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Lauralee Sherwood
Hillar Klandorf
Paul Yancey
Chapter 12
Excretory Systems
Sections 12.1-12.4
Kip McGilliard • Eastern Illinois University
12.1 Evolution of Excretory Systems
 Functions of the excretory systems
• Maintenance of proper internal levels of
inorganic solutes
• Maintenance of proper plasma water volume
• Removal of waste
• Maintenance of osmotic balance
12.1 Evolution of Excretory Systems
 Evolution of basic excretory organs
• Simple aquatic animals depend on diffusion
and membrane transporters
• More complex aquatic animals evolved
specialized excretory tissues with
transport epithelia
• Larger aquatic and all terrestrial animals
evolved specialized tubules lined with
transport epithelia
12.1 Evolution of Excretory Systems
 Nitrogenous wastes
• Result from the metabolism of proteins and nucleic
acids
• Choice of primary nitrogen waste correlates with
water availability
• Ammonia
• Most aquatic animals that breathe water (ammonotely)
• Diluted to nontoxic concentrations
• Urea
• Most terrestrial animals (ureotely)
• More expensive metabolically, less toxic than ammonia
• Uric acid
• Insects, reptiles and birds (uricotely)
• Most expensive metabolically, highly insoluble
12.1 Evolution of Excretory Systems
Cellular proteins
Hydrolysis
Ingested proteins
Amino acids
Growth +
maintenance
Retention (osmolyte)
Glutamine
Catabolism
of excess
HCO3–
HCO3–
Ammonia
Retention
(osmolyte)
Urea
Uric acid
Retention
(buoyancy)
Excreted
Excreted
Excreted
Figure 12-1 p559
Hydrolysis
Cellular proteins
Ingested proteins
Amino acids
Growth +
maintenance
Retention
Glutamine
Catabolism
of excess
HCO3–
HCO3–
Ammonia
Retention
Urea
Retention
Uric acid
Excreted
Excreted
Excreted
Stepped Art
Fig. 12-1, p.559
12.1 Evolution of Excretory Systems
 Transporting salt and water across epithelial layer
• Na+/K+ ATPase pump on basolateral membrane
lowers Na+ concentration inside the cell by pumping
Na+ into the ECF
• Na+ enters cell from apical side by diffusion through
ENaC channel
• Cl– is attracted out of the cell through CIC or CFTR
channels by the charge gradient produced by Na+
efflux
• Increased extracellular solute concentration attracts
water across basolateral membrane
12.1 Evolution of Excretory Systems
Lumen
Tubular cell
Na+
channel or
cotransport
carrier
CFTR
or ClC
channel
Interstitial fluid
Capillary
Basolateral
Na+ /K + pump
1
Lateral space
2
Figure 12-3a p562
Lumen
Proximal tubular cell
Interstitial fluid
Capillary
Osmosis
AQP-1 water
channel
3
AQP-1
water
channel
Hydrostatic
pressure
Osmosis
1
4
2
Figure 12-3b p562
ANIMATION: Tubular reabsorption
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12.2 Renal Excretory Organs: Overview
 Basic processes taking place in renal tubules
• Filtration
• Water and small solutes pass through a barrier while
cells and large molecules remain behind
• Secretion
• Transport epithelia move specific solutes into the tubule
lumen for excretion
• Reabsorption
• Transport epithelia move specific solutes and water
back into the body from the lumen
• Osmoconcentration
• Water is removed from the lumen, leaving solutes
behind, producing a more concentrated excretory fluid
12.2 Renal Excretory Organs: Overview
Secretion Reabsorption
Blood flow
Filtrate
Filtration
Osmoconcentration
Urine
(c) The four basic renal processes (shown for a vertebrate): filtration,
secretion, reabsorption, and osmoconcentration
Figure 12-3c p563
12.2 Renal Excretory Organs: Overview
 Techniques for studying renal excretory systems
• Isolated perfused tubule
• Perfusion studies
• Patch clamp
• Microelectrode
• Plasma clearance
12.2 Renal Excretory Organs: Overview
 Types of renal organs
• Protonephridia
• Blind end ducts project into body cavity
• Ultrafiltration driven by cilia moving fluid outward
• Mesonephridia and metanephridia
•
•
•
•
Ultrafiltration driven by fluid pressure
Requires a circulatory system
Mesonephric tubules in aquatic vertebrates
Metanephric tubules in terrestrial animals
• Malpighian tubules
• Filtration driven by active secretion of ions
• Arthropods
12.2 Renal Excretory Organs: Overview
Flame
cell
Protonephridial
network
Conducting
tubule
Nucleus
Cilia
Fenestration
Nephridiopores
Nephridiopore
Figure 12-5a p566
Urine
Excretory pore Bladder
Eye
Antennal
gland
Antenna
End-sac
Labyrinth—
site of H2O
reabsorption
Fluid flow and
filtration as
fluid flows
across the
end-sac
Nephridial canal—
site of ion
reabsorption
Figure 12-5b p566
12.3 Insect Malpighian Tubules
 Structure of Malpighian tubules
• Blind end epithelial ducts
• One cell layer thick
• Project into hemolymph from the hindgut
 Filtration by ion secretion
• Secrete K+ (and often Na+) into tubule lumen using a
proton pump (V-ATPase, secondary active transport)
• Electrical gradient attracts Cl– into lumen through
CIC channels
• Water from hemolymph moves into tubular fluid by
osmosis through aquaporins
12.3 Insect Malpighian Tubules
Solute secretion
into the Malpighian
tubule
Malpighian
tubule
Midgut
H2O follows
by osmosis
+
CI– K
H2O
H2O
reabsorption
Urine—
semi-solid
Head
Solute
reabsorption
Thorax
Figure 12-6a p567
Perinephric
Hemolymph 750 membrane
Leptophragmata
KCI
300
300
From
midgut
KCI
Malpighian tubule
4,000
Perinephric space
2,500
H2O
800
KCI
H2O
Rectal lumen
<2,500
To
anus
Figure 12-6b p567
12.3 Insect Malpighian Tubules
 Secretion and reabsorption in Malpighian tubules
• Osmotic movement of water creates bulk flow down
the tubule
• Organic wastes (e.g. uric acid) are secreted into the
tubule by transporters
 Antidiuresis and diuresis take place in the
hindgut
• Tubule empties an isosmotic fluid into the gut
• Osmoconcentration involves active transport of
ions from hindgut followed by osmosis
• Diuresis involves active transport of ions without
water uptake
• Regulated by antidiuretic and diuretic hormones
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Vertebrate urinary systems
• Kidneys are the urine-forming organ
• Paired organs on dorsal side of abdominal cavity,
one on each side of the vertebral column
• Blood is supplied by renal artery, exits via renal
vein
• Urine drains into two ureters
• Ureters empty into urinary bladder (in fish,
amphibians, mammals), which stores the urine,
or hindgut (in reptiles and birds)
• Bladder empties to the outside through the ureter
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
Renal artery
Renal
vein
Inferior vena
cava
Urinary
bladder
Kidney
Aorta
Ureter
Urethra
Figure 12-7a p569
Renal
cortex
Renal
pyramid
Renal
medulla
Renal
pelvis
Figure 12-7b p569
Gas bladder
Kidney
Ureter
Urinary bladder
Stomach
Figure 12-7c p569
ANIMATION: Human kidney
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12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Regions of the kidney
• Renal cortex -- outer
• Renal medulla -- inner
• Medulla is divided into renal pyramids in larger
mammals
• Renal pelvis -- drainage area in center of kidney
 Nephron is the smallest functional unit of the
kidney
• 1 million nephrons in human kidney
• Consists of a tubule and associated vascular component
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Vasculature of the nephron
• Afferent arteriole supplies each nephron
• Glomerulus is a ball-like knot of capillaries in
renal cortex -- site of filtration of the blood
• Efferent arteriole exits the glomerulus
• Peritubular capillaries surrounding the
tubules supply the renal tissue with blood and
exchange materials with the tubular fluid
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Functional parts of the renal tubule
• Bowman’s capsule -- glomerular filtration
• Proximal tubule -- tubular reabsorption and secretion
• Loop of Henle -- osmoconcentration
• Descending limb plunges into medulla
• Ascending limb returns to cortex
• Distal tubule -- reabsorption/secretion and
osmoconcentration
• Collecting duct -- osmoconcentration
• Empties into renal pelvis
• Juxtaglomerular apparatus -- sensor in
osmoregulation and blood pressure regulation
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
Distal
tubule
Collecting
duct
Proximal tubule
Juxtaglomerular
apparatus
Efferent
arteriole
Afferent
arteriole
Bowman’s
capsule
Glomerulus
Artery
Vein
Cortex
Medulla
Peritubular
capillaries
Overview of Functions of Parts of a Nephron
Vascular component
• Afferent arteriole —carries blood to the
glomerulus
• Glomerulus —a tuft of capillaries that
filters a protein-free plasma into the
tubular component
• Efferent arteriole —carries blood from
the glomerulus
• Peritubular capillaries —supply the
renal tissue; involved in exchanges
with the fluid in the tubular lumen
Tubular component
• Bowman’s capsule —collects the
glomerular filtrate
• Proximal tubule —uncontrolled
reabsorption and secretion of selected
substances occur here
• Loop of Henle —establishes an osmotic
gradient in the renal medulla that is
important in the kidney’s ability to
produce urine of varying concentration
• Distal tubule and collecting duct —
variable, controlled reabsorption of Na+
and H2O and secretion of K+ and H+
occur here; fluid leaving the collecting
duct is urine, which enters the renal
pelvis
Combined vascular/tubular component
• Juxtaglomerular apparatus —produces
substances involved in the control of
kidney function
Loop of Henle
To renal pelvis
Figure 12-8 p570
ANIMATION: Urine formation
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12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Fish urinary systems
• Elasmobranches are isosmotic or
hyperosmotic relative to seawater
• Retain urea and trimethylamine oxide (TMAO) as
major osmolytes
• Rectal gland in hindgut excretes a hypertonic fluid
high in NaCl
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Fish urinary systems
• Marine bony fishes are hypo-osmotic
• Drink seawater to reverse water loss through the
gills
• Gills actively transport NaCl outward and excrete
nitrogenous waste
• Kidneys remove excess divalent ions
• Freshwater bony fishes are hyperosmotic
• Take in water through gills and mouth
• Excrete a large volume of highly dilute urine
• Gills take in NaCl and excrete NH3 and NH4+
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
Neck
Glomerulus
Proximal
tubule
segment I
Proximal
tubule
segment II
Collecting
duct
Distal tubule
(a) Marine elasmobranch
Figure 12-9a p573
Neck
Proximal tubule segment I
Proximal tubule segment II
Collecting tubule
Collecting duct
(b) Marine glomerular teleost
Figure 12-9b p573
Proximal
tubule
segment II
Collecting
duct
Collecting
tubule
(c) Marine aglomerular teleost
Figure 12-9c p573
Neck
Proximal
tubule
segment I
Proximal
tubule
segment II
Intermediate segment
Distal tubule
Collecting duct
(d) Freshwater glomerular teleost
Figure 12-9d p573
Neck
Proximal tubule
Distal tubule
Collecting duct
Intermediate
segment
(e) Amphibian
Figure 12-9e p573
Neck
Proximal tubule
Intermediate segment
Collecting duct
Distal tubule
(f) Nonavian reptile
Figure 12-9f p573
Proximal tubule
Distal tubule
Collecting duct
(g) Bird, reptilian-type
Figure 12-9g p573
Distal tubule
Proximal tubule
Collecting duct
Descending thin limb
of Henle’s Loop
Ascending thick limb of
Henle’s Loop
(h) Bird, mammalian-type and mammal
Figure 12-9h p573
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Amphibian urinary systems
• Demonstrate the transition to life on land
• Lungs cannot excrete nitrogenous wastes nor regulate
NaCl
• Kidneys maintain a constant ECF
• Metanephric nephrons in adult amphibians
resemble mesonephric ones in freshwater fish with
urea excretion added
• Urinary bladder serves as a temporary water
reservoir in case of dehydration
• Arginine vasotocin (AVT) triggers water uptake
through aquaporins in the bladder wall
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Reptile urinary system
• Nephrons resemble aquatic vertebrates
• Ureters carry urine in liquid or semisolid form
into the cloaca
• Lack a loop of Henle to help conserve water
• Uric acid is the primary nitrogenous waste
• Cloaca and lower intestine can reabsorb
water
• Nasal salt glands secrete a highly salty fluid
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
 Avian urinary system
• Resembles reptiles
• Some mammalian-type nephrons with loops
of Henle further concentrate the urine
• Uric acid crystals are covered with protein
coats to form urate balls
• Marine birds have nasal salt glands located
near the eyes
• Contain blind-end tubules lined with active salt
secreting cells
• Excrete excess salt out of nasal passages
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
Afferent vein
Collecting duct
Central vein
(efferent)
Arterial supply
Reptilian-type
nephron
Capillary plexus
Short loop mammalian-type
nephron
Long loop mammalian-type
nephron
Medullary cone
Ureteral branch
Ureter
Figure 12-10a p575
Ureter
Proctodaeum
Colon
Ureteral
urine
Caecum
Coprodaeum
Ureteral
urine
Urodaeum
Cloaca
Bird
Figure 12-10b p575
12.4 Vertebrate Urinary Systems and
Extrarenal Organs
INSERT FIG 12-11 on page 576
Salt gland
Ducts
Lobe
Central
canal
Figure 12-11 p576