries, leading to the vasodilation necessary to achieve an erection. To reverse the process of vasodilation, an enzyme called phosphodiesterase (PDE) degrades a key component of the NO signaling pathway called cGMP. There are several different forms of this enzyme, and PDE type 5 is the type of PDE found in the tissues of the penis. Scientists discovered that inhibiting PDE5 increases blood flow, and allows vasodilation of the penis to occur.

PDEs and the vasodilation signaling pathway are found in the vasculature in other parts of the body. In the 1990s, clinical trials of a PDE5 inhibitor called sildenafil were initiated to treat hypertension and angina pectoris (chest pain caused by poor blood flow through the heart). The trial showed that the drug was not effective at treating heart conditions, but many men experienced erection and priapism (erection lasting longer than 4 hours). Because of this, a clinical trial was started to investigate the ability of sildenafil to promote erections in men suffering from ED. In 1998, the FDA approved the drug, marketed as Viagra®.

Since the approval of the drug, sildenafil and similar PDE inhibitors now generate over a billion dollars a year in sales and are reported to be effective in treating approximately 70% to 85% of cases of ED. Importantly, men with health problems—especially those with cardiac disease taking nitrates—should avoid Viagra or talk to their physician to find out if they are a candidate for the use of this drug, as deaths have been reported for at-risk users.

terms to know

Penis

The male organ used for copulation.

Semen

Ejaculatory fluid composed of sperm and secretions from the seminal vesicles, prostate, and bulbourethral glands.

Radix Penis

The root of the penis that is the most proximal part of the human penis; it is internal and located within the perineum.

Shaft

The part of the penis that surrounds the urethra; it is located between the root and glans penis.

Corpus Cavernosum

(plural = corpora cavernosa) Either of two columns of erectile tissue in the penis that fill with blood during an erection.

Corpus Spongiosum

The column of erectile tissue in the penis that fills with blood during an erection and surrounds the penile urethra on the ventral portion of the penis.

Glans Penis

The bulbous end of the penis that contains a large number of nerve endings.

Prepuce

The flap of skin that forms a collar around, and thus protects and lubricates, the glans penis; also referred as the foreskin in males. In females, this is the fold of vulvar skin that covers the clitoris and is also referred to as the hood of the clitoris.

3. Scrotum

The testes, which you learned produce sperm, are located in a skin-covered, highly pigmented, muscular sack called the scrotum that extends from the body behind the penis. Sperm are immobile at body temperature; therefore, the testes are external to the body so that a correct temperature is maintained for motility. In land mammals, including humans, the pair of testes must be suspended outside the body so the environment of the sperm is about 2 °C lower than body temperature to produce viable sperm.

The dartos muscle makes up the subcutaneous muscle layer of the scrotum. It continues internally to make up the scrotal septum, a wall that divides the scrotum into two compartments, each of which houses one testis. Descending from the internal oblique muscle of the abdominal wall are the two cremaster muscles, which cover each testis like a muscular net. By contracting simultaneously, the dartos and cremaster muscles can elevate the testes in cold weather (or water), moving the testes closer to the body and decreasing the surface area of the scrotum to minimize heat loss. Alternatively, as the environmental temperature increases, the scrotum relaxes, moving the testes farther from the body core and increasing the scrotal surface area, which promotes heat loss. Externally, the scrotum has a raised medial thickening on the surface called the raphae.

terms to know

Scrotum

The external pouch of skin and muscle that houses the testes.

Dartos Muscle

The muscle that comprises the subcutaneous muscle layer of the scrotum; works in conjunction with the cremaster muscles to retract testes.

Scrotal Septum

A wall composed of tissue that divides the scrotum into two compartments, each of which includes one testis.

Cremaster Muscles

Muscles that cover the testes; works in conjunction with the dartos muscle to retract testes.