Each of the two ureters is a long, slender, muscular tube that extends from the kidney down to and through the inferior portion of the urinary bladder (see Fig. 18-1). The ureters, which are located posterior to the peritoneum and, at the distal portion, below the peritoneum, are entirely extraperitoneal. Their length naturally varies with the size of the individual, and they may be anywhere from 25 cm to 32 cm. (10-13 inches) long. Nearly 2.5 cm (1 inch) of the ureter’s distal portion enters the bladder by passing obliquely (at an angle) through the inferior bladder wall. Because of the oblique direction the ureter takes through the wall, a full bladder compresses the ureter and prevents the backflow of urine. The wall of the ureter includes a lining of epithelial cells, a relatively thick layer of involuntary muscle, and finally, an outer coat of fibrous connective tissue. The epithelium is the transitional type, which flattens from a cuboidal shape as the tube stretches. This same type of epithelium lines the renal pelvis, the bladder, and the proximal portion of the urethra. The ureteral muscles are capable of the same rhythmic contraction (peristalsis) that occurs in the digestive system. Urine is moved along the ureter from the kidneys to the bladder by gravity and by peristalsis at frequent intervals.
Figure 18-1 Male urinary system, showing blood vessels.
The Urinary Bladder
When it is empty, the urinary bladder (Fig. 18-10) is located below the parietal peritoneum and posterior to the pubic joint. When filled, it pushes the peritoneum upward and may extend well into the abdominal cavity proper. The urinary bladder is a temporary reservoir for urine, just as the gallbladder is a storage sac for bile. The bladder wall has many layers. It is lined with mucous membrane containing transitional epithelium. The bladder’s lining, like that of the stomach, is thrown into folds called rugae when the organ is empty. Beneath the mucosa is a layer of connective tissue, followed by a three-layered coat of involuntary muscle tissue that can stretch considerably. Finally, there is an incomplete coat of peritoneum that covers only the superior portion of the bladder. When the bladder is empty, the muscular wall becomes thick, and the entire organ feels firm. As the bladder fills, the muscular wall becomes thinner, and the organ may increase from a length of 5 cm (2 inches) up to as much as 12.5 cm (5 inches) or even more. A moderately full bladder holds about 470 mL (1 pint) of urine. The trigone is a triangular-shaped region in the floor of the bladder. It is marked by the openings of the two ureters and the urethra (see Fig. 18-10). As the bladder fills with urine, it expands upward, leaving the trigone at the base stationary. This stability prevents stretching of the ureteral openings and the possible back flow of urine into the ureters.
The urethra is the tube that extends from the bladder to the outside (see Fig. 18-1) and is the means by which the
bladder is emptied. The urethra differs in men and women; in the male, it is part of both the reproductive system and the urinary system, and it is much longer than is the female urethra.
Figure 18-10 Interior of the male urinary bladder. The trigone is a triangular region in the floor of the bladder marked by the openings of the ureters and the urethra.
The male urethra is about 20 cm. (8 inches) in length. Proximally, it passes through the prostate gland, where it is joined by two ducts carrying male germ cells (spermatozoa) from the testes and glandular secretions. From here, it leads to the outside through the penis, the male organ of copulation. The male urethra serves the dual purpose of conveying semen with the germ cells and draining the bladder. The urethra in the female is a thin-walled tube about 4 cm (1.5 inches) long. It is located posterior to the pubic joint and is embedded in the muscle of the vagina’s anterior wall. The external opening, called the urinary meatus, is located just anterior to the vaginal opening between the labia minora. The female urethra drains the bladder only and is entirely separate from the reproductive system.
The process of expelling (voiding) urine from the bladder is called urination or micturition. This process is controlled both voluntarily and involuntarily with the aid of two rings of muscle (sphincters) that surround the urethra (see Fig. 18-10).
Near the bladder’s outlet is an involuntary internal urethral sphincter formed by a continuation of the smooth muscle of the bladder wall. Below this muscle is a voluntary external urethral sphincter formed by the muscles of the pelvic floor. By learning to control the voluntary sphincter, one can gain control over emptying of the bladder. As the bladder fills with urine, stretch receptors in its wall send impulses to a center in the lower part of the spinal cord. Motor impulses from this center stimulate contraction of the bladder wall, forcing urine outward as both the internal and external sphincters are made to relax. In the infant, this emptying occurs automatically as a simple reflex. Early in life, a person learns to control urination from higher centers in the brain until the time is appropriate, a process known as toilet training. The impulse to urinate will override conscious controls if the bladder becomes too full. The bladder can be emptied voluntarily by relaxing the muscles of the pelvic floor and increasing the pressure in the abdomen. The resulting increased pressure in the bladder triggers the spinal reflex that leads to urination.