The term urinary tract refers collectively to the renal pelvis, the ureters, the urinary bladder, and the urethra. These structures function to carry urine from the kidneys to the external environment. Urine passes from the renal pelvis into the ureter and is carried by peristalsis to the urinary bladder. Urine is voided from the urinary bladder through the urethra.

Ureters

Each ureter is a slender tube about 25 cm (10 in) long that extends from a kidney to the urinary bladder. It begins at the kidney with the funnel-shaped renal pelvis and enters the inferior lateral margin of the urinary bladder.

The wall of a ureter is formed of three layers. The external fibrous layer is composed of dense irregular connective tissue. The middle layer consists of smooth muscle cells that produce peristaltic waves for urine transport. The internal layer is a mucosa that is continuous with that of the renal pelvis and the urinary bladder. A flap like fold of mucosa in the urinary bladder covers the opening of the ureter, and it functions as a valve that prevents backflow of urine into the ureter.

Urinary Bladder

The urinary bladder is a hollow, muscular organ located posterior to the pubic symphysis within the pelvic cavity. It lies inferior to the parietal peritoneum. The urinary bladder provides temporary storage of urine, and its size and shape vary with the volume of urine that it contains. When filled with urine, it is almost spherical as its superior surface expands. When empty, its superior surface collapses, giving a deflated appearance.

The internal floor of the urinary bladder contains the trigone (trf-gon), a smooth, triangular area that contains an opening at each of its angles. The openings of the ureters are located at the two laterally located posterior angles, and the opening of the urethra is located at the anterior angle.

Four layers compose the wall of the urinary bladder. The most internal layer is the mucosa, which is composed of transitional epithelium that is adapted to the repeated stretching of the urinary bladder wall. The epithelium stretches, and its thickness decreases as the urinary bladder fills with urine.

The mucosa is supported by the underlying submucosa formed of areolar connective tissue containing an abundance of elastic fibers. Blood vessels and nerves supplying the urinary bladder are present in the submucosa.

Smooth muscle cells compose the third, and thickest, layer. These cells form a muscle called the detrusor (de-tru t-sor). The detrusor is relaxed as the urinary bladder fills with urine, and it contracts as urine is expelled. Cells of the detrusor form an i nternal urethral sphincter at the junction of the urinary bladder and the urethra.

The external layer consists of the parietal peritoneum, but it covers only the superior portion of the urinary bladder. The remainder of the urinary bladder surface is coated with dense irregular connective tissue.

Urethra

The urethra is a thin-walled tube that carries urine from the urinary bladder to the external environment. The urethral wall contains smooth muscle cells and is supported by connective tissue. The internal lining is a mucosa that is continuous with the mucosa of the urinary bladder. An external urethral sphincter, which is composed of skeletal muscle fibers, is located where the urethra penetrates the pelvic floor.

The female urethra is quite short, about 3 to 4 cm (1.5 in) in length. The external urethral orifice, its external opening, lies anterior to the vaginal orifice. The male urethra is much longer, about 16 to 20 cm (6-8 in) in length, because the urethra runs the length of the penis. The external urethral orifice is at the tip of the penis.

Micturition

Micturition (mik-tu-rish ‘un), or urination, is the act of expelling urine from the urinary bladder. Although the urinary bladder may hold up to 1,000 ml of urine, micturition usually occurs long before that volume is attained. When 200 to 400 ml of urine have accumulated in the urinary bladder, stretch receptors in the urinary bladder wall are stimulated and they trigger the micturition reflex. This reflex sends parasympathetic nerve impulses to the detrusor, causing rhythmic contractions. As this reflex continues, it causes the involuntarily controlled internal urethral sphincter to open and the person becomes aware of the desire to urinate. The act of urinating then becomes a consciously controlled process. If the voluntarily controlled external urethral sphincter is relaxed, micturition occurs; if it is not relaxed, micturition is postponed.

Micturition may be postponed by keeping the external sphincter voluntarily closed, and in a few moments the urge to urinate subsides. After more urine enters the urinary bladder, the micturition reflex is activated again, and the urge to urinate returns. Micturition cannot be postponed for long periods of time. After a while, the reflex overwhelms voluntary control and micturition occurs, ready or not.

An infant is not able to be toilet trained until neural development allows control of the external urethral sphincter muscle. Voluntary control is possible shortly after two years of age.