Download Chapter 13: Urinary System: Section 13.3

UNIT B: Human Body Systems
Chapter 8: Human Organization
Chapter 9: Digestive System
Chapter 10: Circulatory System and
Lymphatic System
Chapter 11: Respiratory System
Chapter 12: Nervous System
Chapter 13: Urinary System: Section 13.3
Chapter 14: Reproductive System
UNIT B Chapter 13: Urinary System
Chapter 13: Urinary System
What is the role of the kidneys in the
body?
How would problems in the
collecting ducts of the nephrons
cause kidney failure?
Born with Bad Kidneys. The kidney on the left
is normal. The kidney on the right is from a
person who has polycystic kidney disease
(PKD). In PKD, cysts form within the collecting
ducts of the nephrons, which can lead to kidney
failure. A kidney transplant is required to treat
PKD.
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Polycystic kidney disease seems to
cause more serious problems in
people of African descent, especially
those who have sickle cell disease.
Sickle cell is mainly a disease of the
red blood cells. What does this have
to do with the kidneys?
UNIT B Chapter 13: Urinary System
Section 13.3
13.3 Regulatory Functions of the Kidneys
Osmoregulation
The kidneys are involved in the maintenance of the water-salt
balance in the blood (osmoregulation). In this way, they also
maintain blood volume and blood pressure.
The excretion of a hypertonic urine (more concentrated than
blood) depends on the reabsorption of water. This requires the
following processes:
• Reabsorption of salt
• Establishment of a solute gradient
• Water reabsorption
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UNIT B Chapter 13: Urinary System
Reabsorption of Salt
The kidneys regulate salt balance by controlling the excretion
and reabsorption of various ions, such as sodium.
Sodium (Na+) reabsorption
• About 99% of Na+ filtered at the glomerulus is reabsorbed
into the blood
o 67% is reabsorbed at the proximal convoluted tubule,
25% is reabsorbed by the loop of Henle, and the rest is
reabsorbed from the distal convoluted tubule
o Two hormones regulate the reabsorption of Na+ at the
distal convoluted tubule: aldosterone and atrial
natriuretic hormone (ANH)
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Section 13.3
UNIT B Chapter 13: Urinary System
Reabsorption of Salt
Aldosterone
Aldosterone is a hormone that
promotes the reabsorption of Na+
• Juxtaglomerular apparatus:
secretes renin, an enzyme that
leads to the secretion of
aldosterone from the adrenal
cortex
• Aldosterone promotes
reabsorption of Na+, which
leads to reabsorption of water
o Blood volume and blood
pressure increase
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Section 13.3
Figure 13.6 Juxtaglomerular
apparatus. The afferent arteriole and the
distal convoluted tubule usually lie next
to each other. The juxtaglomerular
apparatus occurs where they touch. The
juxtaglomerular apparatus secretes
renin, a substance that leads to the
release of aldosterone by the adrenal
cortex. Reabsorption of sodium ions and
water then occurs. Thereafter, blood
volume and blood pressure increase.
UNIT B Chapter 13: Urinary System
Figure 13.7 Regulation
of blood pressure and
volume. Bottom: When
the blood Na+ is low, low
blood pressure causes
the kidneys to secrete
renin. Renin leads to the
secretion of aldosterone
from the adrenal cortex.
Aldosterone causes the
kidneys to reabsorb
Na+, and water follows.
Blood volume and
pressure return to
normal.
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Section 13.3
UNIT B Chapter 13: Urinary System
Reabsorption of Salt
Atrial natriuretic hormone (ANH)
ANH is a hormone that promotes the excretion of Na+.
• Secreted by the atria of the heart when cardiac cells are
stretched due to increased blood volume
• Inhibits the secretion of renin by the juxtaglomerular
apparatus and the secretion of aldosterone by the adrenal
cortex
• Leads to excretion of Na+ (natriuresis), which is followed
by excretion of water into the urine
o Blood volume and blood pressure decrease
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Section 13.3
UNIT B Chapter 13: Urinary System
Figure 13.7 Regulation
of blood pressure and
volume. Top: When the
blood Na+ is high, high
blood volume causes
the heart to secrete
ANH. ANH causes the
kidneys to excrete Na+,
and water follows. The
blood volume and
pressure return to
normal.
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Section 13.3
UNIT B Chapter 13: Urinary System
Establishment of a Solute Gradient
Reabsorption of water at the loop
of Henle and the collecting duct is
due to the establishment of a
solute gradient.
• Ascending limb: Salt (NaCl) is
actively transported out of the
ascending limb and into the
renal medulla
o Less salt is available to
transport as the fluid
moves up the ascending
limb, establishing a solute
gradient that increases
toward the inner medulla
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Section 13.3
UNIT B Chapter 13: Urinary System
Section 13.3
Establishment of a Solute Gradient
• Urea moves out of the
collecting duct, further
contributing to the increasing
solute concentration at the
inner medulla
• Because of this solute
gradient, water leaves the
descending limb and the
collecting duct and returns to
the blood
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Figure 13.8 Reabsorption of water at the loop of Henle and the collecting duct. Salt (NaCl)
diffuses and is actively transported out of the ascending limb of the loop of Henle into the renal
medulla. Also, urea is believed to leak from the collecting duct and to enter the tissues of the renal
medulla. This creates a hypertonic environment, which draws water out of the descending limb and
the collecting duct. This water is returned to the circulatory system. (The thick black outline of the
ascending limb means that it is impermeable to water.) The solute concentration is 300 mOsm/L in
the glomerulus and peritubular capillary network.
UNIT B Chapter 13: Urinary System
Water Reabsorption
The solute gradient results in
water reabsorption at the
descending limb of the loop of
Henle.
Descending limb:
• Permeable to water
• Water diffuses out of the
descending limb and into the
blood of the surrounding
capillaries
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Section 13.3
UNIT B Chapter 13: Urinary System
Water Reabsorption
Ascending limb:
• Not permeable to water
• Solutes diffuse out of the
ascending limb and into the
blood of the surrounding
capillaries
• Water is not reabsorbed
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Section 13.3
UNIT B Chapter 13: Urinary System
Water Reabsorption
Ascending limb (upper, thick
portion)
• Active transport of Na+ and
passive transport of K+ and Clout of the ascending loop and
into the blood of the
surrounding capillaries
• No reabsorption of water
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Section 13.3
UNIT B Chapter 13: Urinary System
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Figure 13.9 Reabsorption in the loop of Henle occurs through both
active and passive transport.
Section 13.3
UNIT B Chapter 13: Urinary System
Section 13.3
Water Reabsorption
Reabsorption of water and ions continues from the loop of Henle to
the distal convoluted tubule and the collecting duct.
The reabsorption of water and ions at this point is regulated by the
needs of the body.
• If the body is dehydrated, the pituitary gland releases
antidiuretic hormone (ADH)
o ADH increases the number of aquaporins (water channels)
of the collecting duct, which promotes increased water
reabsorption and a more concentrated urine
o In the absence of ADH: collecting duct is impermeable to
water, and urine is dilute
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UNIT B Chapter 13: Urinary System
Section 13.3
Diuretics
Diuretics are chemicals that increase the flow of urine.
• Alcohol: inhibits the secretion of ADH
• Caffeine: increases the glomerular filtration rate and
decreases tubular reabsorption of Na+
• Diuretic drugs that lower high blood pressure: inhibit active
transport of Na+ at the loop of Henle or the distal convoluted
tubule
• Diuretics have been abused for quick weight loss (water
loss), and by individuals attempting to pass a urine drug test
• Side effects: electrolyte imbalances, dehydration, death
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UNIT B Chapter 13: Urinary System
Section 13.3
Acid-Base Balance
The normal pH of blood is 7.4.
• Proteins (e.g., cellular enzymes) function best at this pH
• pH can be changed by the foods we eat and by metabolic
processes (e.g., CO2 from cellular respiration combines
with water to form carbonic acid)
• Several mechanisms in the body help maintain blood pH:
o Acid-base buffer systems
o Respiratory centre in the medulla oblongata
o Kidneys
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UNIT B Chapter 13: Urinary System
Section 13.3
Acid-Base Buffer Systems
The pH of the blood is maintained at 7.4 because it is buffered.
• Buffer: a chemical or combination of chemicals that take up
excess hydrogen ions (H+) or excess hydroxide ions (OH-)
• Buffers in blood: carbonic acid (H2CO3) and bicarbonate ions
(HCO3-)
o When the blood is too acidic (excess H+ added):
o When the blood is too basic (excess OH- added):
• These reactions prevent any significant change in blood pH
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UNIT B Chapter 13: Urinary System
Section 13.3
Respiratory Centre
The respiratory centre in the medulla oblongata increases
breathing rate if the H+ concentration of the blood rises.
• Increasing breathing rate rids the body of H+ because of
the following reaction in the pulmonary capillaries:
• When CO2 is exhaled, the reaction shifts to the right, and
H+ is reduced
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UNIT B Chapter 13: Urinary System
Section 13.3
The Kidneys
The kidneys can rid the body of a wide range of acidic and
basic substances to adjust pH.
• Reabsorb bicarbonate ions (HCO3-) and excrete H+ as
needed to maintain blood pH
• Ammonia (NH3) produced in the tubule cells also helps
to buffer and remove H+ in urine:
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UNIT B Chapter 13: Urinary System
Figure 13.10 Acid–base balance. In the kidneys, bicarbonate ions (HCO3–) are
reabsorbed and hydrogen ions (H+) are excreted as needed to maintain the pH of
the blood. Excess hydrogen ions are buffered, for example, by ammonia (NH3).
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Section 13.3
UNIT B Chapter 13: Urinary System
Section 13.3
Check Your Progress
1. Describe the three steps or processes that are required
for the excretion of a hypertonic urine.
2. Explain the relationship between aldosterone, the
juxtaglomerular apparatus, and renin.
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UNIT B Chapter 13: Urinary System
Section 13.3
Check Your Progress
3. Describe some medical uses of diuretics. Why are they
sometimes abused?
4. Explain how the kidneys control ion levels to maintain
blood pH and homeostasis.
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UNIT B Chapter 13: Urinary System
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Section 13.3
UNIT B Chapter 13: Urinary System
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Section 13.3