Urinary Bladder

Urinary Bladder

The urinary bladder is a hollow, muscular organ that stores urine and forces it into the urethra. It is found in the pelvic cavity behind the symphysis pubis, beneath the parietal peritoneum. In males, the prostate gland is inferior to the neck of the bladder, which empties into the urethra. In females, the bladder is anterior to the uterus and vagina. The wall of the bladder has many folds or rugae when it is empty, but these smooth out as it fills. The bladder is pear shaped when full, rising superiorly in the abdominal cavity. The rugae disappear as the bladder walls become thinner due to stretching to contain urine. Because of the flexibility of its walls, there is no large rise in internal pressure as the bladder fills.

The bladder’s internal floor has a triangular area or trigone, which has an opening at each of its three angles. FIGURE 22-10 shows both the male and female urinary bladder and related structures. The urinary bladder wall has four layers—the mucous coat, sub-mucous coat, muscular coat, and serous coat—and its cellular thickness changes based on how much urine it holds. The smooth muscle fibers of the muscular coat are interlaced, comprising the detrusor muscle, part of which surrounds the neck of the bladder to form the internal urethral sphincter. This muscle is innervated­ with parasympathetic nerve fibers that function in the micturition reflex. The mucous coat of the urinary bladder contains transitional epithelium. The urinary bladder is held in place by the median and lateral umbilical ligaments.

Micturition, also called urination or voiding, is the process of expelling urine from the urinary bladder. The detrusor muscle contracts along with the abdomi-nal wall and pelvic floor muscles, and the external ure-thral sphincter relaxes. The micturition reflex center in the spinal cord sends parasympathetic motor impulses to the detrusor muscle, causing it to rhythmically contract. In infants and young children, micturition is reflexive. However, training to control micturition usually is successful between ages 2 and 3, because descending brain circuits have matured sufficiently to take control of micturition replacing ­reflexive micturi-tion. In older children and adults, the normal urge to urinate usually occurs before urine volume in the blad-der exceeds 500 mL. After urination, there is usually only approximately 10 mL of urine left over.

The urinary bladder may hold up to nearly 1,000 mL or 2 pints of urine, if necessary. However, the urge to urinate usually occurs once it contains around 200 mL. When approximately half full, the bladder is about 12 cm or 5 inches long, although it can greatly increase in size. The external urethral sphincter is under con-scious control, allowing the micturition reflex to occur once the person decides to urinate. The detrusor muscle contracts and urine flows through the urethra.