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Chapter 35
The Urinary System
Lecture Outlines by Gregory Ahearn,
University of North Florida
Copyright © 2011 Pearson Education Inc.
Chapter 35 At a Glance
 35.1 What Are the Basic Functions of Urinary
Systems?
 35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 35.3 What Are the Functions of the Human Urinary
System?
 35.4 What Are the Structures of the Human Urinary
System?
 35.5 How Is Urine Formed and Concentrated?
 35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
Biology: Life on Earth, 9e
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35.1 What Are the Basic Functions of Urinary
Systems?
 Urinary systems help maintain homeostasis—
the relatively constant internal environment
required to preserve health and life
– Urinary systems regulate the composition of the
blood and extracellular fluid, the watery
substance that bathes all cells
– They also help control the concentration, or
osmolarity, of dissolved substances in cells and
in their extracellular environment
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35.1 What Are the Basic Functions of Urinary
Systems?
 Urinary systems help maintain homeostasis—
the relatively constant internal environment
required to preserve health and life (continued)
– A second function of urinary systems is
excretion, or the removal of unwanted
substances from the body
–Urinary systems produce urine, which
contains the waste products of cellular
metabolism
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35.1 What Are the Basic Functions of Urinary
Systems?
 Urinary system functions are performed through
three basic processes
– Blood or extracellular fluid is filtered, removing
water and small dissolved substances
– Nutrients are selectively reabsorbed back into
the filtered fluid
– Excess water, excess nutrients, and dissolved
wastes are excreted from the body in urine
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35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 Protonephridia filter extracellular fluid in flatworms
 The earliest excretory system probably served to
maintain water balance, which is the primary function of
the simple excretory system of flatworms
– This early excretory system consists of protonephridia,
which are tubules that branch throughout extracellular
fluid that surrounds the flatworm’s tissues
– This excretory system serves to collect excess water
from the extracellular fluid using ciliated “flame cells,” and
forces the fluid out of the animal through excretory pores
– The large body surface of flatworms also serves as an
excretory structure through which most cellular waste
diffuse out
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Flatworms Use Protonephridia
excretory pore
eye spot
tubule
extracellular
fluid
cilia
excretory
pore
nucleus
flame
cell
(a) Flatworms use protonephridia
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Fig. 35-1a
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35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 Malphigian tubules filter the hemolymph of
insects
– Insects have an open circulatory system where
hemolymph (a fluid that serves as both blood
and extracellular fluid) fills the hemocoel (the
body cavity) and bathes the internal tissues and
organs directly
– Insect excretory systems consist of Malpighian
tubules, small tubes that extend outward from
the intestine and end blindly within the
hemolymph
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35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 Malphigian tubules filter the hemolymph of
insects (continued)
– Wastes and nutrients move from the hemolymph
into the tubules by diffusion and active transport,
and water follows by osmosis
– Urine is conducted into the intestine, where
important solutes are secreted into the
hemolymph by active transport
– Insects produce very concentrated urine, which
is excreted along with feces
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Insects Use Malphigian Tubules
abdomen
Malpighian tubules
intestine
hemocoel
(filled with
hemolymph)
rectum
cellular and
digestive wastes
(b) Insects use Malpighian tubules
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Fig. 35-1b
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35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 Nephridia filter extracellular fluid in earthworms
– In earthworms, mollusks, and several other
invertebrates, excretion is performed by tubular
structures called nephridia
– In the earthworm, the body cavity (the coelom) is
filled with extracellular fluid into which wastes
and nutrients from the blood diffuse
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35.2 What Are Some Examples of Invertebrate
Excretory Systems?
 Nephridia filter extracellular fluid in earthworms
(continued)
– Each nephridium begins with a funnel-like opening, the
nephrostome, ringed with cilia that direct extracellular
fluid into a narrow, twisted tubule surrounded by
capillaries
– As the fluid traverses the tubule, salts and nutrients are
reabsorbed back into the capillary blood, leaving the
wastes and water behind
– The resulting urine is then excreted through an opening
in the body called a nephridiopore
– Each segment in an earthworm’s body contains a pair of
nephridia
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Earthworms Use Nephridia
coelom (filled with
extracellular fluid)
nephridium
capillary
bed
nephrostome
nephridiopore
(c) Earthworms use nephridia
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Fig. 35-1c
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35.3 What Are the Functions of Vertebrate Urinary
Systems?
 Kidneys are the organs of the vertebrate
urinary system where the blood is filtered and
urine is produced
– Because vertebrates live in such a wide variety
of habitats, vertebrate kidneys face radically
different challenges in maintaining constant
conditions within their bodies
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35.3 What Are the Functions of Vertebrate Urinary
Systems?
 The kidneys of humans and other mammals perform
many homeostatic functions
– The mammalian urinary system consists of the kidneys,
ureters, bladder, and urethra
– These organs filter the blood, collecting and then
excreting the dissolved waste products in urine
– During filtration, water and dissolved molecules are
forced out of the blood
– The kidneys then return to the blood nearly all of the
water and nutrients required by the body
– The urine retains wastes, which are expelled from the
body
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35.3 What Are the Functions of Vertebrate Urinary
Systems?
 The mammalian urinary system helps maintain
homeostasis in several ways, by:
– Regulating blood levels of ions such as sodium,
potassium, chloride, and calcium
– Maintaining proper pH of the blood by regulating
hydrogen and bicarbonate ion concentrations
– Regulating the water content of the blood
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35.3 What Are the Functions of Vertebrate Urinary
Systems?
 The mammalian urinary system helps maintain
homeostasis in several ways, by: (continued)
– Retaining important nutrients such as glucose
and amino acids in the blood
– Eliminating cellular waste products such as urea
– Secreting substances that help regulate blood
pressure and blood oxygen levels
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35.3 What Are the Functions of Vertebrate Urinary
Systems?
 Urea is a waste product of protein digestion
– An important function of most urinary systems is
to eliminate nitrogenous (nitrogen-containing)
wastes that are formed when cells break down
amino acids
– Nitrogenous wastes from cells enter the blood as
ammonia (NH3), which is toxic
– The livers of humans and other mammals
convert ammonia into urea, which is less toxic
– Urea is filtered from the blood by the kidneys and
then excreted in the urine
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Urea Formation and Excretion
1 Proteins in food are digested
4 The liver converts ammonia
to urea, which is less toxic
urea
2 Amino acids are carried in
the blood to body cells
amino acid
3 The cells convert the
amino groups (-NH2) to
ammonia, which is carried
in the blood to the liver
ammonia
NH3
5 Urea is carried in the blood
to the kidneys
6 In kidney nephrons, urea
is filtered into the urine
Fig. 35-2
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35.4 What Are the Structures of the Human
Urinary System?
 The urinary system consists of the kidneys,
ureters, bladder, and urethra
– Kidneys are paired organs located on either side
of the spinal column, just above the waist
– Blood enters each kidney through the renal
artery, and after the blood has been filtered, it
exits through the renal vein
– Urine leaves each kidney through a narrow,
muscular tube called the ureter
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35.4 What Are the Structures of the Human
Urinary System?
 The urinary system consists of the kidneys,
ureters, bladder, and urethra (continued)
– Rhythmic contractions of the ureter transports
urine to the bladder, a hollow, muscular
chamber that collects and stores blood
– The bladder wall is lined with smooth muscle and
is capable of considerable expansion,
accommodating up to a pint of urine
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35.4 What Are the Structures of the Human
Urinary System?
 The urinary system consists of the kidneys, ureters,
bladder, and urethra (continued)
– Urine is contained within the bladder by two sphincter
muscles
– The internal sphincter, located where the bladder joins
the urethra, opens automatically during the reflexive
contractions of the smooth muscle
– The external sphincter, located slightly below the
internal sphincter, is under voluntary control, allowing
the brain to suppress urination unless the bladder
becomes overly full
– When open, the sphincters allow urine to flow into the
urethra, a single narrow tube that conducts urine
outside the body
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Author Animation: Overview of the Kidney
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The Human Urinary System
left renal artery
left kidney
left renal vein
aorta
left ureter
vena cava
urinary
bladder
urethra
(in penis)
Fig. 35-3
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35.4 What Are the Structures of the Human
Urinary System?
 The structure of the kidney supports its function
of producing urine
– Each kidney contains a solid outer layer
consisting of the renal cortex, which overlies an
inner layer called the renal medulla
– The renal medulla surrounds a branched, funnellike chamber called the renal pelvis, which
collects urine and funnels it into the ureter
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Cross-Section of a Kidney
renal pelvis
(cut away to
show the
path of urine)
renal
artery
renal
cortex
renal
medulla
renal
pelvis
renal
vein
ureter
collecting
duct
nephron
enlargement of a
single nephron and
collecting duct
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renal
medulla
renal
cortex
urine
to the
bladder
Fig. 35-4
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35.4 What Are the Structures of the Human
Urinary System?
 The renal cortex of each kidney is made up of
more than 1 million microscopic filters called
nephrons
– Each nephron has two major parts
–The glomerulus, which is a dense knot of
capillaries where fluid is filtered out of the
blood through the porous capillary walls
–A long, twisted tubule, where urine formation
occurs
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35.4 What Are the Structures of the Human
Urinary System?
 The tubule has four major sections
– The tubule begins with Bowman’s capsule, a
cup-like chamber that surrounds the glomerulus
and receives fluid filtered out of the blood from
the glomerular capillaries
– The remaining sections of the tubule return water
and nutrients to the blood while retaining wastes
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35.4 What Are the Structures of the Human
Urinary System?
 The tubule has four major sections (continued)
– From Bowman’s capsule, the fluid is conducted to the
proximal tubule
– The loop of Henle carries the filtered fluid from the
cortex deep into the medulla and back to the cortex
– The distal tubule is in the cortex, and collects the filtrate
from the loop of Henle and passes it on to the collecting
ducts
– The collecting duct is not part of the nephron, but
collects fluid from many nephrons and deposits it in the
renal pelvis
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Author Animation: Parts of the Nephron
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An Individual Nephron and Its Blood Supply
collecting duct
distal tubule
proximal tubule
Bowman’s
capsule
glomerulus
arterioles
venule
branch of
the renal
branch of the artery
renal vein
loop of Henle
capillaries
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Fig. 35-5
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35.4 What Are the Structures of the Human
Urinary System?
 The kidney’s blood supply allows it to fine-tune blood
composition
– To support their role in maintaining homeostasis, the
kidneys have an enormous blood supply, receiving more
than one quart of blood every minute
– Blood flows to each kidney from the renal artery, which
branches into arterioles that each supply single nephrons
with blood for filtration
– The arterioles branch into capillaries that form the
glomerulus of each nephron
– The capillaries empty into an outgoing arteriole that
branches into more capillaries that surround the tubule
– The capillaries carry blood into a venule that takes the
blood to the renal vein and then the inferior vena cava
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35.5 How Is Urine Formed and Concentrated?
 Urine is produced in the nephrons of the
kidneys in three stages
– Filtration, during which water and most small
dissolved molecules are filtered out of the blood
– Tubular reabsorption, the process by which
water and necessary nutrients are restored to
the blood
– Tubular secretion, during which wastes and
excess ions that still remain in the blood are
secreted into the urine
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Urine Formation and Concentration
1 Filtration: Water, nutrients,
and wastes are filtered from the
glomerular capillaries into the
Bowman’s capsule of the nephron
2 Tubular reabsorption: In the
proximal tubule, most water and nutrients
are reabsorbed into the blood
proximal
tubule
blood
leaving the
glomerulus
3 Tubular secretion:
Additional wastes are
actively transported into
the proximal and distal
tubules from the blood
collecting
duct
distal tubule
blood entering
the glomerulus
Bowman’s
capsule
loop of
Henle
4 Concentration: The loop of
Henle produces a salt concentration
gradient in the extracellular fluid;
in the collecting duct, urine may
become more concentrated than the
blood as water leaves by osmosis
Fig. 35-6
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35.5 How Is Urine Formed and Concentrated?
 As urine is formed, essentially all small organic
nutrients, including amino acids and glucose,
are filtered out of and then returned to the blood
– Large quantities of water and many ions are also
filtered out, but their return rate is continuously
adjusted to meet the body’s changing needs
–These ions include sodium (Na+), chloride (Cl–
), potassium (K+), calcium (Ca++), hydrogen
(H+), and bicarbonate
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35.5 How Is Urine Formed and Concentrated?
 Urine is formed in the glomerulus and tubule of each
nephron
– Filtration, the first step in urine formation, occurs when
water carrying small dissolved molecules and ions is
forced through the walls of the capillaries that form the
glomerulus
– Blood cells and large proteins are too large to leave
the capillaries, and so remain behind in the blood
– The fluid filtered out of the glomerular capillaries, called
filtrate, is collected in Bowman’s capsule and then
continues through the tubule
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35.5 How Is Urine Formed and Concentrated?
 Urine is formed in the glomerulus and tubule of each
nephron (continued)
– Tubular reabsorption occurs primarily in the proximal
tubule, although water and other nutrients are also
reabsorbed in other tubule areas
– It returns nearly all organic nutrients (such as glucose,
amino acids, and needed vitamins) and most of the
ions (Na+, Cl–, K+, Ca++, H+ and HCO3–) to the blood
– It restores most of the water that was filtered from the
blood, as water follows the nutrients and ions by
osmosis through aquaporins, proteins that form water
pores
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35.5 How Is Urine Formed and Concentrated?
 Urine is formed in the glomerulus and tubule of each
nephron (continued)
– During tubular secretion, remaining wastes and excess
ions move from the blood into the proximal and distal
tubules
– These wastes include excess K+ and H+, small
quantities of ammonia, many drugs, food additives,
pesticides, and toxic substances, such as nicotine
– Tubular secretion, which occurs primarily by active
transport, takes place in both the proximal and distal
tubules
– When the filtrate leaves the distal tubule, it has become
urine
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35.5 How is Urine Formed and Concentrated?
 The loop of Henle creates an extracellular
concentration gradient in the renal medulla
– The function of the loop of Henle is twofold
–Some water and salt is reabsorbed from the
filtrate as it passes through the loop
–Most importantly, it creates a high salt and
urea concentration in the extracellular fluid
within the medulla
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35.5 How Is Urine Formed and Concentrated?
 The loop of Henle creates an extracellular
concentration gradient in the renal medulla (continued)
– To understand why a high salt concentration is important,
we must start with an important role of the human kidney:
water regulation
– The kidneys help to maintain appropriate water
content in body tissues by producing dilute, watery
urine when fluid intake is high, and concentrated urine
when fluid intake is low
– Water can be conserved by allowing it to move out of
the collecting duct by osmosis down its concentration
gradient
– The more concentrated the extracellular fluid, the
more water that can leave the urine as it moves
through the collecting duct
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35.5 How Is Urine Formed and Concentrated?
 The loop of Henle creates an extracellular
concentration gradient in the renal medulla (continued)
– The loop of Henle produces and maintains a high salt
concentration gradient in the extracellular fluid in the
medulla by transporting salt out of the filtrate
– The salt and urea gradient causes an osmotic gradient
between the filtrate and the surrounding extracellular fluid
– The most concentrated fluid surrounds the bottom of the
loop
– The collecting duct passes through this gradient as it
conducts urine from the distal tubule in the renal cortex
into the renal pelvis
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35.5 How Is Urine Formed and Concentrated?
 The loop of Henle creates an extracellular
concentration gradient in the renal medulla (continued)
– As the filtrate descends into the loop of Henle and
collecting duct, the following occur
– It is exposed to the osmotic gradient surrounding the
nephron
– Water leaves the filtrate by osmosis and enters the
surrounding capillaries
– Filtrate becomes urine when it enters the collecting duct,
and can be more than four times as concentrated as the
blood
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Details of Urine Formation
TUBULAR REABSORPTION
& TUBULAR SECRETION
FILTRATION
HCO3– H+
Ca2+ NH3
Cl– some
Na+
nutrients H2O K+ drugs
URINE
CONCENTRATION
H+
K+
NaCl some H2O*
Ca2+ drugs
1
H2O*
7
6 distal
tubule
2 proximal
tubule
Bowman’s
capsule
renal cortex
NaCI
H2 O
renal medulla
5
NaCI
H2 O
NaCI
3
H2 O
4
(extracellular fluid)
NaCI
urea
8
H2O*
H2 O
osmosis
active transport
loop of Henle
diffusion
collecting duct
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Fig. E35-2
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 The kidneys regulate the osmolarity of the blood
– One important function of the kidney is to
regulate the water content of the blood
–Human kidneys filter out about half a cup of
fluid from the blood each minute, fine-tuning
the composition of the blood and helping to
maintain homeostasis
–If the kidneys did not reabsorb this water, the
rate of filtration would require that we drink
nearly 50 gallons of water a day
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 The kidneys regulate the osmolarity of the blood
(continued)
– Consequently, the urinary system needs to restore nearly
all of the water that is initially filtered out of the glomeruli
– The amount of reabsorption is precisely regulated, and
the ability of kidneys to reabsorb water is under the
influence of antidiuretic hormone (ADH)
– ADH is secreted by the posterior pituitary gland and is
carried in the bloodstream
– It stimulates cells of the distal tubule and collecting
ducts to insert more aquaporin proteins into their
membranes
– The abundance of aquaporin membranes determines
the permeability of the membranes to water
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 The kidneys regulate the osmolarity of the blood
(continued)
– Under normal conditions, some ADH is always
present in the blood
– Within the hypothalamus, receptors monitor
blood osmolarity, which increases when water is
lost
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Author Animation: Urine Formation
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 The kidneys regulate the osmolarity of the blood
(continued)
– For example, when water is lost during dehydration:
– If blood osmolarity exceeds an optimal level, the
hypothalamus stimulates the pituitary gland to release
ADH into the bloodstream
– In response to ADH, cells of the distal tubule and
collecting duct insert more aquaporins into their
membranes, increasing their permeability to water
– The more concentrated extracellular fluid draws water
out by osmosis, restoring water to the blood through
nearby capillaries
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Dehydration Stimulates ADH Release and Water
Retention
1 Heat causes water loss and
dehydration through sweating
2 Receptors in the hypothalamus
detect the increased blood
osmolarity and signal the pituitary
gland
3 The pituitary gland
releases ADH into the
bloodstream
4 ADH increases the
permeability of the distal tubule
and the collecting duct, allowing
more water to be reabsorbed
into the blood
5 Water is retained in the body
and concentrated urine is
produced
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Fig. 35-7
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Kidneys release substances that help regulate
blood pressure and oxygen levels
– When blood pressure falls, the kidneys release
the enzyme renin into the bloodstream
– Renin catalyzes the formation of the hormone
angiotensin in the blood
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Angiotensin helps combat low blood pressure in
three major ways
– It stimulates the proximal tubules of the
nephrons to reabsorb more Na+ into the blood,
causing water to follow by osmosis
– It stimulates ADH release, causing more water to
be reclaimed from the distal tubule and collecting
duct
– It causes arterioles throughout the body to
constrict, which directly increases blood pressure
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Kidneys release substances that help regulate
blood pressure and oxygen levels (continued)
– When blood oxygen levels are low, the kidneys
release the hormone erythropoietin
–This hormone stimulates the bone marrow to
make more red blood cells
–The higher number of red blood cells
increases the oxygen carrying capacity of the
blood
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Vertebrate kidneys are adapted to diverse
environments
– Mammals have structurally different nephrons,
depending upon the availability of water in their natural
habitat
– Mammals adapted to dry climates generally have long
loops of Henle
– Longer loops of Henle allow for allow a higher
concentration of salt to be produced in the extracellular
fluid of the medulla, allowing more water to be reclaimed
from the collecting duct
– An example of a mammal with very long-looped
nephrons is the desert kangaroo rat
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A Well-Adapted Desert Dweller
Fig. 35-8
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Vertebrate kidneys are adapted to diverse
environments
– In contrast, mammals adapted to habitats with an
abundance of fresh water typically have short
loops of Henle
–For example, beavers, which live along
streams, can only concentrate their urine to
about twice their blood osmolarity
– Humans have a mixture of long- and shortlooped nephrons, and can concentrate urine to
about four times the osmolarity of blood
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Freshwater and saltwater environments pose special
challenges for water regulation
– Animals have evolved homeostatic mechanisms,
including kidney adaptations, to maintain water and salt
within their bodies, a process called osmoregulation
– For example, freshwater fish live in a hypotonic
environment
– Water continuously leaks into their bodies by
osmosis
– Salts diffuse out
– Freshwater fish acquire salt from their food and
through their gills but never drink
– Their kidneys retain salt and excrete large quantities
of extremely dilute urine
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Osmoregulation in Fish
fresh water
water
salt
Water moves in by
osmosis; salt diffuses out
Salt is pumped in
by active transport
Salt and some
water enters
in food
(a) Freshwater fish
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The kidneys conserve salt
and excrete large amounts
of dilute urine
Fig. 35-9a
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35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Freshwater and saltwater environments pose special
challenges for water regulation (continued)
– Saltwater fish live in a hypertonic environment; seawater
has a solute concentration of two to three times that of
their body fluids
– Water is constantly leaving their tissues by osmosis,
and salt is constantly diffusing in and being taken in
with food
– To compensate for these effects, saltwater fish drink to
restore their lost water, and excess salt they take in is
excreted by active transport through their gills
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Osmoregulation in Fish
salt water
Salt and water enter
in food and by drinking
seawater
Water moves out by
osmosis; salt diffuses in
Salt is pumped out
by active transport
water
salt
(b) Saltwater fish
Biology: Life on Earth, 9e
Some salt is excreted in
small quantities of urine
Fig. 35-9b
Copyright © 2011 Pearson Education Inc.
35.6 How Do Vertebrate Kidneys Help Maintain
Homeostasis?
 Freshwater and saltwater environments pose
special challenges for water regulation
(continued)
– Fish nephrons completely lack loops of Henle,
and so fish cannot produce concentrated urine
– To conserve water, the kidneys of most saltwater
fish excrete very small quantities of urine
containing salts not eliminated by their gills
Biology: Life on Earth, 9e
Copyright © 2011 Pearson Education Inc.