Urinary bladder

The urinary bladder (Pic. 1) is a reservoir for urine—this triangle-shaped, hollow organ is in the lower abdomen. Ligaments, attached to other organs and the pelvic bone, hold the bladder in place (Pic. 2). The bladder’s wall relaxes and expands to hold urine and contacts and flattens to release the urine down the urethra. The average adult bladder can hold up to two cups, or 500 mL, of urine for two to five hours.

Function

Urine, excreted by the kidneys, collects in the bladder before disposal by urination (micturition). Urine, one of the body’s waste products, is mainly comprised of water and urea. Urea is a nitrogenous waste that the body must constantly remove. 

When the bladder fills with urine (about half full), stretch receptors send nerve impulses to the spinal cord, which then sends a reflex nerve impulse back to the sphincter (muscular valve) at the neck of the bladder, causing it to relax and allow the flow of urine into the urethra. The internal urethral sphincter is involuntary. The ureters enter the bladder diagonally from its dorsolateral floor in an area called the trigone. The trigone is a triangular shaped area on the postero-inferior wall of the bladder. The urethra exits at the lowest point of the triangle of the trigone.

The urine in the bladder also helps regulate body temperature. A bladder when operating normally empties completely upon a complete discharge, otherwise it is a sign that its elasticity is compromised, when it becomes completely void of fluid, it may cause a chilling sensation due to the rapid change of body temperature. As urine accumulates, the rugae flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure. 

Anatomical structure

The urinary bladder is partially retroperitoneal (outside the peritoneal cavity) with its peritoneal-covered “dome” projecting into the abdomen when the bladder is distended with urine. The bladder is a highly distensible organ comprised of irregular crisscrossing bands of smooth muscle collectively called the detrusor muscle. 

The interior surface is made of transitional cellular epithelium that is structurally suited for the large volume fluctuations of the bladder. When empty, it resembles columnar epithelia, but when stretched, it “transitions” to a squamous appearance. Volumes in adults can range from nearly zero to 500–600 ml. 

The detrusor muscle is a layer of the urinary bladder wall (Pic. 3) made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. When the bladder is stretched, this signals the parasympathetic nervous system to contract the detrusor muscle. This encourages the bladder to expel urine through the urethra. The fundus of the bladder is the base of the bladder, formed by the posterior wall. It is lymphatically drained by the external iliac lymph nodes. The peritoneum lies superior to the fundus. Detrusor muscle contracts with significant force in the young.

The bladder’s strength diminishes with age, but voluntary contractions of abdominal skeletal muscles can increase intra-abdominal pressure to promote more forceful bladder emptying. Such voluntary contraction is also used in forceful defecation and childbirth.

Development

The urinary bladder is derived in embryo from the urogenital sinus (a part of the human body only present in the development of the urinary and reproductive organs) and, it is initially continuous with the allantois. In males, the base of the bladder lies between the rectum and the pubic symphysis. In females, the bladder sits inferior to the uterus and anterior to the vagina; thus, its maximum capacity is lower than in males. It is separated from the uterus by the vesicouterine excavation. In infants and young children, the urinary bladder is in the abdomen even when empty.