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Chapter 17
Urinary System
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 Introduction
A. The urinary system consists of two
kidneys that filter the blood, two ureters,
a urinary bladder, and a urethra to
convey waste substances to the outside.
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 Kidneys
A. The kidney is a reddish brown, bean-shaped organ
12 centimeters long; it is enclosed in a tough,
fibrous capsule.
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B. Location of the Kidneys
1.
The kidneys are positioned
retroperitoneally on either side of
the vertebral column between the
twelfth thoracic and third lumbar
vertebrae, with the left kidney
slightly higher than the right.
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C. Kidney Structure
1.
A medial depression in the kidney
leads to a hollow renal sinus into
which blood vessels, nerves,
lymphatic vessels, and the ureter
enter.
2.
Inside the renal sinus lies a renal
pelvis that is subdivided into major
and minor calyces; small renal
papillae project into each minor
calyx.
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3.
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Two distinct regions are found
within the kidney: a renal medulla
and a renal cortex.
a.
The renal medulla houses
tubes leading to the papillae.
b.
The renal cortex contains the
nephrons, the functional units
of the kidney.
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D. Kidney Functions
1.
The kidneys function to regulate the
volume, composition, and pH of
body fluids and remove metabolic
wastes from the blood in the
process.
2.
The kidneys also help control the
rate of red blood cell formation by
secreting erythropoietin, and
regulate blood pressure by secreting
renin.
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 Urine Formation
A. Urine formation involves glomerular
filtration, tubular reabsorption, and
tubular secretion.
B. Glomerular Filtration
1.
Urine formation begins when the
fluid portion of the blood is filtered
by the glomerulus and enters the
glomerular capsule as glomerular
filtrate.
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C. Filtration Pressure
1.
The main force responsible for
moving substances by filtration
through the glomerular capillary
wall is the hydrostatic pressure of
the blood inside.
2.
Due to plasma proteins, osmotic
pressure of the blood resists
filtration, as does hydrostatic
pressure inside the glomerular
capsule.
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D. Filtration Rate
1.
The factors that affect the filtration
rate are filtration pressure,
glomerular plasma osmotic
pressure, and hydrostatic pressure
in the glomerular capsule.
2.
When the afferent arteriole
constricts in response to
sympathetic stimulation,
filtration pressure, and thus filtration
rate, declines.
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3.
4.
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When the efferent arteriole
constricts, filtration pressure
increases, increasing the rate of
filtration.
When osmotic pressure of the
glomerular plasma is high, filtration
rate decreases.
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5.
6.
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When hydrostatic pressure inside
the glomerular capsule is high,
filtration rate declines.
On average, filtration rate is 125
milliliters per minute or 180 liters in
24 hours, most of which is
reabsorbed farther down the
nephron.
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E. Regulation of Filtration Rate
1.
Glomerular filtration rate is relatively
constant, although sympathetic
impulses may decrease the rate of
filtration.
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2.
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Another control over filtration rate is
the renin-angiotensin system, which
regulates sodium excretion.
a.
When the sodium chloride
concentration in the tubular
fluid decreases, the macula
densa senses these changes
and causes the
juxtaglomerular cells to
secrete renin.
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b.
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Secretion of renin triggers a
series of reactions leading to
the production of angiotensin
II, which acts as a
vasoconstrictor; this may, in
turn, affect filtration rate.
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c.
d.
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Presence of angiotensin II also
increases the secretion of
aldosterone, which stimulates
reabsorption of sodium.
The heart can also increase
filtration rate when blood
volume is high.
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F. Tubular Reabsorption
1.
Changes in the fluid composition
from the time glomerular filtrate is
formed to when urine arrives at the
collecting duct are largely the result
of tubular reabsorption of selected
substances.
2.
Most of the reabsorption occurs in
the proximal convoluted tubule,
where cells possess microvilli with
carrier proteins.
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3.
4.
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Carrier proteins have a limited
transport capacity, so excessive
amounts of a substance will be
excreted into the urine.
Glucose and amino acids are
reabsorbed by active transport,
water by osmosis, and proteins by
pinocytosis.
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G. Sodium and Water Reabsorption
1.
Sodium ions are reabsorbed by
active transport, and negatively
charged ions follow passively.
2.
As sodium is reabsorbed, water
follows by osmosis.
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H. Regulation of Urine Concentration and
Volume
1.
Most of the sodium ions are
reabsorbed before the urine is
excreted, and sodium is
concentrated in the renal medulla
by the countercurrent mechanism.
2.
Normally the distal convoluted
tubule and collecting duct are
impermeable to water unless the
hormone ADH is present.
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I. Urea and Uric Acid Excretion
1.
Urea is a by-product of amino acid
metabolism; uric acid is a byproduct of nucleic acid metabolism.
2.
Urea is passively reabsorbed by
diffusion but about 50% of urea is
excreted in the urine.
3.
Most uric acid is reabsorbed by
active transport and a small amount
is secreted into the renal tubule.
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J. Tubular Secretion
1.
Tubular secretion transports certain
substances from the plasma into the
renal tubule.
2.
Active transport mechanisms move
excess hydrogen ions into the renal
tubule along with various organic
compounds.
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3.
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Potassium ions are secreted both
actively and passively into the distal
convoluted tubule and the collecting
duct.
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K. Urine Composition
1.
Urine composition varies from time
to time and reflects the amounts of
water and solutes that the kidneys
eliminate to maintain homeostasis.
2.
Urine is 95% water, and also
contains urea, uric acid, a trace of
amino acids, and electrolytes.
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 Urine Elimination
A. After forming in the nephrons, urine
passes from the collecting ducts to the
renal papillae, then to the minor and
major calyces, and out the renal pelvis to
the ureters, urinary bladder, and finally to
the urethra, which conveys urine to the
outside.
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B. Ureters
1.
The ureters are muscular tubes
extending from the kidneys to the
base of the urinary bladder.
2.
The wall of the ureter is composed
of three layers: mucous coat,
muscular coat, and outer fibrous
coat.
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3.
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Muscular peristaltic waves convey
urine to the urinary bladder where it
passes through a flaplike valve in
the mucous membrane of the
urinary bladder.
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C. Urinary Bladder
1.
The urinary bladder is a hollow,
distensible, muscular organ lying in
the pelvic cavity.
2.
The internal floor of the bladder
includes the trigone, which is
composed of the openings of the
two ureters and the urethra.
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3.
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The wall of the urinary bladder is
made up of four coats: inner
mucous coat, submucous coat,
muscular coat made up of detrusor
muscle, and outer serous coat.
a.
The portion of the detrusor
muscle that surrounds the
neck of the bladder forms an
internal sphincter muscle.
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D. Micturition
1.
Urine leaves the bladder by the
micturition reflex.
2.
The detrusor muscle contracts and
the external urethral sphincter (in
the urogenital diaphragm) must also
relax.
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3.
4.
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Stretching of the urinary bladder
triggers the micturition reflex center
located in the sacral portion of the
spinal cord.
Return parasympathetic impulses
cause the detrusor muscle to
contract in waves, and an urge to
urinate is sensed.
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5.
6.
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When these contractions become
strong enough, the internal urethral
sphincter is forced open.
The external urethral sphincter is
composed of skeletal muscle and is
under conscious control.
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E. Urethra
1.
The urethra is a tube that conveys
urine from the urinary bladder to
the outside.
2.
It is a muscular tube with urethral
glands that secrete mucus into the
urethral canal.
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Kidney Stones
A kidney stone is a solid mass made up of
tiny crystals. One or more stones can be in
the kidney or ureter at the same time.
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Symptoms
The main symptom is severe pain that starts suddenly and may go away
suddenly:
Pain may be felt in the belly area or side of the back
Pain may move to groin area (groin pain) or testicles (testicle pain)
Other symptoms can include:
Abnormal urine color
Blood in the urine
Chills
Fever
Nausea
Vomiting
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Treatment
The goal of treatment is to relieve
symptoms and prevent further symptoms.
(Kidney stones that are small enough
usually pass on their own.) Treatment varies
depending on the type of stone and how
severe the symptoms are. People with
severe symptoms might need to be
hospitalized.
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Treatment
When the stone passes, the urine should be strained and the stone saved and tested to
determine the type.
Drink at least 6 - 8 glasses of water per day to produce a large amount of urine. Some
people might need to get fluids through a vein (intravenous).
Pain relievers can help control the pain of passing the stones (renal colic). For severe
pain, you may need to take narcotic pain killers or nonsteroidal anti-inflammatory drugs
(NSAIDS) such as ibuprofen.
Depending on the type of stone, your doctor may prescribe medicine to decrease stone
formation or help break down and remove the material that is causing the stone.
Medications can include:
Allopurinol (for uric acid stones)
Antibiotics (for struvite stones)
Diuretics
Phosphate solutions
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Treatment
Surgery is usually needed if:
The stone is too large to pass on its own
The stone is growing
The stone is blocking urine flow and cuasing an infection or kidney damage
Today, most treatments are much less invasive than in the past.
Extracorporeal shock-wave lithotripsy is used to remove stones slightly smaller than a
half an inch that are located near the kidney. This method uses ultrasonic waves or
shock waves to break up stones. Then, the stones leave the body in the urine.
Percutaneous nephrolithotomy is used for large stones in or near the kidney, or when the
kidneys or surrounding areas are incorrectly formed. The stone is removed with an
endoscope that is inserted into the kidney through a small opening.
Ureteroscopy may be used for stones in the lower urinary tract.
Standard open surgery (nephrolithotomy) may be needed if other methods do not work
or are not
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Urinary Tract Infections
Urinary tract infections are a serious health problem
affecting millions of people each year.
Infections of the urinary tract are the second most common
type of infection in the body. Urinary tract infections
(UTIs) account for about 8.3 million doctor visits each
year.* Women are especially prone to UTIs for reasons
that are not yet well understood. One woman in five
develops a UTI during her lifetime. UTIs in men are not as
common as in women but can be very serious when they
do occur.
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What are the causes of UTI?
Normally, urine is sterile. It is usually free of bacteria, viruses, and fungi but
does contain fluids, salts, and waste products. An infection occurs when tiny
organisms, usually bacteria from the digestive tract, cling to the opening of the
urethra and begin to multiply. The urethra is the tube that carries urine from
the bladder to outside the body. Most infections arise from one type of
bacteria, Escherichia coli (E. coli), which normally lives in the colon.
In many cases, bacteria first travel to the urethra. When bacteria multiply, an
infection can occur. An infection limited to the urethra is called urethritis. If
bacteria move to the bladder and multiply, a bladder infection, called cystitis,
results. If the infection is not treated promptly, bacteria may then travel further
up the ureters to multiply and infect the kidneys. A kidney infection is called
pyelonephritis.
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What are the causes of UTI?
Microorganisms called Chlamydia and Mycoplasma may also cause
UTIs in both men and women, but these infections tend to remain
limited to the urethra and reproductive system. Unlike E. coli,
Chlamydia and Mycoplasma may be sexually transmitted, and
infections require treatment of both partners.
The urinary system is structured in a way that helps ward off infection.
The ureters and bladder normally prevent urine from backing up
toward the kidneys, and the flow of urine from the bladder helps wash
bacteria out of the body. In men, the prostate gland produces secretions
that slow bacterial growth. In both sexes, immune defenses also
prevent infection. But despite these safeguards, infections still occur.
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How is UTI diagnosed?
To find out whether you have a UTI, your doctor will test a sample of urine for
pus and bacteria. You will be asked to give a "clean catch" urine sample by
washing the genital area and collecting a "midstream" sample of urine in a
sterile container. This method of collecting urine helps prevent bacteria around
the genital area from getting into the sample and confusing the test results.
Usually, the sample is sent to a laboratory, although some doctors' offices are
equipped to do the testing.
In the urinalysis test, the urine is examined for white and red blood cells and
bacteria. Then the bacteria are grown in a culture and tested against different
antibiotics to see which drug best destroys the bacteria.
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How is UTI treated?
UTIs are treated with antibacterial drugs.
The choice of drug and length of treatment
depend on the patient's history and the urine
tests that identify the offending bacteria.
The sensitivity test is especially useful in
helping the doctor select the most effective
drug.
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