What Is the Function of the Bladder?

The bladder, part of the urinary system, is a hollow stretchable organ in the lower abdomen. Ureters carry urine from the kidneys to the bladder, and the bladder empties through the urethra. In the female, the bladder lies against the uterus and vagina; and in the male, against the rectum. The bladder has a moist mucosal inner lining that resists absorption, so the bladder doesn't leak urine. A second layer consists of elastic connective tissue that provides stretch, and the outer thick muscular layer, the detrusor muscle, controls bladder contraction.

Storage

• The primary function of the bladder is to serve as a reservoir to store urine produced when the kidneys filter waste products from the blood. According to "Hole's Human Anatomy and Physiology," the bladder has a usual capacity of about 2 cups, but in some cases may stretch to hold up to 6 cups. Urine fills the bladder from the trigone area near the base of the bladder, and the bladder distends upward toward the umbilicus. The rugae (or folds) smooth out and the detrusor muscle stretches and thins as the bladder fills.

Sensation

• When the bladder fills to about 1/2 cup of urine, stretch receptors (nerves) in the bladder wall send messages to the brain, signalling the sensation of fullness. Most people begin to feel discomfort when the bladder holds about a cup of urine, but urination is under voluntary control, so, according to the National Kidney and Urologic Diseases Information Clearinghouse, most people can wait two to five hours between urinations.

Contraction

• When the bladder expands, stretch receptors also send messages to the micturition (urination) reflex center in the lower part of the spinal cord, which in turn sends an impulse back along motor nerves to the detrusor muscle, signalling it to contract. The detrusor muscle begins to contract rhythmically, causing a sense of urgency.

Urination

• The motor nerves also signal the internal sphincter muscle, an extension of the detrusor muscle that wraps about the opening to the urethra, to relax. However, the external sphincter muscle at the bottom of the urethra is under voluntary control, so you should be able to prevent urination until you consciously relax the external sphincter muscle.

Recovery

• As the bladder empties of urine, the motor nerves stop sending messages to contract, and the detrusor muscle relaxes, folds and shrinks until it begins to fill with urine again and the cycle repeats.