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15 Urinary System
LEARNING OBJECTIVES
1 State six functions of the urinary system.
2 Describe the location and structural features of the kidneys.
3 Draw and label the parts of a nephron.
4 State the two parts of the juxtaglomerular apparatus.
5 Describe the location, structure, and function of the ureters, urinary bladder, and urethra.
6 List and describe the three steps in urine formation.
7 Identify the hormones that affect kidney function, and explain how they do so.
8 Explain the function of renin.
9 Describe ways in which the aging of an individual affects the urinary system.
10 Identify pathology related to the urinary system.
Key Terms
glomerular capsule (gloh-M ER-yoo-lar KAP-sool)
juxtaglomerular apparatus (juks-tah-gloh-M ER-yoo-lar ap-pah-RAT-us)
nephron (NEFF-rahn)
renal tubule (REE-nal TOOB-yool)
217 218
Introduction to the Urinary System
The overall function of the urinary system is to maintain the volume and composition of body fluids within normal limits. The urinary system
accomplishes this by excreting the waste products that accumulate as a result of cellular metabolism. Because of this, the urinary system is
sometimes referred to as the excretory system. Although the urinary system has a major role in excretion, other organs contribute to the
excretory function. Some waste products, such as carbon dioxide and water, are excreted by the lungs through the respiratory system. The skin
excretes wastes through the sweat glands. The liver and intestines excrete bile pigments that result from the destruction of hemoglobin. The
major task of excretion, however, still belongs to the urinary system. If the urinary system fails, the other organs cannot take over and
compensate adequately. In addition to eliminating waste products, the urinary system maintains an appropriate fluid volume. It does this by
regulating the amount of water that is excreted in the urine. Other functions of the urinary system include regulating the concentrations of
various electrolytes in the body fluids and maintaining normal pH of the blood.
In addition to maintaining fluid balance in the body, the urinary system controls red blood cell production by secreting the hormone
erythropoietin (ee-rith-roh-poy-EE-tin). The urinary system also plays a role in maintaining normal blood pressure by secreting the enzyme
renin.
Components of the Urinary System
The urinary system consists of the kidneys, ureters, urinary bladder, and urethra. The kidneys produce the urine. The ureters transport the
urine away from the kidneys to the urinary bladder. The urinary bladder stores the urine until it is excreted from the body. The urethra is a
tubular structure that carries the urine from the urinary bladder to the outside of the body. The components of the urinary system are illustrated
in Figure 15-1.
Kidneys
The kidneys are the primary organs of the urinary system. They are the organs that filter the blood, remove the wastes, and excrete the wastes
into the urine. They are the organs that perform the functions of the urinary system. The other components of the urinary system are accessory
structures to help eliminate the urine from the body.
Location
The kidneys are located between the twelfth thoracic and third lumbar vertebrae, one on each side of the vertebral column. The right kidney is
usually slightly lower than the left because the liver displaces it downward. The kidneys are partially protected by the lower ribs and lie in
shallow
Figure 15-1 Components of the urinary system.
depressions against the posterior abdominal wall behind the peritoneum (retroperitoneal). Each kidney is held in place by connective tissue,
called renal fascia.
Highlight on the Urinary System
Hangover: Alcohol inhibits the secretion of antidiuretic hormone, so when people drink alcohol, they experience diuresis, or excessive urination.
Experts believe that the dehydration caused by diuresis contributes to “hangover” symptoms.
Kidney stones: Kidney stones develop when uric acid or calcium salts precipitate instead of remaining dissolved in the urine. The stones
usually form in the renal pelvis, but they may also develop in the urinary bladder. If small enough, they may pass naturally with urine flow but
usually cause a lot of discomfort. If kidney stones cause a serious obstruction, they may need to be surgically removed. A newer method of
treatment called lithotripsy uses high-frequency sound waves to break the stone into small pieces so that it may pass naturally. The formation of
stones in the urine is called urolithiasis.
Nephrons: The number of nephrons does not increase after birth. Growth of the kidney occurs from enlargement of the individual nephrons.
When nephrons are damaged they are not replaced.
Nephroptosis: Nephroptosis, commonly referred to as a floating kidney, occurs when the kidney is no longer held in place by the renal fascia
and it drops out of its normal position. This may make the kidney more vulnerable to injury if it is no longer protected by the ribs. Another
danger is that the ureter may become twisted and block the flow of urine. Nephroptosis occurs more frequently in horseback riders, truck
drivers, and people who ride motorcycles.
Polycystic kidney disease: This inherited condition affects the tubular portion of the nephrons. Swelling or cysts develop along the tubules,
and as the cysts enlarge they displace and damage functional kidney tissue. This eventually leads to a total loss of kidney function. When this
occurs in both kidneys, a transplant is necessary.
Uremia: When the kidneys do not function properly and fail to remove the waste products from the blood, uremia may result. Uremia is a
condition in which there is a toxic level of urea in the blood.
Urinary incontinence: Urinary incontinence is the inability to control urination and to retain urine in the bladder. Temporary incontinence
may result when the muscles around the bladder and urethra become weakened and lose muscle tone. This is sometimes caused by stretching of
the muscles during childbirth. Because these muscles help restrict the outlet of the bladder, their weakness contributes to a leakage of urine. A
cough or sneeze may increase pressure within the bladder sufficiently to force urine to escape. Permanent incontinence is usually caused by
damage to the central nervous system or by extensive damage to the bladder or urethra.
Urinary tract infection (UTI): UTIs occur more frequently in women than in men because of differences in the urethra. In females the urethral
opening is in close proximity to the anal opening, which gives intestinal bacteria easier access to the urethra. The female urethra is short, which
allows any infection to spread to the urinary bladder. An infection of the urethra is called urethritis, and one of the urinary bladders is called
cystitis. ▪
A thick layer of adipose tissue surrounds each kidney. This is called perirenal fat, and it helps to protect the kidney. A tough, fibrous
connective tissue encases each kidney and is called the renal capsule. The renal capsule provides support for the soft tissue that is inside.
M acroscopic Structure
In the adult, each kidney is approximately 3 cm thick, 6 cm wide, and 12 cm long (1.2 × 2.5 × 5 inches). The kidney is bean-shaped with an
indentation, called the hilum. The hilum leads to a large cavity within the kidney called the renal sinus. The ureter and renal vein leave the
kidney at the hilum, and the renal artery enters the kidney at the hilum.
The macroscopic internal structure of the kidney is illustrated in Figure 15-2. The outer, reddish region is the renal cortex. The renal cortex
surrounds a darker reddish-brown region called the renal medulla. The renal medulla consists of a series of renal pyramids. The renal
pyramids appear striated because they contain straight tubular structures and blood vessels. The wide bases of the pyramids are adjacent to the
pyramids appear striated because they contain straight tubular structures and blood vessels. The wide bases of the pyramids are adjacent to the
cortex. The pointed ends of the pyramids, called renal papillae, are directed toward the center of the kidney. Portions of the renal cortex extend
into the spaces between adjacent pyramids to form renal columns. The cortex and medulla make up the functional tissue of the kidney.
The central region of the kidney contains the renal pelvis, a large cavity that collects the urine as it is produced. 218219
Figure 15-2 Coronal (frontal) section through the kidney.
The periphery of the renal pelvis is interrupted by cuplike projections called calyces. A minor calyx surrounds the renal papillae of each
pyramid and collects urine from that pyramid. Several minor calyces converge to form a major calyx. From the major calyces the urine flows
into the renal pelvis and from there into the ureter.
Nephrons
Each kidney contains more than 1 million functional units, called nephrons, located in the cortex and medulla. The nephron is where the blood is
filtered and urine is formed. A nephron consists of a renal corpuscle and a renal tubule (Figure 15-3).
The renal corpuscle consists of the glomerulus (gloh-M ER-yoo-lus) and the glomerular capsule (Bowman capsule). The glomerulus is a
cluster of capillaries. Blood enters the glomerulus through an afferent arteriole and is filtered. The blood then leaves the glomerulus through an
efferent arteriole (Figure 15-4). As the blood is filtered, the filtrate enters the glomerular capsule, which continues as the renal tubule. Renal
corpuscles are located in the cortex of the kidney and give it a granular appearance.
The renal tubule, which carries fluid away from the glomerular capsule, consists of a proximal convoluted tubule, a nephron loop (Henle loop),
and a distal convoluted tubule.
The first portion of the tubule, located in the cortex, is highly coiled and is known as the proximal convoluted tubule. Next the tubule
straightens and dips into the medulla, makes a U-turn, and ascends back toward the cortex. This forms the nephron loop (Henle loop). The
portion of the loop that descends from the proximal convoluted tubule into the medulla is the descending limb, 219220 220221
Figure 15-3 A section of a kidney showing the structures in the cortex and those in the medulla. The renal pyramids in the medulla contain the
nephron loops and collecting ducts.
Figure 15-4 Juxtaglomerular apparatus and its relationship to the nephron. The juxtaglomerular apparatus is in the boxes. In the region of
contact, the cells of the ascending limb are modified to form the macula densa, and the cells of the afferent arteriole are modified to form the
juxtaglomerular cells. Together, these modified regions are the juxtaglomerular apparatus.
and the part that ascends back toward the cortex is the ascending limb. The final region of the tubule, which is also coiled and found in the
cortex, is known as the distal convoluted tubule (see Figure 15-4).
Collecting Ducts
Urine passes from the distal convoluted tubules of the nephrons into collecting ducts. These straight tubules, with the nephron loops and
blood vessels, give the medulla its striated appearance. Fluid flows from the collecting ducts into the minor calyces that surround the renal
papillae.
Juxtaglomerular Apparatus
The ascending limb of the nephron loop, in the region where it continues into the distal convoluted tubule, comes into contact with the
The ascending limb of the nephron loop, in the region where it continues into the distal convoluted tubule, comes into contact with the
glomerular afferent arteriole of the same nephron (Figure 15-4). In the region of contact, the cells of the ascending limb are modified to form the
macula densa, and those in the afferent arteriole are modified to form the juxtaglomerular (juks-tah-gloh-M ER-yoo-lar) cells. The macula
densa monitors sodium chloride concentration in the urine and also influences the juxtaglomerular cells. In the afferent arteriole, the
juxtaglomerular cells produce the enzyme renin, which has a role in the regulation of blood pressure. Together, the macula densa and
juxtaglomerular cells make up the juxtaglomerular apparatus.
Figure 15-5 Ureter, urinary bladder, and urethra. A, Urinary tract. B, Cross section through the ureter. C, Cross section of the bladder wall. D,
Regions of the male urethra.
Blood Flow through the Kidney
Blood flows through the kidneys at an approximate rate of 1200 mL/min. This is about one fourth of the total cardiac output. Blood is brought
to the kidneys by the renal arteries, which are branches from the abdominal aorta. The blood flows through the arteries of the kidney until it
enters the afferent arterioles. Each of these tiny vessels continues into a glomerulus, where the blood is filtered. The blood leaves the glomerulus
through an efferent arteriole and enters a series of veins. The renal vein exits the kidney and takes blood to the inferior vena cava.
Ureters
Each ureter is a small tube, about 25 cm (10 inches) long, that carries urine from the renal pelvis to the urinary bladder. It descends from the
renal pelvis and enters the urinary bladder on the posterior inferior surface.
The wall of the ureter consists of three layers (Figure 15-5). The outer layer is a supporting layer of fibrous connective tissue known as the
fibrous coat. The middle layer is known as the muscular coat. It consists of smooth muscle. The main function of this layer is peristalsis to
propel the urine through the ureter. The inner layer is the mucosa. This layer secretes mucus, which coats and protects the surface of the cells.
221222
Urinary Bladder
The urinary bladder is located in the pelvic cavity and is a temporary storage reservoir for urine (see Figure 15-5). The size and shape of the
urinary bladder vary with the amount of urine it contains and with the pressure from surrounding organs.
The inner lining of the urinary bladder consists of a mucous membrane. When the bladder is empty, the mucosa has numerous folds called
rugae. The rugae allow the bladder to expand as it fills. The next layer is the muscularis, which is composed of smooth muscle. The smooth
muscle fibers in the muscular layer are interwoven in all directions, and collectively these are called the detrusor (dee-TROO-sor) muscle.
Contraction of this muscle expels urine from the bladder.
A triangular area, called the trigone, is formed by three openings in the floor of the urinary bladder. Two of the openings are from the ureters
and form the base of the trigone. Small flaps of mucosa cover these openings and act as valves that allow urine to enter the bladder but prevent it
from backing up from the bladder into the ureters. The third opening, at the apex of the trigone, is the opening into the urethra. A band of the
detrusor muscle encircles this opening to form the internal urethral sphincter.
Urethra
The final passageway for the flow of urine is the urethra. The urethra consists of a thin-walled tube that conveys urine from the floor of the
urinary bladder to outside of the body (see Figure 15-5). The opening to the outside is known as the external urethral orifice.
The beginning of the urethra, where it leaves the urinary bladder, is surrounded by the internal urethral sphincter. This sphincter is smooth
(involuntary) muscle. Another sphincter, the external urethral sphincter, is skeletal (voluntary) muscle and encircles the urethra where it
passes through the pelvic floor. These two sphincters control the flow of urine through the urethra.
In females the urethra is short, only 3 to 4 cm (about
opening for the vagina.
inches) long. The external urethral orifice opens to the outside just anterior to the
In males the urethra is much longer, about 20 cm (7 to 8 inches) in length, and transports both urine and semen. The first part of the male urethra
passes through the prostate gland and is called the prostatic urethra. The second part is a short region that penetrates the pelvic floor and
enters the penis. This short region is known as the membranous urethra. The third part of the male urethra is the longest region and is called
the spongy urethra. This portion of the urethra extends the entire length of the penis, and the external urethral orifice opens to the outside at
the spongy urethra. This portion of the urethra extends the entire length of the penis, and the external urethral orifice opens to the outside at
the tip of the penis.
Urine Formation
The work of the kidneys, performed by the nephrons, is to maintain the volume and composition of body fluids,
Figure 15-6 Steps in urine formation. Urine consists of the substances that enter the tubules in glomerular filtration minus substances that are
reabsorbed in the tubules plus substances that are secreted into the tubules.
regulate the pH of the blood, and remove waste products from the blood. The result of this work is the formation of urine. As urine is excreted
to the outside of the body, it carries with it the wastes, excess water, and excess electrolytes. At the same time the kidneys conserve other
electrolytes to maintain the appropriate balance. The formation of urine involves glomerular filtration, tubular reabsorption, and tubular
secretion, which are illustrated in Figure 15-6.
Glomerular Filtration
The first step in the formation of urine is glomerular filtration. During this process, blood plasma leaves the glomerulus and enters the
glomerular capsule. The force that moves the fluid across the membrane is filtration pressure, and the fluid that enters the capsule is the
filtrate.
Blood flows through the kidneys at an average rate of 1200 mL/min. As the blood passes through the glomeruli, about 19% of the plasma enters
the glomerular capsule as filtrate. This is equivalent to forming filtrate at a rate of 125 mL/min, or 180 L (45 gal) per day. This is the total value
for all the nephrons in both kidneys. The filtration membrane acts as a barrier that prevents blood cells and 222223protein molecules from
entering the capsule; therefore they are absent from the filtrate.
Tubular Reabsorption
If the volume and composition of the filtrate in the glomerular capsule are compared with the volume and composition of urine, it is obvious that
changes occur after filtration. First of all, about 180 L (45 gal) of filtrate are formed in a 24-hour period. This volume is reduced to 1 to 2 L of
urine. Glucose is present in the filtrate but normally absent in the urine. Urea and uric acid are present in higher concentrations in the urine than
in the filtrate.
Tubular reabsorption is the first process that changes the volume and composition of the filtrate. Tubular reabsorption is the movement of
substances from the filtrate in the kidney tubules into the blood. Only about 1% of the filtrate remains in the tubules and becomes urine. In
general, water and other substances that are useful to the body are reabsorbed. Wastes remain in the filtrate and are excreted in the urine.
Tubular Secretion
The final process in the formation of urine is the transport of molecules and ions into the filtrate. This is called tubular secretion. M ost of
these substances are waste products of cellular metabolism that become toxic if allowed to accumulate in the body. Tubular secretion is the
method by which some drugs, such as penicillin, are removed from the body. The tubular secretion of hydrogen ions plays an important role in
regulating the pH of the blood. Other molecules and ions that may enter the filtrate by tubular secretion include potassium ions, creatinine, and
histamine.
The final product, urine, produced by the nephrons of the kidney consists of the substances that are filtered, minus the substances that are
reabsorbed in the tubules, plus the substances that are added by tubular secretion. If kidney function is impaired by disease or injury, dialysis
reabsorbed in the tubules, plus the substances that are added by tubular secretion. If kidney function is impaired by disease or injury, dialysis
may be necessary to maintain body fluid composition. Dialysis is a procedure used to separate waste material from the blood and to maintain
fluid, electrolyte, and acid-base balance in the body.
Regulation of Urine Concentration and Volume
The concentration and volume of urine depend on conditions in the internal environment of the body. Cells in the hypothalamus are sensitive to
changes in the composition of the blood and initiate appropriate responses that affect the kidneys. If the concentration of solutes in the blood
increases above normal, the kidneys excrete a small volume of concentrated urine. This conserves water in the body and gets rid of solutes to
restore the blood to normal. If the blood solute concentration decreases below normal, the kidneys conserve solutes and get rid of water by
producing large quantities of dilute urine. Urine production plays an important role in maintaining homeostasis of blood concentration and
volume. By regulating blood volume, the kidneys also play a role in regulating blood pressure because volume is directly related to pressure.
Under average conditions, the kidneys produce about 1500 mL of urine in a 24-hour period, but the volume may vary from 1 to 2 L. The pH
may vary from 4.6 to 8, with an average of about 6. This means that urine is usually slightly acidic but may become alkaline under certain
conditions such as vegetarian diets.
Three hormones—aldosterone, antidiuretic hormone (ADH), and atrial natriuretic hormone—influence urine concentration and volume.
Aldosterone, secreted by cells of the adrenal cortex, acts on the kidney tubules to increase the reabsorption of sodium. When sodium is
reabsorbed, water follows by osmosis. This reduces urine output.
Antidiuretic hormone (ADH) is produced by cells in the hypothalamus and is released from the posterior lobe of the pituitary gland. ADH
makes the kidney tubules more permeable to water. When ADH is present, more water is reabsorbed, which reduces the volume of urine and
makes it more concentrated. Water is conserved in the body. In the absence of ADH, the tubules are less permeable to water and there is less
reabsorption. This results in large quantities of dilute urine, and water is lost from the body.
Special cells in the heart produce a hormone called atrial natriuretic hormone, or atriopeptin, which is secreted when the atrial cells are stretched.
This hormone promotes the excretion of sodium and water by acting directly on the kidney tubules and by inhibiting the secretion of ADH,
renin, and aldosterone. The result of atrial natriuretic hormone is a decrease in both blood volume and blood pressure.
Renin is an enzyme that is produced by the juxtaglomerular cells in the kidney in response to low blood pressure or decreased blood sodium
concentration. Renin promotes the production of angiotensin II in the blood. Angiotensin II is a powerful vasoconstrictor, a substance that
increases the blood pressure. Angiotensin II also stimulates the adrenal gland to secrete aldosterone, which acts on the kidney tubules to
conserve sodium and water. This increases blood volume and consequently increases blood pressure.
M icturition
Micturition (mik-too-RISH-un), commonly called urination or voiding, is the act of expelling urine from the bladder. The bladder can hold up
to a liter of urine, but normally when it contains 200 to 400 mL, stretch receptors in the bladder wall trigger impulses that initiate the
micturition reflex. This is an automatic and involuntary response that is coordinated in the spinal cord. Impulses are transmitted along
parasympathetic nerves to the detrusor muscle. Even though the micturition reflex is involuntary, it can be inhibited or stimulated by higher
brain centers.
It is desirable to completely empty the bladder when urinating. Residual urine is what remains in the bladder if an individual is unable to
completely empty the bladder. This may be indicative of a pathologic condition such as a urinary tract infection or, in males, an enlarged
prostate. 223224
Aging of the Urinary System
Some of the more obvious and familiar aging changes occur in the urinary bladder and urethra. M uscles in the walls of these structures tend to
weaken and become less elastic with age. As a person ages, the bladder is unable to expand or contract as much as in younger people. This
reduces the capacity of the bladder and makes it more difficult to completely empty it during urination. Awareness of the need to urinate, which
usually occurs when the bladder is half full in younger people, may be delayed in the elderly until the bladder is nearly full. Thus urgency
accompanies awareness. The external urethral sphincter also weakens, which adds to the problems.
Several anatomic changes occur in the kidneys as a person ages, and these changes are reflected in their related functions. There is a general
atrophy of nephrons so that by the age of 80, the kidney is about 80% of its young, but mature, size. Some of the remaining glomeruli are
modified, and this, along with the decrease in number, results in a decreased glomerular filtration rate so that the blood is not filtered as quickly
as before.
The tubules also undergo changes as a person ages. In general, the tubule walls thicken, which makes them less able to reabsorb water to form
concentrated urine. The collecting ducts are less responsive to ADH, and this, along with a diminished thirst mechanism, may result in
dehydration. The ability to reabsorb glucose and sodium is also diminished. The tubules become less efficient in the secretion of ions and drugs.
They have a diminished ability to compensate for drastic changes in acid-base balance. Drugs that are normally eliminated from the body by
tubular secretion may accumulate to toxic levels because they are not cleared from the blood as quickly as they are in younger people.
Amazingly, even with the changes caused by aging, the kidneys of elderly persons are capable of maintaining relatively stable balances in the
blood and body fluids under normal conditions. However, their ability to compensate for drastic changes and abnormal conditions is diminished.
Terminology Review
M edical Term Word
Parts
Definition
Glomerular
capsule
Double-layered epithelial cup that surrounds the glomerulus in a nephron; also called Bowman capsule.
Juxtaglomerularjuxta-:
Complex of modified cells in the afferent arteriole and the ascending limb and distal tubule in the kidney; helps regulate
Juxtaglomerularjuxta-: Complex of modified cells in the afferent arteriole and the ascending limb and distal tubule in the kidney; helps regulate
apparatus
near to blood pressure by secreting renin; consists of the macula densa and juxtaglomerular cells.
Nephron
nephr/o: Functional unit of the kidney consisting of a renal corpuscle and a renal tubule.
kidney
Renal tubule
ren/o:
kidney
Tubular portion of the nephron that carries the filtrate away from the glomerular capsule; site where tubular
reabsorption and secretion occur.
On the Web
For information on the urinary system:
Inner Body: www.innerbody.com/html/body.html
The Internet Pathology Laboratory for M edical Education—Renal Pathology: library.med.utah.edu/WebPath/ORGAN.html (Click on “Renal
Pathology”)
Urinary Incontinence: www.seekwellness.com/incontinence
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