Bladder - what is, definition, function, diseases and urination

What is Bladder and Definition

A muscular, saclike structure in the lower pelvis that serves as a reservoir for the URINE the KIDNEYS produce. In women the bladder is in front of and slightly below the UTERUS. During PREGNANCY the expanding uterus limits the bladder’s ability to expand, accounting for the URINARY FREQUENCY common in pregnancy’s last trimester. In men the bladder is in front of the RECTUM, with the PROSTATE GLAND encircling the first segment of the URETHRA as it exits the bladder. Swelling of the prostate gland, such as typically occurs with advancing age, as in BENIGN PROSTATIC HYPERPLASIA (BPH), can constrict the flow of urine through the urethra in a man, accounting for symptoms such as urinary frequency and dribbling.

Three layers of tissue form the bladder. The outermost and innermost layers are membranous, the outer being a continuation of the peritoneum that lines the abdominal cavity and the inner being mucous-secreting epithelium. The bladder’s middle layer is smooth MUSCLE called the detrusor muscle that itself has three layers, the fibers of each running differently. The outer muscle fibers run longitudinally (lengthwise), the middle muscle fibers form patterns of circles that ultimately culminate in the sphincter muscle that encloses the bladder’s neck, and the inner muscle fibers run laterally (crosswise). Together these muscle layers allow the bladder to expand to accommodate the urine draining from the kidneys and also to contract, in coordination with relaxation of the urethral sphincter, to expel urine from the bladder through the urethra.

The ureters drain urine from the kidneys into the bladder; the urethra drains urine from the bladder to the outside of the body. One URETER, a narrow tubelike structure, drops from each kidney and enters the back wall of the bladder near its midline. Urine drips continuously from the ureters into the bladder. The urethra, a somewhat muscular tube, carries urine from the bladder to outside the body. When empty the bladder is about the size of a large lemon; when filled to its capacity of about 500 milliliters (32 to 34 ounces) the bladder can reach the size of a small cantaloupe. As the bladder expands it extends upward into the abdominal cavity.

Function of Bladder

URINATION, the process of expelling urine from the bladder (also called micturition), is an involuntary function that becomes an action of learned control. NEURON sensors in the muscle fibers of the bladder wall send NERVE signals to the sacral portion of the SPINAL CORD. This activates the micturition REFLEX, which sends nerve signals via the spinal cord to micturition centers in the BRAIN. These centers activate nerve impulses that cause the urethral sphincter to relax and the detrusor muscle to begin a series of wavelike contractions. These involuntary actions create the urge to urinate, experienced as a sensation of pressure.

Bladder and Urination

Learning to control the pubococcygeal muscle, which forms the floor of the pelvis, serves to override the micturition reflex for a period of time. Relaxing the pubococcygeal muscle and contracting the abdominal muscles synchronize with the involuntary responses of the micturition reflex, and urination occurs. Most children acquire the developmental ability, a blend of conscious effort and neuromuscular maturity, to learn to control urination (commonly called bladder control) between the ages of three and five. With advanced age this control may diminish, a consequence of a weakened urethral sphincter, neurologic conditions, and other factors.

For further discussion of the bladder within the context of the urinary system’s structure and function please see the overview section “The Urinary System.”

See also AGING, URINARY SYSTEM CHANGES THAT OCCUR WITH; BLADDER CATHETERIZATION; CYSTOSCOPY; FECAL INCONTINENCE; KEGEL EXERCISES.

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