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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 13.1
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
13.1 The Urinary System
The urinary system is involved in
excretion, which is the removal of
metabolic wastes from the body.
The urinary system produces urine
and conducts it outside the body.
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Section 13.1
UNIT B Chapter 13: Urinary System
Section 13.1
Functions of the Urinary System
Excretion of Metabolic Wastes
• The kidneys excrete metabolic wastes (mostly nitrogenous
wastes: urea, ammonium, creatinine, uric acid)
o Urea is formed when ammonia released during amino
acid breakdown combines with CO2
o Some ammonia (NH3) is excreted as ammonium ion
(NH4+)
o Creatinine is a breakdown product of creatine phosphate,
a high-energy phosphate reserve molecule
o Uric acid is produced from the breakdown of nucleotides
− Gout can occur if too much uric acid in the blood has
crystallized and moved into the joints
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UNIT B Chapter 13: Urinary System
Section 13.1
Osmoregulation
• The kidneys are involved in osmoregulation (maintenance
of the balance of water and salt in the blood)
o Salts can cause osmosis (diffusion of water) into the
blood, causing blood volume and blood pressure to
increase
• Kidneys also maintain levels of other ions, such as potassium
(K+), bicarbonate (HCO3-), and calcium (Ca2+), in the blood
Regulation of Acid-Base Balance
• The kidneys help regulate the acid-base balance of the blood
o Monitor and keep blood pH at 7.4 by excreting hydrogen
ions (H+) and reabsorbing bicarbonate ions (HCO3-)
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UNIT B Chapter 13: Urinary System
Section 13.1
Secretion of Hormones
• The kidneys help the endocrine system in hormone
secretion
o Secrete renin, an enzyme that stimulates the adrenal
cortex to secrete the hormone aldosterone, which
promotes the absorption of sodium ions (Na+) by the
kidneys
o Secrete the hormone erythropoietin (EPO) to simulate
red blood cell production when oxygen demand
increases
o Help activate Vitamin D, a hormone-like molecule that
promotes calcium (Ca2+) absorption from the digestive
tract
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UNIT B Chapter 13: Urinary System
Section 13.1
Organs of the Urinary System
The urinary system consists of the kidneys, ureters, urinary bladder,
and urethra.
Figure 13.1 The
urinary system. Urine
is found only within the
kidneys, the ureters, the
urinary bladder, and the
urethra. The kidneys are
important organs of
homeostasis because
they excrete metabolic
wastes and adjust both
the water–salt and acid–
base balance of the
blood.
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UNIT B Chapter 13: Urinary System
Kidneys
• Paired, bean-shaped organs
that regulate acid-base
balance and water-salt balance
of blood
• Each kidney is covered by a
tough connective tissue layer
called a renal capsule
• Each kidney has a depression
(called a hilium) on the
concave side where a renal
artery enters and a renal vein
and ureter exit
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Section 13.1
UNIT B Chapter 13: Urinary System
Ureters
• Small muscular tubes that
transport urine from the
kidneys to the bladder
• Wall of each ureter has
three layers: inner mucosa,
smooth muscle layer, outer
fibrous connective tissue
• Peristaltic contractions in
the ureters cause urine to
enter the bladder
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Section 13.1
UNIT B Chapter 13: Urinary System
Urinary Bladder
• Stores urine until it is
expelled from the body
• Has three openings: two for
the ureters, and one for the
urethra, which drains the
bladder
• Has two sphincters that lie
close to where the urethra
exits the bladder
o External sphincter is
under voluntary control
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Section 13.1
UNIT B Chapter 13: Urinary System
Urethra
• Small tube opening that
extends from the bladder to
an external opening
• Removes urine from the
body
• Males: 20 cm long; urethra
carries urine and semen
• Females: 4 cm long; urethra
carries urine (not
connected to reproductive
system)
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Section 13.1
UNIT B Chapter 13: Urinary System
Section 13.1
Urination
• When the bladder fills with
about 250 mL of urine,
stretch receptors send nerve
impulses to the spinal cord
• Motor nerve impulses from
the spinal cord cause the
bladder to contract and
sphincters to relax, allowing
urination to occur
• The brain controls this reflex
in older children and adults,
allowing urination to be
delayed
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Figure 13.2 Urination. As the bladder fills with
urine, sensory impulses go to the spinal cord and
then to the brain. When urination occurs, motor
nerve impulses cause the bladder to contract and
internal and external sphincters to relax.
UNIT B Chapter 13: Urinary System
Check Your Progress
1. Define excretion.
2. Describe the functions of the urinary system.
3. Describe a function of the kidneys.
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Section 13.1
UNIT B Chapter 13: Urinary System
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Section 13.1
UNIT B Chapter 13: Urinary System
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Section 13.1