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Light micrograph of the human urinary bladder wall (6x).

In a male, the urethra, which functions both as a urinary canal and a passageway for cells and secretions from the reproductive organs, can be divided into three sections: the prostatic urethra, the membranous urethra, and the penile urethra (see fig. 20.32b and reference plate 60).

The prostatic urethra is about 2.5 centimeters long and passes from the urinary bladder through the prostate gland, which is located just below the bladder. Ducts from reproductive structures join the urethra in this region.

Urethral glands Muscle layer

Lumen of urethra Mucous membrane

Figure 20.31

Cross section of the urethra (10x).

Urethral glands Muscle layer

Lumen of urethra Mucous membrane

Figure 20.31

Cross section of the urethra (10x).

The membranous urethra is about 2 centimeters long. It begins just distal to the prostate gland, passes through the urogenital diaphragm, and is surrounded by the fibers of the external urethral sphincter muscle.

The penile urethra is about 15 centimeters long and passes through the corpus spongiosum of the penis, where erectile tissue surrounds it. This portion of the urethra terminates with the external urethral orifice at the tip of the penis.

H Describe the structure of the urethra.

How does the urethra of a male differ from that of a female?

Micturition

Urine leaves the urinary bladder by the micturition (mik"tu-rish'un) (urination) reflex. The detrusor muscle contracts, and contractions of muscles in the abdominal wall and pelvic floor may help, as well as fixation of the thoracic wall and diaphragm. In micturition, the external urethral sphincter also relaxes. This muscle, which is part of the urogenital diaphragm (see chapter 9, p. 336), surrounds the urethra about 3 centimeters from the bladder and is composed of voluntary skeletal muscle tissue.

Distension of the bladder wall as it fills with urine stimulates the urge to urinate. The wall expands, stimulating stretch receptors, which triggers the micturition reflex.

The micturition reflex center is located in the sacral portion of the spinal cord. When sensory impulses from the stretch receptors signal the reflex center, parasympa-thetic motor impulses travel out to the detrusor muscle, which contracts rhythmically in response. A sensation of urgency accompanies this action.

Trigone

External urethral orifice (a)

Trigone

External urethral orifice (a)

Trigone

Prostatic urethra

Membranous urethra

Penile urethra

Trigone

Prostatic urethra

Membranous urethra

Penile urethra

Ureter

-Urinary bladder

Prostate gland

Bulbourethral gland

Penis

Figure 20.32

(a) Longitudinal section of the female urinary bladder and urethra.

(b) Longitudinal section of the male urinary bladder and urethra.

Ureter

-Urinary bladder

Prostate gland

Bulbourethral gland

Penis

External urethral orifice

Figure 20.32

(a) Longitudinal section of the female urinary bladder and urethra.

(b) Longitudinal section of the male urinary bladder and urethra.

Major Events of Micturition

1. Urinary bladder distends as it fills with urine.

2. Stretch receptors in the bladder wall are stimulated, and they signal the micturition center in the sacral spinal cord.

3. Parasympathetic nerve impulses travel to the detrusor muscle, which responds by contracting rhythmically.

4. The need to urinate is sensed as urgent.

5. Voluntary contraction of the external urethral sphincter and inhibition of the micturition reflex by impulses from the brain stem and the cerebral cortex prevent urination.

6. Following the decision to urinate, the external urethral sphincter is relaxed, and impulses from the pons and the hypothalamus facilitate the micturition reflex.

7. The detrusor muscle contracts, and urine is expelled through the urethra.

8. Neurons of the micturition reflex center fatigue, the detrusor muscle relaxes, and the bladder begins to fill with urine again.

Major Events of Micturition

Incontinence is the loss of control of micturition. Stress incontinence, caused by pressure on the bladder, is particularly common among women who have had children, especially if they have gained weight. An effective treatment is at least two months of doing Kegel exercises, in which a woman contracts the muscles that support the bladder, several times daily. Treatments for severe cases include a tampon-like cone inserted into the vagina to raise the pelvic floor; a small foam pad placed over the urethra to catch small amounts of urine; collagen injections around the urethra to tighten it; and surgery. Many people use absorbent pads.

Nighttime bedwetting was noted as long ago as 1500 b.c. Treatments have ranged from drinking the broth from boiled hens combs, to blocking the urethra at night, to punishment and ridicule. In many cases, this nocturnal enuresis is inherited. Drug treatment and pads to absorb urine help to manage affected children, who usually outgrow the condition.

turition reflex. Consequently, the detrusor muscle contracts, and urine is excreted through the urethra. Within a few moments, the neurons of the micturition reflex tire, the detrusor muscle relaxes, and the bladder begins to fill with urine again. Table 20.5 outlines the micturition process, and Clinical Application 20.5 discusses urinaly-sis and health.

Damage to the spinal cord above the sacral region may abolish voluntary control of urination. However, if the micturition reflex center and its sensory and motor fibers are uninjured, micturition may continue to occur reflexively. In this case, the bladder collects urine until its walls stretch enough to trigger a micturition reflex, and the detrusor muscle contracts in response. This condition is called an automatic bladder.

The urinary bladder may hold as much as 600 milliliters of urine. The desire to urinate usually appears when it contains about 150 milliliters. Then, as urine volume increases to 300 milliliters or more, the sensation of fullness becomes increasingly uncomfortable.

As the bladder fills with urine and its internal pressure increases, contractions of its wall intensify. When these contractions become strong enough to force the internal urethral sphincter open, another reflex signals the external urethral sphincter to relax, and the bladder may empty. However, because the external urethral sphincter is composed of skeletal muscle, it is under conscious control. Thus, the sphincter muscle ordinarily remains contracted until a decision is made to urinate. Nerve centers in the brain stem and cerebral cortex that inhibit the micturition reflex aid this control. When a person decides to urinate, the external urethral sphincter relaxes, and the micturition reflex is no longer inhibited. Nerve centers within the pons and the hypothalamus heighten the mic-

Describe micturition.

How is it possible to consciously inhibit the micturition reflex?

Life-Span Changes

As with other organ systems, the urinary system is sufficiently redundant, in both structure and function, to mask aging-related changes. However, overall, the kidneys are slower to remove nitrogenous wastes and toxins and to compensate for changes to maintain homeostasis.

From the outside, the kidneys change with age, appearing scarred and grainy as arterioles serving the cortex constrict, and fibrous connective tissue accumulates around the capsules. On the inside, kidney cells begin to die as early as age 20 years, but the gradual shrinkage is not generally noticeable until after age 40. By 80 years, the kidneys have lost about a third of their mass.

Urinalysis: Clues to Health

Urine has long fascinated medical minds. As a folk remedy, urine has been used as a mouthwash, toothache treatment, and a cure for sore eyes. Hippocrates (460-377 b.c.) was the first to observe that the condition of the urine can reflect health, noting that frothy urine denoted kidney disease. During the Middle Ages, health practitioners consulted charts that matched certain urine colors to certain diseases. In the seventeenth century, British physicians diagnosed diabetes by having their medical students taste sugar in patients' urine. Today, urine composition is still used as a window on health and also to check for illicit drug use.

Certain inherited disorders can alter urine quite noticeably. The name maple syrup urine disease vividly describes what this inborn error of metabolism does to the urine. This condition, which causes mental retardation, results from a block in the breakdown pathways for certain amino acids. In alkaptonuria, one of the first inborn errors to be described, urine turns black when it is left to stand. This condition also produces painful arthritis and blackened ear tips. People with Wilson disease have an inherited inability to excrete copper. If they are properly diagnosed and given the drug penicillamine, they excrete some of the built-up copper, and their urine takes on a coppery appearance.

Other genetic conditions alter urine without causing health problems. People with beeturia excrete dark pink urine after they eat beets. The problem for people with urinary excretion of odoriferous component of asparagus is obvious. Parents of newborns who have inherited blue diaper syndrome are in for a shock when they change their child's first diaper. Due to a defect in transport of the amino acid tryptophan in the small intestine, bacteria degrade the partially digested tryptophan, producing a compound that turns blue on contact with oxygen ■

Kidney shrinkage is largely due to the gradual loss of glomeruli—they may atrophy, cease functioning, become blocked with fibrous connective tissue, or untwist. About five percent of glomeruli are abnormal by age 40; 37 percent are abnormal by age 90. The progressive shut down of glomeruli decreases the surface area available for filtration, and as a result, glomerular filtration rate (GFR) begins to drop in the fourth decade of life. By age 75, GFR is about half that in a young adult, falling from about 125 milliliters/minute to about 60. With this decline in function, proteins are more likely to get into the urine. About a third of the elderly have proteinuria.

Further along the nephron, the renal tubules thicken, accumulating coats of fat. They may shorten, forming small outpouches as cell death disrupts their sleek symmetry. Urine may become more dilute as reabsorption of sodium and glucose and other molecules becomes less efficient. The renal tubules also slow in their processing of certain drugs, which therefore remain in the circulation longer. It becomes harder to clear non-steroidal anti-inflammatory drugs such as aspirin, as well as opiates, antibiotics, urea, uric acid, creatinine, and various toxins. Therefore, a person's age should be taken into account when prescribing drugs. The pharmaceuti cal industry is beginning to test new drugs on people of a range of ages.

Circulatory changes slow the journey of blood through the kidneys. A college student's kidneys may process about a fourth of the cardiac output, or about 1,200 milliliters, per minute. Her 80 year old grandfather's kidneys can handle about half that volume. Starting at about age 20, renal blood flow rate diminishes by about one percent per year. The blood vessels that serve the kidneys become slower to dilate or constrict in response to body conditions. At the same time, the kidneys' release of renin declines, hampering control of osmotic pressure, blood pressure, and sodium and potassium ion concentrations in the blood. The kidneys are also less able to activate vitamin D, which may contribute to the higher prevalence of osteoporosis among the elderly.

The bladder, ureters, and urethra change with the years too. These muscular organs lose elasticity and recoil with age, so that in the later years, the bladder holds less than half of what it did in young adulthood, and may retain more urine after urination. In the elderly, the urge to urinate may become delayed, so that when it does happen, it is sudden. Older individuals have to urinate at night more than younger people.

Controlling bladder function is a challenge at the beginning of life and much later too. A child usually learns to control urination by about age 2 or 3 years. Loss of bladder control, or incontinence, becomes more common in advanced years, although it is not considered a normal part of aging. It results from loss of muscle tone in the bladder, urethra and ureters. Incontinence affects 15 to 20 percent of women over 65, and half of all men. In women, incontinence reflects the stresses of childbirth and the effects of less estrogen during menopause. Bladder sphincter muscles atrophy, muscles in the pelvic floor weaken, and poor muscle tone develops in the smooth muscle of the urethra. In males, incontinence is usually a consequence of an enlarged prostate gland that presses on the bladder.

O How do the kidneys change in appearance with advancing years?

^9 What happens to glomeruli as a person ages?

^9 How does kidney function change with age?

□ How do aging-related changes in the circulatory system affect the kidneys?

Q How do the ureters, bladder, and urethra change with age?

Clinical Terms Related to the Urinary System anuria (ah-nu're-ah) Absence of urine due to failure of kidney function or to an obstruction in a urinary pathway. bacteriuria (bak-te"re-u're-ah) Bacteria in the urine. cystectomy (sis-tek'to-me) Surgical removal of the urinary bladder.

cystitis (sis-ti'tis) Inflammation of the urinary bladder. cystoscope (sis'to-skop) Instrument to visually examine the interior of the urinary bladder. cystotomy (sis-tot'o-me) Incision of the wall of the urinary bladder.

diuresis (di"u-re'sis) Increased production of urine. diuretic (di"u-ret'ik) Substance that increases urine production.

dysuria (dis-u're-ah) Painful or difficult urination. enuresis (en"u-re'sis) Uncontrolled urination. hematuria (hem"ah-tu're-ah) Blood in the urine. incontinence (in-kon'ti-nens) Inability to control urination and/or defecation reflexes. nephrectomy (ne-frek'to-me) Surgical removal of a kidney. nephrolithiasis (nef"ro-li-thi'ah-sis) Kidney stones. nephroptosis (nefrop-to'sis) Movable or displaced kidney. oliguria (ol"i-gu're-ah) Scanty output of urine. polyuria (pol"e-u're-ah) Excess urine.

pyelolithotomy (pi"e-lo-li-thot'o-me) Removal of a stone from the renal pelvis. pyelonephritis (pi"e-lo-ne-fri'tis) Inflammation of the renal pelvis.

pyelotomy (pi"e-lot'o-me) Incision into the renal pelvis. pyuria (pi-u're-ah) Pus (excess white blood cells) in the urine. uremia (u-re'me-ah) Condition in which substances ordinarily excreted in the urine accumulate in the blood. ureteritis (u-re"ter-i'tis) Inflammation of the ureter. urethritis (u"re-thri'tis) Inflammation of the urethra.

1 nnerConnections

Urinary System

Integumentary System Cardiovascular System

The urinary system compensates for water loss due to sweating. The kidneys and skin both play a role in vitamin D production.

The urinary system controls blood volume. Blood volume and blood pressure play a role in determining water and solute excretion.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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