The urethra is a fibromuscular tube that conveys urine from the urinary bladder to the exterior of the body. It marks the final pathway of the urinary tract. While sharing this role in both sexes, the tube also serves the reproductive system in males by providing an exit for semen during ejaculation. Understanding its layout is vital for safe catheterization procedures in clinical practice.
Significant sexual dimorphism characterizes the length and path of the human urethra. The female version is short, measuring roughly 4 centimeters, and runs a straight course directly anterior to the vagina. Conversely, the male version spans roughly 20 centimeters, making a convoluted S-shaped path through the pelvis and penis. This drastic difference in length explains why women experience urinary tract infections much more frequently than men.
The long male urethra divides into four distinct anatomical segments from proximal to distal:
The female urethra travels downward and forward behind the pubic symphysis. Its posterior wall is closely bound to the anterior wall of the vagina, creating a shared connective tissue septum. The external urethral orifice opens into the vestibule of the vulva, located directly anterior to the vaginal opening and posterior to the clitoris. Because of these close anatomical relations, pelvic floor trauma during childbirth can impact urethral support.
The cellular lining of the urethra changes along its pathway to withstand varying mechanical and chemical stresses. Near the bladder neck, transitional epithelium predominates. As it extends distally, the lining shifts to pseudostratified or stratified columnar epithelium, and finally transitions to stratified squamous epithelium near the external orifice. Small mucous glands, known as urethral glands, empty into the lumen to lubricate and protect the mucosa.
Continence relies on two distinct muscular rings. The internal urethral sphincter consists of involuntary smooth muscle fibers located at the bladder neck, controlled by the autonomic nervous system. The external urethral sphincter is composed of voluntary skeletal muscle situated within the urogenital diaphragm, innervated by the pudendal nerve. Coordinated relaxation of these sphincters alongside bladder contraction allows normal voiding.
Blood reaches the urethra via branches of the internal iliac artery, including the inferior vesical and internal pudendal arteries. Venous blood drains into matching plexuses that empty into the internal iliac veins. Nerve supply is mixed: sympathetic and parasympathetic fibers control involuntary components, while somatic fibers manage the external sphincter. Lymphatic drainage flows primarily into internal and external iliac lymph nodes.
Beyond carrying urine, the male urethra plays a dual role during sexual intercourse. It acts as a conduit for seminal fluid, propelled through the canal during ejaculation. Specialized glands along its path secrete mucus to neutralize any residual acidic urine traces before semen passes through. In females, the urethra remains dedicated solely to urinary elimination.