CHAPTER 51
LECTURE
SLIDES
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Osmotic Regulation and the
Urinary System
Chapter 51
Osmolarity and Osmotic
Balance
• Water in a multicellular body distributed
between
– Intracellular compartment
– Extracellular compartment
• Most vertebrates maintain homeostasis for
– Total solute concentration of their extracellular
fluids
– Concentration of specific inorganic ions
3
Osmolarity and Osmotic
Balance
• Important ions
– Sodium (Na+) is the major cation in
extracellular fluids
– Chloride (Cl–) is the major anion
– Divalent cations, calcium (Ca2+) and
magnesium (Mg2+), the monovalent cation K+,
as well as other ions, also have important
functions and are maintained at constant
levels
4
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External environment
Animal body
H2O
(Sweat)
Solutes
and H2O
Solutes
and H2O
Intracellular
compartment
Extracellular compartment
(including blood)
CO2 and H2O
O2
CO2 and H2O
Solutes
and H2O
Solutes
and H2O
O2
Food
and
H2O
Solutes
and H2O
Urine (excess H2O)
Solutes
and H2O
Waste
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Osmolarity and Osmotic
Balance
• Osmotic pressure
– Measure of a solution’s tendency to take in water by
osmosis
• Osmolarity
– Number of osmotically active moles of solute per liter
of solution
• Tonicity
– Measure of a solution’s ability to change the volume
of a cell by osmosis
– Solutions may be hypertonic, hypotonic, or isotonic
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Osmolarity and Osmotic
Balance
• Osmoconformers
– Organisms that are in osmotic equilibrium with their
environment
– Among the vertebrates, only the primitive hagfish are
strict osmoconformers
– Sharks and relatives (cartilaginous fish) are also
isotonic
• All other vertebrates are osmoregulators
– Maintain a relatively constant blood osmolarity
despite different concentrations in their environment
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Osmolarity and Osmotic
Balance
• Freshwater vertebrates
– Hypertonic to their environment
– Have adapted to prevent water from entering
their bodies, and to actively transport ions
back into their bodies
• Marine vertebrates
– Hypotonic to their environment
– Have adapted to retain water by drinking
seawater and eliminating the excess ions
through kidneys and gills
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Osmolarity and Osmotic
Balance
• Terrestrial vertebrates
– Higher concentration of water than
surrounding air
– Tend to lose water by evaporation from skin
and lungs
– Urinary/osmoregulatory systems have
evolved in these vertebrates that help them
retain water
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Osmoregulatory Organs
• In many animals, removal of water or salts
is coupled with removal of metabolic
wastes through the excretory system
• A variety of mechanisms have evolved to
accomplish this
– Single-celled protists and sponges use
contractile vacuoles
– Other multicellular animals have a system of
excretory tubules to expel fluid and wastes
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Osmoregulatory Organs
• Invertebrates
– Flatworms
• Use protonephridia which branch into bulblike
flame cells
• Open to the outside of the body, but not to the
inside
– Earthworms
• Use nephridia
• Open both to the inside and outside of the body
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Osmoregulatory Organs
• Insects
– Use Malpighian tubules
• Extensions of the digestive tract
– Waste molecules and K+ are secreted into
tubules by active transport
– Create an osmotic gradient that draws water
into the tubules by osmosis
– Most of the water and K+ is then
reabsorbed into the open circulatory system
through hindgut epithelium
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15
Osmoregulatory Organs
• Vertebrate kidneys
– Create a tubular fluid by filtering the blood
under pressure through the glomerulus
– Filtrate contains many small molecules, in
addition to water and waste products
– Most of these molecules and water are
reabsorbed into the blood
• Selective reabsorption provides great flexibility
– Waste products are eliminated from the body
in the form of urine
16
Evolution of the
Vertebrate Kidney
• Made up of thousands of repeating units –
nephrons
• Although the same basic design has been
retained in all vertebrate kidneys, a few
modifications have occurred
• All vertebrates can produce a urine that is
isotonic or hypotonic to blood
• Only birds and mammals can make a
hypertonic urine
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18
Evolution of the
Vertebrate Kidney
• Kidneys are thought to have evolved among the
freshwater teleosts, or bony fishes
• Body fluids are hypertonic with respect to
surrounding water, causing two problems
1. Water enters body from environment
• Fishes do not drink water and excrete large
amounts of dilute urine
2. Solutes tend to leave the body
• Reabsorb ions across nephrons
• Actively transport ions across gills into blood
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Evolution of the
Vertebrate Kidney
• In contrast, marine bony fishes have body
fluids that are hypotonic to seawater
• Water tends to leave their bodies by
osmosis across their gills
• Drink large amounts of seawater
• Eliminate ions through gill surfaces and
urine
• Excrete urine isotonic to body fluids
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Evolution of the Vertebrate Kidney
21
Evolution of the
Vertebrate Kidney
• Cartilaginous fish, including sharks and
rays, reabsorb urea from the nephron
tubules
• Maintain a blood urea concentration that is
100 times higher than that of mammals
• Makes blood isotonic to surrounding sea
• These fishes do not need to drink
seawater or remove large amounts of ions
from their bodies
22
Evolution of the
Vertebrate Kidney
• Amphibian kidney is identical to that of
freshwater fish
• Kidneys of reptiles are very diverse
– Marine reptiles drink seawater and excrete an
isotonic urine
• Eliminate excess salt via salt glands
– Terrestrial reptiles reabsorb much of the salt
and water in their nephron tubules
• Don’t excrete urine, but empty it into cloaca
23
Evolution of the
Vertebrate Kidney
• Mammals and birds are the only
vertebrates that can produce urine that is
hypertonic to body fluids
• Accomplished by the loop of Henle
• Birds have relatively few or no nephrons
with long loops, and so cannot produce
urine as concentrated as that of mammals
• Marine birds excrete excess salt from salt
glands near the eyes
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Evolution of the Vertebrate Kidney
25
Nitrogenous Wastes
• When amino acids and nucleic acids are
catabolized, they produce nitrogenous
wastes that must be eliminated from the
body
• First step is deamination
– Removal of the amino (―NH2) group
– Combined with H+ to form ammonia (NH3) in
the liver
• Toxic to cells, and thus it is only safe in dilute
concentrations
26
Nitrogenous Wastes
• Bony fishes and amphibian tadpoles eliminate
most of the ammonia by diffusion via gills
• Elasmobranchs, adult amphibians, and
mammals convert ammonia into urea, which is
soluble in water
• Birds, reptiles, and insects convert ammonia into
the water-insoluble uric acid
– Costs most energy, but saves most water
27
Nitrogenous Wastes
• Mammals also produce uric acid, but from
degradation of purines, not amino acids
• Most have an enzyme called uricase,
which convert uric acid into a more soluble
derivative called allantoin
• Humans lack this enzyme
• Excessive accumulation of uric acid in
joints causes gout
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29
The Mammalian Kidney
• Each kidney receives blood from a renal
artery
• Produces urine from this blood
• Urine drains from each kidney through a
ureter into a urinary bladder
• Urine is passed out of the body through
the urethra
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The Mammalian Kidney
• Within the kidney, the mouth of the ureter
flares open to form the renal pelvis
• Receives urine from the renal tissue
• Divided into an outer renal cortex and
inner renal medulla
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The Mammalian Kidney
32
The Mammalian Kidney
• The kidney has three basic functions
– Filtration
• Fluid in the blood is filtered out of the glomerulus
into the tubule system
– Reabsorption
• Selective movement of solutes out of the filtrate
back into the blood via peritubular capillaries
– Secretion
• Movement of substances from the blood into the
extracellular fluid, then into the filtrate in the tubular
system
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34
The Mammalian Kidney
• Each kidney is made up of about 1 million
functioning nephrons
– Juxtamedullary nephrons have long loops that dip
deeply into the medulla
– Cortical nephrons have shorter loops
• Blood is carried by an afferent arteriole to the
glomerulus
• Blood is filtered as it is forced through porous
capillary walls
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The Mammalian Kidney
• Blood components that are not filtered
drain into an efferent arteriole, which
empties into peritubular capillaries
– Vasa recta in juxtamedullary nephrons
• Glomerular filtrate enters the first region of
the nephron tubules – Bowman’s capsule
• Goes into the proximal convoluted tubule
• Then moves down the medulla and back
up into cortex in the loop of Henle
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The Mammalian Kidney
• After leaving the loop, the fluid is delivered
to a distal convoluted tubule in the cortex
• Drains into a collecting duct
• Merges with other collecting ducts to
empty its contents, now called urine, into
the renal pelvis
37
38
Reabsorption and Secretion
• Approximately 2000 L of blood passes
through the kidneys each day
• 180 L of water leaves the blood and enters
the glomerular filtrate
• Most of the water and dissolved solutes
that enter the glomerular filtrate must be
returned to the blood by reabsorption
• Water is reabsorbed by the proximal
convoluted tubule, descending loop of
Henle, and collecting duct
39
Reabsorption and Secretion
• Reabsorption of glucose and amino acids
is driven by active transport and
secondary active transport
– Maximum rate of transport
– Glucose remains in the urine of untreated
diabetes mellitus patients
• Secretion of waste products involves
transport across capillary membranes and
kidney tubules into the filtrate
– Penicillin must be administered several times
40
a day
Excretion
• Major function of the kidney is elimination
of a variety of potentially harmful
substances that animals eat and drink
• In addition, urine contains nitrogenous
wastes, and may contain excess K+, H+,
and other ions that are removed from
blood
• Kidneys are critically involved in
maintaining acid–base balance of blood
41
Transport in the Nephron
• Proximal convoluted tubule
– Reabsorbs virtually all nutrient molecules in
the filtrate, and two-thirds of the NaCl and
water
– Because NaCl and water are removed
from the filtrate in proportionate
amounts, the filtrate that remains in the
tubule is still isotonic to the blood
plasma
42
Transport in the Nephron
• Loop of Henle
– Creates a gradient of increasing osmolarity
from the cortex to the medulla
– Actively transports Na+, and Cl– follows from
the ascending loop
• Creates an osmotic gradient
– Allows reabsorption of water from descending
loop and collecting duct
– Two limbs of the loop form a countercurrent
multiplier system
• Creates a hypertonic renal medulla
43
44
Counter Current Multiplier system – Loop of Henle
45
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Transport in the Nephron
• Distal convoluted tubule and collecting duct
– Filtrate that enters is hypotonic
– Hypertonic interstitial fluid of the renal medulla
pulls water out of the collecting duct and into
the surrounding blood vessels
• Permeability controlled by antidiuretic hormone
(ADH)
– Kidneys also regulate electrolyte balance in
the blood by reabsorption and secretion
• K+, H+, and HCO3–
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48
Hormones Control
Osmoregulation
• Kidneys maintain relatively constant levels
of blood volume, pressure, and osmolarity
• Also regulate the plasma K+ and Na+
concentrations and blood pH within narrow
limits
• These homeostatic functions of kidneys
are coordinated primarily by hormones
49
Hormones Control
Osmoregulation
• Antidiuretic hormone (ADH)
– Produced by the hypothalamus and secreted
by the posterior pituitary gland
– Stimulated by an increase in the osmolarity of
blood
– Causes walls of distal tubule and collecting
ducts to become more permeable to water
• Aquaporins
– More ADH increases reabsorption of water
• Makes a more concentrated urine
50
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Hormones Control
Osmoregulation
• Aldosterone
– Secreted by the adrenal cortex
– Stimulated by low levels of Na+ in the blood
– Causes distal convoluted tubule and
collecting ducts to reabsorb Na+
– Reabsorption of Cl– and water follows
– Low levels of Na+ in the blood are
accompanied by a decrease in blood volume
• Renin-angiotensin-aldosterone system is activated
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Hormones Control
Osmoregulation
• Atrial natriuretic hormone (ANH)
– Opposes the action of aldosterone in
promoting salt and water retention
– Secreted by the right atrium of the heart in
response to an increased blood volume
– Promotes the excretion of salt and water in
the urine and lowering blood volume
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