Urethra

Urethra

The urethra is a relatively thin-walled, muscular tube that conveys urine from the urinary bladder to outside the body. It is lined with mucous membrane known as pseudostratified columnar epithelium and a thick layer of smooth muscle tissue. The portions of the ure-thra nearer the urinary bladder become transitional epithelium. The wall of the urethra has many mucous glands, also called urethral glands, which secrete mucus into the urethral canal. In females, the urethra is only about 3–4 cm or 1.5 inches long, opening via the external urethral orifice or urinary meatus.

In males, the urethra is about 20 cm or 8 inches long. It functions as part of both the urinary and repro-ductive systems, extending from the bladder to the tip of the penis. In both males and females, the portion of the urethra near the external opening becomes protective stratified squamous epithelium. There are three regions of the urethra in males: the prostatic, membranous, and spongy urethra. The prostatic urethra is inside the prostate gland and is about 2.5 cm or 1 inch in length. The membranous urethra is its intermediate portion, located inside the urogenital diaphragm. It extends from the prostate for about 2 cm to the beginning of the penis. The spongy urethra is about 15 cm in length, continu-ing throughout the penis up to its opening at the tip.

The detrusor muscle originating in the ­bladder becomes thicker, forming the internal urethral sphincter at the junction of the urethra and the blad-der. This sphincter is involuntary and is controlled by the autonomic nervous system to prevent leaking of urine. As the urethra passes through the ­urogenital diaphragm, it is surrounded by the external urethral sphincter. This sphincter is voluntarily controlled. Another muscle, the levator ani, helps in voluntary constriction of the urethra.

1. Describe the structures needed for urination.

2. What is the amount of urine in the bladder that usually triggers the urge to urinate?

3. Contrast the female and male urethra.