Composition of Urine

• Consists of 95% water; 5% dissolved substances (nitrogenous wastes—such as urea, uric acid, ammonia, and creatinine—as well as other solutes, such as sodium, potassium, and sulfates. 

• Reveals information about health of kidneys and other organs

Storage and Elimination of Urine

• The ureters and urethra serve as passageways for conducting urine away from the kidneys and out of the body while the bladder stores urine until it can be eliminated.

• The ureters connect the renal pelvis of each kidney with the bladder.

Urinary Bladder

• The wall of the bladder is the detrusor muscle; it consists of three layers of smooth muscle.

• Mucous transitional epithelium lines the bladder. When the bladder relaxes, this layer forms folds called rugae. As urine fills the bladder, the rugae flatten and the epithelium thins, allowing the bladder to expand. 

• The floor of the bladder has three openings: two from the ureters (which pass behind the bladder to enter from below) and one from the urethra. They form a triangular-shaped, smooth area on the floor of the bladder called the trigone. Infections commonly attack this area.

• At the point where the urethra leaves the bladder is a ring of smooth muscle that forms the internal urethral sphincter. This sphincter contracts involuntarily to retain urine in the bladder.

• A second sphincter (the external urinary sphincter) exists where the urethra passes through the pelvic floor; this sphincter consists of skeletal muscle and is, therefore, under voluntary control.

• The urethra is a small tube that conveys urine away from the bladder and out of the body; it opens to the outside of the body at the external urinary meatus.

• In females, the urethra is 3 cm (1.2 in.) long and exits the body just in front of the vaginal orifice

• In males, the urethra measures about 20 cm (7.9 in.). The urethra passes from the bladder through the center of the prostate gland, curves around to enter the penis, and then exits the body at the tip of the penis. In men, the urethra conveys both urine and semen. 

Urination

• When the bladder contains 200 mL or more of urine, stretch receptors in the bladder wall send impulses to the sacral region of the spinal cord.

• The spinal cord sends motor impulses to the bladder wall to contract and the internal sphincter to relax; voiding will occur involuntarily unless the brain overrides the impulse. 

• The brain can override the impulse because stretch receptors in the bladder also send impulses to the micturition center in the pons. The pons integrates that information with information from other parts of the brain, such as the cerebrum, and evaluates whether the time is appropriate to urinate.

• If the time is not appropriate to urinate, the brain sends impulses to inhibit urination and to keep the external urinary sphincter contracted. If the time is appropriate, the brain signals the bladder wall to contract and the external urethral sphincter to relax, and voluntary urination occurs.