23.2: Anatomy of the Genitourinary System II: Bladder and Urethra

The lower urinary system consists of the urinary bladder and urethra, which are essential in storing and expelling urine from the body. Together with the internal and external sphincters, these structures work together to regulate urination effectively.

Anatomy of the Bladder

The urinary bladder is a muscular, stretchable organ behind the pubic bone and in front of the rectum. In females, the bladder is positioned anterior to the vagina and inferior to the uterus, while in males, it is located above the prostate gland. The primary function of the bladder is to store urine, with a typical capacity of 400 to 500 mL.

The bladder wall consists of four layers:

1. Adventitia: The outer connective tissue layer that supports the bladder.
2. Detrusor Muscle: A layer of smooth muscle fibers that contract to expel urine during urination.
3. Submucosa: A connective tissue layer containing blood vessels and nerves.
4. Urothelium: The innermost lining of transitional epithelium, which prevents urine from being reabsorbed into the body.

Within the bladder, the trigone, a triangular area formed by the ureteral and urethral openings, helps funnel urine into the urethra.

Anatomy of the Urethra

The urethra is a slender tube that transports urine from the bladder to the outside of the body. In females, the urethra is approximately 2.5 to 5 cm long, while in males, it extends about 20 to 25 cm through the penis and is surrounded by the prostate gland. The urethra serves as a pathway for both urine and semen in males.

Sphincters and Their Control Mechanisms

Two sphincters control the flow of urine:

1. Internal Urethral Sphincter: This involuntary smooth muscle sphincter is located at the bladder neck. It remains closed to retain urine and relaxes during urination. In males, it also prevents semen from entering the bladder during ejaculation.
2. External Urethral Sphincter: This sphincter is composed of voluntary striated muscle. It provides conscious control over urination and surrounds the urethra in both males and females, allowing individuals to start or stop urine flow.