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.
A. 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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B. Kidney Structure con’t
3. 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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C. 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.
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.
E. Renal Blood Vessels
1.The abdominal aorta gives rise to renal
arteries leading to the kidneys.
2.As renal arteries pass into the kidneys, they
branch into successively smaller arteries:
interlobar arteries, arcuate arteries,
interlobular arteries, and afferent arterioles
leading to the nephrons.
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3.
Venous blood is returned through a
series of vessels that generally
correspond to the arterial pathways.
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F. Nephrons
1. Nephron Structure
a. A kidney contains
one million
nephrons, each of
which consists
of a renal
corpuscle and a
renal tubule.
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b. The renal corpuscle is
the filtering portion of
the nephron; it is
made up of a ball of
capillaries called the
glomerulus and a
glomerular
capsule
that
receives
the
filtrate.
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c. The renal tubule leads
away from the glomerular
capsule and first
becomes a highly coiled
proximal convoluted
tubule, then leads to the
nephron loop, and finally
to the distal convoluted
tubule.
d. Several distal convoluted
tubules join to become a
collecting duct.
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2.
Blood Supply of a Nephron
a.
The glomerulus receives blood
from a fairly large afferent
arteriole and passes it to a smaller
efferent arteriole.
b.
The efferent arteriole gives rise to
the peritubular capillary system,
which surrounds the renal tubule.
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3.
Juxtaglomerular Apparatus
a.
At the point of contact between
the afferent and efferent arterioles
and the distal convoluted tubule,
the epithelial cells of the distal
tubule form the macula densa.
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b.
Near the macula densa on the
afferent arteriole are smooth
muscle cells called
juxtaglomerular cells.
c.
The macula densa together with
the juxtaglomerular cells make up
the juxtaglomerular apparatus.
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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 filter by the glomerulus and
enters the glomerular
capsule as glomerular filtrate.
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.
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.
When the efferent arteriole constricts,
filtration pressure increases, increasing
the rate of filtration.
4.
When osmotic pressure of the
glomerular plasma is high, filtration rate
decreases.
5.When hydrostatic pressure inside the
glomerular capsule is high, filtration rate
declines.
6.On the average, filtration rate is 125
milliliters per minute or 180 liters in 24
hours, most of which is reabsorbed further
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.
2. Another control over filtration rate is the reninangiotensin 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.
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.
Presence of angiotensin II also
increases the secretion of
aldosterone, which stimulates
reabsorption of sodium.
d.
The heart can also increase
filtration rate when blood volume
is high.
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.
Carrier proteins have a limited transport
capacity, so excessive amounts of a
substance will be excreted into the
urine.
4.
Glucose and amino acids are reabsorbed
by active transport, water by osmosis,
and proteins by pinocytosis.
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.
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.
I. Urea and Uric Acid Excretion
1. Urea is a by-product of amino acid
metabolism; uric acid is a by-product 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.
Potassium ions are secreted both
actively and passively into the distal
convoluted tubule and the collecting
duct.
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.
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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D. Micturition
1.
Urine leaves the bladder by the
micturation reflex.
2.
The detrusor muscle contracts and the
external urethral sphincter (in the
urogenital diaphragm) must also relax.
3. Stretching of the urinary bladder
triggers the micturation reflex center
located in the sacral portion of the
spinal cord.
4. Return parasympathetic impulses cause
the detrusor muscle to contract in waves,
and an urge to urinate is sensed.
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5. When these contractions become strong
enough, the internal urethral sphincter is
forced open.
6. 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.
Water, Electrolyte, and
Acid-Base Balance
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 Introduction
A. To be in balance, the quantities of fluids and
electrolytes leaving the body should be equal
to the amounts taken in.
B. Anything that alters the concentrations of
electrolytes will also alter the concentration of
water, and vice versa.
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 Distribution of Body Fluids
A. Fluids occur in compartments in the body, and
movement of water and electrolytes between
compartments is regulated.
B. Fluid Compartments
1.
The average adult female is 52% water
by weight, while a male is 63% water,
the difference due to the female's
additional adipose tissue.
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2.
3.
The intracellular fluid compartment
includes all the water and electrolytes
within cells.
The extracellular fluid compartment
includes all water and electrolytes
outside of cells (interstitial fluid,
plasma, and lymph).
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4.
Transcellular fluid includes the
cerebrospinal fluid of the central
nervous system, fluids within the
eyeball, synovial fluid of the joints,
serous fluid within body cavities, and
exocrine gland secretions.
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C. Body Fluid Composition
1.
Extracellular fluids have high
concentrations of sodium, chloride, and
bicarbonate ions, and lesser amounts of
potassium, calcium, magnesium,
phosphate, and sulfate ions.
2.
Intracellular fluid has high
concentrations of potassium, phosphate,
and magnesium ions, and lesser
amounts of sodium, chloride, and
bicarbonate ions.
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D. Movement of Fluid between Compartments
1.
Hydrostatic pressure and osmotic
pressure regulate the movement of
water and electrolytes from one
compartment to another.
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2.
Although the composition of body
fluids varies from one compartment to
another, the total solute concentrations
and water amounts are normally equal.
3.
A net gain or loss of water will cause
shifts affecting both the intracellular
and extracellular fluids due to osmosis.
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Water Balance
A. Water balance exists when water intake equals
water output.
B. Water Intake
1.
The volume of water gained each day
varies from one individual to the next.
2.
About 60% of daily water is gained
from drinking, another 30% comes from
moist foods, and 10% from the water of
metabolism.
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C. Regulation of Water Intake
1.
The thirst mechanism is the primary
regulator of water intake.
2.
The thirst mechanism derives from the
osmotic pressure of extracellular fluids
and a thirst center in the hypothalamus.
3.
Once water is taken in, the resulting
distention of the stomach will inhibit the
thirst mechanism.
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D. Water Output
1.
Water is lost in urine, feces,
perspiration, evaporation from skin
(insensible perspiration), and from the
lungs during breathing.
2.
The route of water loss depends on
temperature, relative humidity, and
physical exercise.
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E. Regulation of Water Output
1.
The distal convoluted tubules and
collecting ducts of the nephrons
regulate water output.
2.
Antidiuretic hormone from the
posterior pituitary causes a reduction in
the amount of water lost in the urine.
3.
When drinking adequate water, the
ADH mechanism is inhibited, and more
water is expelled in urine.
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Electrolyte Balance
A. An electrolyte balance exists when the
quantities of electrolytes gained equals the
amount lost.
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B. Electrolyte Intake
1.
The electrolytes of greatest importance
to cellular metabolism are sodium,
potassium, calcium, magnesium,
chloride, sulfate, phosphate,
bicarbonate, and hydrogen ions.
2.
Electrolytes may be obtained from food
or drink or produced as a by-product of
metabolism.
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C. Regulation of Electrolyte Intake
1.
2.
A person ordinarily obtains sufficient
electrolytes from foods eaten.
A salt craving may indicate an
electrolyte deficiency.
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D. Electrolyte Output
1.
Losses of electrolytes occur through
sweating, in the feces, and in urine.
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E. Regulation of Electrolyte Output
1.
The concentrations of the cations,
especially sodium, potassium, and
calcium, are very important.
2.
Sodium ions account for 90% of the
positively charged ions in extracellular
fluids; the action of aldosterone on the
kidneys regulates sodium reabsorption.
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3.
4.
Aldosterone also regulates potassium
ions; potassium ions are excreted when
sodium ions are conserved.
Calcium concentration is regulated by
parathyroid hormone, which increases
the concentrations of calcium and
phosphate ions in extracellular fluids
and by calcitonin which does basically
the reverse.
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5.
Generally, the regulatory mechanisms
that control positively charged ions
secondarily control the concentrations
of anions.
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 Acid-Base Balance
A. Electrolytes that ionize in water and release
hydrogen ions are acids; those that combine
with hydrogen ions are bases.
B. Maintenance of homeostasis depends on the
control of acids and bases in body fluids.
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C. Sources of Hydrogen Ions
1.
Most hydrogen ions originate as byproducts of metabolic processes,
including: the aerobic and anaerobic
respiration of glucose, incomplete
oxidation of fatty acids, oxidation of
amino acids containing sulfur, and the
breakdown of phosphoproteins and
nucleic acids.
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D. Strengths of Acids and Bases
1.
Acids that ionize more completely are
strong acids; those that ionize less
completely are weak acids.
2.
Bases release hydroxyl and other ions,
which can combine with hydrogen ions,
thereby lowering their concentration.
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E. Regulation of Hydrogen Ion Concentration
1.
Acid-base buffer systems, the
respiratory center in the brain stem, and
the kidneys regulate pH of body fluids.
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2.
Acid-Base Buffer Systems
a.
The chemical components of a
buffer system can combine with a
strong acid and convert it to a
weaker one.
b.
The chemical buffer systems in
body fluids include the
bicarbonate buffer system, the
phosphate buffer system, and the
protein buffer system.
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3.
The Respiratory Center
a.
The respiratory center in the brain
stem helps to regulate hydrogen
ion concentration by controlling
the rate and depth of breathing.
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b.
During exercise, the carbon
dioxide, and thus the carbonic
acid, levels in the blood increase.
c.
In response, the respiratory center
increases the rate and depth of
breathing, so the lungs excrete
more carbon dioxide.
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4. The Kidneys
a.
Nephrons secrete excess hydrogen ions
in the urine.
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5. Rates of Regulation
a.
Chemical buffers are considered the
body's first line of defense against shifts
in pH; physiological buffer systems
(respiratory and renal mechanisms)
function more slowly and constitute
secondary defenses.
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Acid-Base Imbalances
A. Chemical and physiological buffer systems
usually keep body fluids within very narrow
pH ranges but abnormal conditions may
prevent this.
1.
A pH below 7.35 produces acidosis
while a pH above 7.45 is called
alkalosis.
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B. Acidosis
1.
Two major types of acidosis are
respiratory and metabolic acidosis.
a.
Respiratory acidosis results from
an increase of carbonic acid
caused by respiratory center
injury, air passage obstructions of
problems with gas exchange.
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b.
c.
Metabolic acidosis is due to
either an accumulation of acids of
a loss of bases and has many
causes including kidney disease,
vomiting, diarrhea and diabetes
mellitus.
Increasing respiratory rate or the
amount of hydrogen ions released
by the kidney can help
compensate for acidosis.
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C. Alkalosis
1.
Alkalosis also has respiratory and
metabolic causes.
a.
Respiratory alkalosis results from
hyperventilation causing an
excessive loss of carbon dioxide.
b.
Metabolic alkalosis is caused by a
great loss of hydrogen ions or a
gain in base perhaps from
vomiting or use of drugs.
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