The Urethra

The urethra conveys urine out of the body. In the female, it is a tube 3 to 4 cm long bound to the anterior wall of the vagina by connective tissue. Its opening, the external ure-thral orifice, lies between the vaginal orifice and clitoris. The male urethra is about 18 cm long and has three regions: (1) The prostatic urethra begins at the urinary bladder and passes for about 2.5 cm through the prostate gland. During orgasm, it receives semen from the reproductive glands. (2) The membranous urethra is a short (0.5 cm), thin-walled portion where the urethra passes through the muscular floor of the pelvic cavity. (3) The spongy (penile) urethra is about 15 cm long and passes through the penis to the external urethral orifice. It is named for the corpus spongiosum of the penis, through which it passes. The male urethra assumes an S shape: it passes downward from the bladder, turns anteriorly as it enters the root of the penis, and then turns about 90° downward again as it enters the external, pendant part of the penis. The mucosa has a transitional epithelium near the bladder, a pseudostratified columnar epithelium for most of its length, and finally stratified squamous near the external urethral orifice. There are mucous urethral glands in its wall.

In both sexes, the detrusor muscle is thickened near the urethra to form an internal urethral sphincter, which compresses the urethra and retains urine in the bladder. Since this sphincter is composed of smooth muscle, it is under involuntary control. Where the urethra passes through the pelvic floor, it is encircled by an external ure-thral sphincter of skeletal muscle, which provides voluntary control over the voiding of urine.

28ruga = fold, wrinkle

Saladin: Anatomy & I 23. The Urinary System I Text I I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 23 The Urinary System 905

Ureter-

Rugae-

Ureteral openings

Trigone

Urogenital diaphragm

Bulbourethral -gland

Spongy (penile)-urethra

Penis

Ureter-

Rugae-

Ureteral openings

Trigone

Spongy (penile)-urethra

Penis

External Urethral Orifice

Parietal peritoneum

Detrusor muscle

Membranous urethra

External urethral sphincter

External urethral-1

orifice (a)

Figure 23.20 Anatomy of the Urinary Bladder and Urethra. (a) Male; (b) female. Why are women more susceptible to bladder infections than men are?

Parietal peritoneum

Detrusor muscle

Internal urethral sphincter

*-Prostate gland

Prostatic urethra

External urethral-1

orifice (a)

Membranous urethra

External urethral sphincter

Parietal peritoneum

Ureter-

Detrusor — muscle

Ureteral openings

Trigone -

External urethral orifice

Female Bladder Detrusor
Internal urethral sphincter External urethral sphincter

Figure 23.20 Anatomy of the Urinary Bladder and Urethra. (a) Male; (b) female. Why are women more susceptible to bladder infections than men are?

Insight 23.3 Clinical Application

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