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The Physiologic Anatomy of Urethra

Urethra overview

The urethra is a thin muscular canal that extends from the neck of the bladder to the exterior, at the external urethral orifice. Urethra is longer in males than in females. The external urethral sphincter is under voluntary control, and it's composed of the surrounding skeletal muscle of the pelvic floor.

Urethral sphincters

There are two urethral sphincters:

  1. Internal urethral sphincter
  2. External urethral sphincter

The bladder neck (internal urethral or posterior urethra) is 2 to 3 centimeters long and its wall is composed of detrusor muscle interlaced with a significant amount of elastic tissue. The muscle in this region is called the internal urethral sphincter (or bladder sphincter), which is innervated by autonomic nerve fibers (it is involuntary). This sphincter is located between the neck of the bladder and the upper end of the urethra. The natural tone normally keeps the bladder neck and posterior urethra free of urine and therefore prevents the bladder from emptying before the pressure in the main part of the bladder rises above the critical threshold (this sphincter closes the urethra when the bladder is emptied).

Beyond the posterior urethra, the urethra passes through the urogenital diaphragm, which comprises a layer of muscle called (external urethral sphincter) of the bladder. This sphincter consists of circular skeletal muscle fibers, that are innervated by somatic nerve fibers. in contrast to the bladder body muscle and the bladder neck, which is entirely smooth muscle, the external sphincter muscle is a voluntary skeletal muscle. So, it is under voluntary control of the nervous system and can be used to consciously prevent urination even when involuntary controls are attempting to empty the bladder.

  1. Internal Urethral sphincter
  2. External Urethral sphincter

Male urethra

The male urethra has both urinary and reproductive functions. It holds s*e*m*e*n and urine. The urethra of men is 7 to 8 inches (17 to 20 cm) long. The first part outside the bladder is surrounded by the prostate gland, that’s why it is referred to as the prostatic urethra (3 cm). The next inch is the membranous urethra (1.25 cm) around which is the external urethral sphincter. The longest part is the cavernous urethra (or spongy or penile urethra) (15.75 cm) that passes through the cavernous (or erectile) tissue of the p*e*n*i*s.

The prostatic urethra is 3 cm long and passes through the prostate gland. The prostatic fluid is emptied through the prostatic sinuses into this part of the urethra. Vas deferens sperm and seminal vesicle fluid are also emptied into the prostatic urethra through ejaculatory ducts.

The membranous urethra is approximately 1 to 2 cm long. It extends from the base of the prostate gland through the urogenital diaphragm up to the bulb of the urethra. The membranous urethra is surrounded by the external sphincter.

The spongy urethra is also known as the cavernous urethra and has a length of around 15 cm. The spongy urethra is surrounded by a p*e*n*i*s spongiosum corpus. It's divided into: Proximal bulbar urethra and distal penile urethra. The penile urethra is narrow, about 6 cm long. It ends with an external urethral meatus or an orifice situated at the end of the p*e*n*i*s. Bilateral bulbourethral glands open to the spongy urethra. Bulbourethral glands are sometimes referred to as (Cowper glands).

  1. Prostatic urethra
  2. Membranous urethra
  3. Spongy urethra

Female urethra

The female urethra has only urinary function and only carries urine. Thus, the male urethra is structurally distinct from the female urethra. The urethra in women is 1 to 1.5 inches (2.5 to 4 cm) long and 6 mm in diameter. It runs down and forward behind the symphysis pubis and opens at the external urethral orifice just in front of the v*a*g*i*n*a. The external urethral sphincter guards the external urethral orifice, which is under voluntary control. Unlike males, females have a shorter urethra, which is why females are more likely to have urinary tract infection.

The wall of the female urethra has two main layers: the outer muscle layer and the inner lining of the mucosa, which is continuous to that of the bladder. The muscle layer has two portions, an outer layer of striated muscle surrounding it. The striated muscle is under voluntary control and it forms the external urethral sphincter. And an inner layer of smooth muscle under autonomic nerve control. The mucosa is protected by loose fibroelastic connective tissue that includes blood vessels and nerves. Proximately, it consists of transitional epithelium, while distally, it is composed of stratified epithelium.

female urethra

Summary

The Urethra carries the urine from the bladder to the outside. Male urethra, approximately 20 cm long is divided into three parts: prostatic urethra (3 cm), membranous urethra (1,25 cm), and penile urethra (15,75 cm). The membranous urethra is surrounded by an external sphincter. The female urethra is approximately 3.8 cm long. It extends from the bladder neck to the external meatus. It crosses the external sphincter and lies directly in front of the v*a*g*i*n*a.

The urethra has two sphincters:

The circular smooth muscle fibers in the region of the bladder neck is thickened to form an internal sphincter (sphincter vesicae). The normal tone of the internal sphincter prevents the bladder from emptying before the pressure in the body of the bladder increases above the threshold.

Beyond the bladder neck, when the urethra passes through the urogenital diaphragm, it is encircled by a ring of voluntary (skeletal type) muscle known as the external bladder sphincter. The external sphincter has voluntary control over micturition.

  1. The Internal sphincter
  2. External sphincter.
References

1.GUYTON AND HALL, Textbook of Medical Physiology, 12th edition, Jackson, Mississippi, University of Mississippi Medical Center, [2011]

2.K SEMBULINGAM AND PREMA SEMBULINGAM, Essentials of Medical Physiology, Sixth Edition, New Delhi, Panama City, London, Dhaka, Kathmandu, JAYPEE BROTHERS MEDICAL PUBLISHERS (P) LTD, [2012]

3.INDU KHURANA AND ARUSHI KHURANA, Textbook of Medical Physiology, 2nd Edition, India, Elsevier India, [December 1, 2015]

4.VALERIE C. SCANLON, TINA SANDERS, Essentials of Anatomy and Physiology, fifth edition, New York, F. A. Davis Company, [January 1, 2006]

5.KIM E. BARRETT, SUSAN M. BARMAN, HEDDWEN L. BROOKS, JASON YUAN, Ganong's Review of Medical Physiology, 26th edition, New York, Chicago, San Francisco, Athens London, Madrid, Mexico City, Milan, New Delhi, Singapore, Sydney, Toronto, Mc Graw Hill Education, [January 29, 2019]

6.ANNE WAUGH, ALLISON GRANT, Ross and Wilson ANATOMY and PHYSIOLOGY in Health and Illness, 11th edition, Edinburgh, London, New York, Oxford, Philadelphia, St Louis Sydney, Toronto, Churchill Livingstone, [September 7, 2010]

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