The spongy urethra, also known as the penile urethra, is the longest portion of the male urethra. It extends from the end of the membranous urethra, which passes through the urogenital diaphragm, all the way to the external urethral orifice at the tip of the penis. The spongy urethra is approximately 15–20 cm in length, though this can vary depending on individual anatomy. It is surrounded by the corpus spongiosum, a spongy, erectile tissue that helps protect the urethra and facilitates the passage of urine and semen. The spongy urethra lies within the shaft of the penis and terminates in the navicular fossa.
Structure and Anatomy
The spongy urethra, also referred to as the penile urethra, is a critical segment of the male urethra. It is the longest portion of the urethral structure and is unique due to its encapsulation within the erectile tissue of the penis, the corpus spongiosum. Below is a detailed description of its anatomical features.
Location and Length
The spongy urethra extends from the distal end of the membranous urethra, which is located just beyond the urogenital diaphragm, to the external urethral orifice located at the tip of the glans penis. Its length typically measures about 15–20 cm, though this varies among individuals based on the size and length of the penis.
Relation to Surrounding Structures
The spongy urethra is completely surrounded by the corpus spongiosum, one of the three cylindrical bodies of erectile tissue in the penis. The corpus spongiosum serves to protect the urethra, especially during erection, ensuring that it remains open for the passage of fluids. The spongy urethra runs along the ventral (underside) of the penis in the midline.
At the proximal end, the spongy urethra connects with the membranous urethra, while at the distal end, it enters the glans penis where it widens to form the navicular fossa, just before terminating at the external urethral orifice.
Structure and Shape
The spongy urethra has a relatively uniform diameter along most of its length, except at two points:
- The Bulbous Portion: The initial part of the spongy urethra, closest to the root of the penis, is wider and referred to as the bulbous urethra. It is located within the bulb of the penis and is the thickest part of the spongy urethra.
- The Navicular Fossa: Towards the distal end, the spongy urethra widens again inside the glans penis to form the navicular fossa, just before narrowing at the external urethral orifice.
Glandular Openings
Scattered along the length of the spongy urethra are small glandular openings called the ducts of Littre’s glands. These glands, also known as urethral glands, secrete mucus that helps lubricate the urethral passage. Additionally, the bulbourethral glands (Cowper’s glands) open into the proximal part of the spongy urethra, near the bulbous portion. These glands contribute to the lubrication and preparation of the urethra prior to ejaculation.
Histological Features
The lining of the spongy urethra is composed of different types of epithelium. The majority of its length is lined by pseudostratified columnar epithelium, which transitions into stratified squamous epithelium as it approaches the distal end near the external urethral orifice. This gradual transition in epithelial type is important as the urethra moves from an internal structure to one exposed to the external environment.
Blood Supply and Innervation
The spongy urethra receives its blood supply primarily from branches of the internal pudendal artery, which is responsible for supplying blood to much of the external genitalia. The dorsal artery of the penis also contributes to its vascularization.
Innervation of the spongy urethra is provided by the dorsal nerve of the penis, a branch of the pudendal nerve, which is responsible for both sensory and motor innervation to this area. The autonomic innervation (sympathetic and parasympathetic) plays a key role in maintaining the functional tone of the erectile tissues that surround the spongy urethra.
Sections of the Spongy Urethra
- Bulbar Urethra: The proximal part of the spongy urethra located in the bulb of the penis. It is wider and contributes to the ejaculation process by receiving fluids from the bulbourethral glands.
- Penile Urethra Proper: The longest portion, running through the corpus spongiosum of the penile shaft, where it remains protected during erection.
- Navicular Fossa: The distal widening of the spongy urethra as it passes through the glans penis, leading to the external urethral orifice.
Function
The spongy urethra, also known as the penile urethra, plays a crucial role in both the urinary and reproductive systems. Its functions are centered on the passage of urine and semen, as well as ensuring the protection and integrity of the urethra during sexual activity.
Urine Conduction
The primary function of the spongy urethra is to conduct urine from the bladder to the external urethral orifice, allowing for its excretion from the body. The spongy urethra serves as the final passageway for urine, which travels through the membranous urethra before entering the spongy urethra. Its relatively long length provides an effective channel for urine to flow through the penis and exit the body.
The urethra is kept open during urination due to the surrounding corpus spongiosum, which prevents the urethra from collapsing during penile erection or contraction of the muscles. This ensures that urine can flow without obstruction, even under the mechanical pressures of the surrounding tissue.
Semen Conduction
The spongy urethra also serves as the final conduit for semen during ejaculation. It carries semen, which is composed of sperm and secretions from the prostate and seminal vesicles, from the prostate urethra to the outside of the body. The bulbourethral glands, which empty into the spongy urethra, also contribute to semen by secreting a lubricating fluid that helps neutralize the acidity of the urethra and facilitates the smooth passage of sperm.
During ejaculation, coordinated contractions of the pelvic floor muscles and other genital structures propel the semen through the spongy urethra and out of the penis. The anatomy of the spongy urethra, particularly the wider bulbous portion, allows for efficient semen conduction during ejaculation.
Protection of the Urethra During Erection
The spongy urethra is encapsulated within the corpus spongiosum, an erectile tissue that becomes engorged with blood during sexual arousal and erection. This protective tissue ensures that the urethra remains open and functional during erection, preventing it from being compressed by the surrounding tissues. This is a unique feature of the spongy urethra, as the other erectile tissues of the penis (corpora cavernosa) swell to allow for erection, while the corpus spongiosum provides structural support to keep the urethral lumen intact.
Contribution to Ejaculate Pre-Lubrication
The spongy urethra plays a role in preparing the urethra for the passage of semen by receiving secretions from the bulbourethral glands (Cowper’s glands). These glands secrete a pre-ejaculate fluid that enters the proximal portion of the spongy urethra, serving to lubricate the urethral lining and neutralize any residual acidity from urine. This pre-ejaculate fluid creates a more favorable environment for sperm, aiding in their survival and movement through the urethra during ejaculation.
Stream Control and Direction
Due to its length and the encapsulation by the corpus spongiosum, the spongy urethra also plays a role in directing and controlling the flow of urine and semen. The smooth passage through the penile urethra ensures that urine and semen exit the body without disruption. The slight widening of the spongy urethra at the navicular fossa helps manage the final output, allowing for a smoother and more controlled flow of fluids, whether during urination or ejaculation.
Barrier Protection from External Environment
The spongy urethra is lined by epithelium that serves as a protective barrier, particularly near the external urethral orifice, where it transitions to stratified squamous epithelium. This epithelial lining plays a key role in protecting the delicate tissues of the urethra from external pathogens and environmental factors. Additionally, the urethral glands (Littre’s glands) along the spongy urethra secrete mucus that aids in this protective function by lubricating and maintaining the integrity of the urethral lining.
Coordination with Other Urethral Segments
While the spongy urethra operates as the final segment of the urethra, its function is dependent on coordination with other segments, including the prostatic and membranous urethra. Fluids (urine or semen) are passed through these sections before entering the spongy urethra, which then completes the conduction of these fluids out of the body. The spongy urethra’s wider sections, such as the bulbous portion, allow for the smooth transfer of these fluids without obstruction.
Clinical Significance
The spongy urethra is clinically significant due to its location and involvement in both the urinary and reproductive systems. It is prone to a variety of conditions and injuries:
- Urethral Stricture: The spongy urethra is a common site for urethral strictures, which involve the narrowing of the urethra due to scar tissue from trauma, infections, or inflammation. This condition can lead to difficulty in urination, a weak stream, or complete urinary obstruction.
- Trauma: The spongy urethra is vulnerable to injury, especially in cases of pelvic fractures, perineal trauma, or during the insertion of medical instruments like catheters. Injuries can result in urethral tears or ruptures, requiring surgical intervention.
- Hypospadias: A congenital abnormality where the urethral opening is located on the underside of the penis rather than at the tip. This affects the spongy urethra and often requires surgical correction to restore normal function.
- Urethritis: Infections, including sexually transmitted infections (STIs) like gonorrhea or chlamydia, can cause inflammation of the spongy urethra, leading to pain, burning sensations during urination, and discharge.
- Fistulas: Abnormal connections between the urethra and surrounding tissues (urethral fistulas) can occur in the spongy urethra, often due to trauma or surgical complications, and may result in urine leakage into other areas.
