The urinary system can simply be described as the body’s drainage system for filtering and eliminating wastes from the body, as well as, maintaining the homeostasis of water, ions, pH, blood pressure, calcium and red blood cells.
The organs of the urinary system are the (Figure 1):
- Kidneys which secrete the urine
- Ureters or tubes that convey the urine to the bladder where it is temporarily stored
- Urethra, through which the urine is discharged from the body.
Organs of the Urinary System
The kidneys are paired organs lying along the posterior wall of the abdominal cavity on either side of the vertebral column and behind the peritoneum. They are often described as bean-shaped each about the size of a fist. The left kidney is located slightly higher than the right kidney because it is displaced upwards by the liver. The kidneys, unlike the other organs of the abdominal cavity, are located posterior to the peritoneum and touch the muscles of the back. The kidneys are surrounded by a layer of adipose tissue that holds them in place and protects them from physical damage.
The kidneys are responsible for regulating the acid-base balance in the blood and hence in the body as a whole, maintaining the water balance in the body and excreting the waste products of metabolism from the blood. Their excretory activities conserve the proper concentration of essential organic and inorganic substances in the blood. Essentially the kidneys role of excretion keeps the body’s internal environment within physiological limits. The kidneys filter about 3 ounces of blood every minute. The waste products of metabolism along with extra water make up approximately the 1 to 2 quarts of urine a person produces each day.
The ureters are a pair of tubes that carry urine from the kidneys to the urinary bladder. Each is about 10 to 12 inches long and run along the left and right sides of the body parallel to the vertebral column. They descend beneath the peritoneum on the posterior abdominal wall and cross the pelvic floor to reach the bladder. Gravity and peristalsis of smooth muscle tissue in the walls of the ureters move urine toward the urinary bladder. The ends of the ureters extend slightly into the urinary bladder and are sealed at the point of entry to the bladder by the ureterovesical valves. These valves prevent urine from flowing back upwards to the kidneys.
The urinary bladder is pear-shaped, becoming more oval as it fills with urine. This muscular sac lays midline at the inferior end of the pelvis behind the symphysis pubis, in front of the rectum in the male and in front of the vagina and uterus in the female. When empty, the bladder inside is arranged in folds (called rugae), which disappear as the bladder expands with urine. Urine entering the urinary bladder from the ureters fills the hollow space of the bladder slowly and stretches its elastic walls. The walls of the bladder allow it to stretch to hold anywhere from 500 to 800 milliliters of urine. After emptying, the bladder may still retain about 50 cc residual volume. At about 150 cc of volume, stretch receptors in the detrusor muscle begin signaling the central nervous system via afferent nerves; at 400 cc we are "seeking" an appropriate toilet.
Thus, the bladder serves as a reservoir in which the urine is stored until it is eliminated from the body. There are three openings on the floor of the bladder: one in front for the urethra and two at the sides for the ureters (called urethral openings).
At the base of the bladder is the bladder neck, which opens into the urethra, through which urine is expelled to the external environment. The bladder neck and the proximal urethra (nearest part) are supported by pubourethral ligaments and the levator ani muscles of the pelvic floor. The anatomy of the male urethra and female urethra vary considerably in both length and structure. The female urethra is around 2 inches long and ends inferior to the clitoris and superior to the vaginal opening. In males, the urethra is about 8 to 10 inches long and ends at the tip of the penis. The urethra is also an organ of the male reproductive system as it propels sperm out of the body through the penis. The first 3 - 4 cm of the male urethra passes through the prostate gland, which lies below the bladder and is attached to its base.
The flow of urine through the urethra is controlled by the internal and external urethral sphincter muscles. The internal urethral sphincter is made of smooth muscle and opens involuntarily when the bladder reaches a certain set level of distention. The opening of the internal sphincter results in the sensation of needing to urinate. The external urethral sphincter is made of skeletal muscle and may be opened to allow urine to pass through the urethra or may be held closed to delay urination.
Micturition is the process by which urine is expelled from the bladder. It can be broken down into 5 basic steps:
- Urine is made in the kidneys
- Urine is stored in the bladder
- The urethral sphincter muscles relax
- The bladder muscle (detrusor muscle) contracts
- The bladder is emptied through
Beyond those 5 steps, there are elaborate layers of neurologic control which illustrate the micturition process (Figure 2).
Neurologic Innervation of Urinary System
The bladder is composed of bands of interlaced smooth muscle called the detrusor muscle. The innervation of the body of the bladder is different from that of the bladder neck. The body of the bladder is rich in beta-adrenergic receptors which are stimulated by the sympathetic component of the autonomic nervous system (ANS). Beta stimulation, via fibers of the hypogastric nerve, suppress contraction of the detrusor muscle, i.e., allows the detrusor muscle to relax (the bladder fills with urine). Conversely, parasympathetic stimulation, by fibers in the pelvic nerve causes the detrusor muscle to contract (the bladder empties). Sympathetic stimulation is predominant during bladder filling, and the parasympathetic causes emptying.
The ureters pass between the layers of the detrusor muscle and enter the bladder through the trigone. The ureters propel urine into the bladder. The bladder passively expands to accept urine. As the bladder expands and intravesicular pressure increases, the ureters are compressed between the layers of muscle, creating a valve mechanism. This valve mechanism limits the backflow of urine up towards the kidneys.
Two sphincters control the bladder outlet:
- The internal sphincter is composed of smooth muscle like the detrusor muscle and extends into the bladder neck. Like the detrusor muscle, the internal sphincter is controlled by the ANS and is normally closed. The primary receptors in the bladder neck are alpha-adrenergic. Sympathetic stimulation of these alpha receptors, via fibers in the hypogastric nerve, contributes to urinary continence.
- The external sphincter is striated muscle. Like skeletal muscle, it is under voluntary control. It receives its innervation from the pudendal nerve, arising from the ventral horns of the sacral cord. During micturition, supraspinal centers block stimulation by the hypogastric and pudendal nerves. This relaxes the internal and external sphincters and removes the sympathetic inhibition of the parasympathetic receptors. The result is the unobstructed passage of urine when the detrusor muscle contracts.
Under normal circumstances, we are able to control where and when we urinate. This is largely because the cerebrum is able to suppress the sacral micturition reflex. If the sacral reflex is unrestrained, parasympathetic stimulation via the pelvic nerve causes detrusor muscle contraction. Detrusor muscle contraction is suppressed by alpha and beta sympathetic stimulation via the hypogastric nerve. In response to afferent stimulation, the cerebrum becomes aware of the need to void. If it is appropriate, the cerebrum relaxes the external sphincter, blocks sympathetic inhibition, the bladder contracts, and urine is expelled.