Excretion

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Chapter 44
Osmoregulation and
Excretion

• Overview: A balancing act
• The physiological systems of animals
– Operate in a fluid environment
• The relative concentrations of water and
solutes in this environment
– Must be maintained within fairly narrow limits

• Freshwater animals
– Show adaptations that reduce water uptake and
conserve solutes
• Desert and marine animals face desiccating
environments
– With the potential to quickly deplete the body water
Figure 44.1

• Osmoregulation
– Regulates solute concentrations and balances
the gain and loss of water
• Excretion
– Gets rid of metabolic wastes

• Concept 44.1: Osmoregulation balances the
uptake and loss of water and solutes
• Osmoregulation is based largely on controlled
movement of solutes
– Between internal fluids and the external
environment

Osmosis
• Cells require a balance
– Between osmotic gain and loss of water
• Water uptake and loss
– Are balanced by various mechanisms of
osmoregulation in different environments

Osmotic Challenges
• Osmoconformers, which are only marine
animals
– Are isoosmotic with their surroundings and do
not regulate their osmolarity
• Osmoregulators expend energy to control
water uptake and loss
– In a hyperosmotic or hypoosmotic environment

• Most animals are said to be stenohaline
– And cannot tolerate substantial changes in
external osmolarity
• Euryhaline animals
– Can survive large fluctuations in external
osmolarity
Figure 44.2

Marine Animals
• Most marine invertebrates are osmoconformers
• Most marine vertebrates and some
invertebrates are osmoregulators

• Marine bony fishes are hypoosmotic to sea
water
– And lose water by osmosis and gain salt by both
diffusion and from food they eat
• These fishes balance water loss
– By drinking seawater
Figure 44.3a
Gain of water and
salt ions from food
and by drinking
seawater
Osmotic water loss
through gills and other parts
of body surface
Excretion of
salt ions
from gills
Excretion of salt ions
and small amounts
of water in scanty
urine from kidneys
(a) Osmoregulation in a saltwater fish

Freshwater Animals
• Freshwater animals
– Constantly take in water from their
hypoosmotic environment
– Lose salts by diffusion

• Freshwater animals maintain water balance
– By excreting large amounts of dilute urine
• Salts lost by diffusion
– Are replaced by foods and uptake across the gills
Figure 44.3b
Uptake of
water and some
ions in food
Osmotic water gain
through gills and other parts
of body surface
Uptake of
salt ions
by gills
Excretion of
large amounts of
water in dilute
urine from kidneys
(b) Osmoregulation in a freshwater fish

Animals That Live in Temporary Waters
• Some aquatic invertebrates living in temporary
ponds
– Can lose almost all their body water and
survive in a dormant state
• This adaptation is called anhydrobiosis
Figure 44.4a, b (a) Hydrated tardigrade (b) Dehydrated tardigrade
100 µm
100 µm

Land Animals
• Land animals manage their water budgets
– By drinking and eating moist foods and by
using metabolic water
Figure 44.5
Water
balance in
a human
(2,500 mL/day
= 100%)
Water
balance in a
kangaroo rat
(2 mL/day
= 100%)
Ingested
in food (0.2)
Ingested
in food (750)
Ingested
in liquid
(1,500)
Derived from
metabolism (250)
Derived from
metabolism (1.8)
Water
gain
Feces (0.9)
Urine
(0.45)
Evaporation (1.46)
Feces (100)
Urine
(1,500)
Evaporation (900)
Water
loss

• Desert animals
– Get major water savings from simple anatomical
features
Figure 44.6
Control group
(Unclipped fur)
Experimental group
(Clipped fur)
4
3
2
1
0
Waterlostperday
(L/100kgbodymass)
Knut and Bodil Schmidt-Nielsen and their colleagues from Duke University observed that the
mels exposed to full sun in the Sahara Desert could reach temperatures of over 70°C, while the
skin remained more than 30°C cooler. The Schmidt-Nielsens reasoned that insulation of the skin
ay substantially reduce the need for evaporative cooling by sweating. To test this hypothesis, they
d the water loss rates of unclipped and clipped camels.
EXPERIMENT
RESULTSRemoving the fur of a camel increased the rate
of water loss through sweating by up to 50%.
The fur of camels plays a critical role in
their conserving water in the hot desert
environments where they live.
CONCLUSION

Transport Epithelia
• Transport epithelia
– Are specialized cells that regulate solute
movement
– Are essential components of osmotic
regulation and metabolic waste disposal
– Are arranged into complex tubular networks

• An example of transport epithelia is found in the
salt glands of marine birds
– Which remove excess sodium chloride from the
blood
Figure 44.7a, b
Nasal salt gland
Nostril
with salt
secretions
Lumen of
secretory tubule
NaCl
Blood
flow
Secretory cell
of transport
epithelium
Central
duct
Direction
of salt
movement
Transport
epithelium
Secretory
tubule
Capillary
Vein
Artery
(a) An albatross’s salt glands
empty via a duct into the
nostrils, and the salty solution
either drips off the tip of the
beak or is exhaled in a fine mist.
(b) One of several thousand
secretory tubules in a salt-
excreting gland. Each tubule
is lined by a transport
epithelium surrounded by
capillaries, and drains into
a central duct.
(c) The secretory cells actively
transport salt from the
blood into the tubules.
Blood flows counter to the
flow of salt secretion. By
maintaining a concentration
gradient of salt in the tubule
(aqua), this countercurrent
system enhances salt
transfer from the blood to
the lumen of the tubule.

• Concept 44.2: An animal’s nitrogenous wastes
reflect its phylogeny and habitat
• The type and quantity of an animal’s waste
products
– May have a large impact on its water balance

Proteins Nucleic acids
Amino acids Nitrogenous bases
–NH2
Amino groups
Most aquatic
animals, including
most bony fishes
Mammals, most
amphibians, sharks,
some bony fishes
Many reptiles
(including
birds), insects,
land snails
Ammonia Urea Uric acid
NH3
NH2
NH2
O C
C
C
N
C
O N
H H
C O
N
C
HN
O
H
• Among the most important wastes
– Are the nitrogenous breakdown products of
proteins and nucleic acids
Figure 44.8

Forms of Nitrogenous Wastes
• Different animals
– Excrete nitrogenous wastes in different forms

Ammonia
• Animals that excrete nitrogenous wastes as
ammonia
– Need access to lots of water
– Release it across the whole body surface or
through the gills

Urea
• The liver of mammals and most adult
amphibians
– Converts ammonia to less toxic urea
• Urea is carried to the kidneys, concentrated
– And excreted with a minimal loss of water

Uric Acid
• Insects, land snails, and many reptiles,
including birds
– Excrete uric acid as their major nitrogenous
waste
• Uric acid is largely insoluble in water
– And can be secreted as a paste with little
water loss

The Influence of Evolution and Environment on
Nitrogenous Wastes
• The kinds of nitrogenous wastes excreted
– Depend on an animal’s evolutionary history
and habitat
• The amount of nitrogenous waste produced
– Is coupled to the animal’s energy budget

• Concept 44.3: Diverse excretory systems are
variations on a tubular theme
• Excretory systems
– Regulate solute movement between internal
fluids and the external environment

Excretory Processes
• Most excretory systems
– Produce urine by refining a filtrate derived from
body fluids
Figure 44.9
Filtration. The excretory tubule collects a filtrate from the blood.
Water and solutes are forced by blood pressure across the
selectively permeable membranes of a cluster of capillaries and
into the excretory tubule.
Reabsorption. The transport epithelium reclaims valuable substances
from the filtrate and returns them to the body fluids.
Secretion. Other substances, such as toxins and excess ions, are
extracted from body fluids and added to the contents of the excretory
tubule.
Excretion. The filtrate leaves the system and the body.
Capillary
Excretory
tubule
FiltrateUrine
1
2
3
4

• Key functions of most excretory systems are
– Filtration, pressure-filtering of body fluids
producing a filtrate
– Reabsorption, reclaiming valuable solutes from
the filtrate
– Secretion, addition of toxins and other solutes
from the body fluids to the filtrate
– Excretion, the filtrate leaves the system

Survey of Excretory Systems
• The systems that perform basic excretory
functions
– Vary widely among animal groups
– Are generally built on a complex network of
tubules

Nucleus
of cap cell
Cilia
Interstitial fluid
filters through
membrane where
cap cell and tubule
cell interdigitate
(interlock)
Tubule cell
Flame
bulb
Nephridiopore
in body wall
Tubule
Protonephridia
(tubules)
Protonephridia: Flame-Bulb Systems
• A protonephridium
– Is a network of dead-end tubules lacking
internal openings
Figure 44.10

• The tubules branch throughout the body
– And the smallest branches are capped by a
cellular unit called a flame bulb
• These tubules excrete a dilute fluid
– And function in osmoregulation

Metanephridia
• Each segment of an earthworm
– Has a pair of open-ended metanephridia
Figure 44.11 Nephrostome Metanephridia
Nephridio-
pore
Collecting
tubule
Bladder
Capillary
network
Coelom

• Metanephridia consist of tubules
– That collect coelomic fluid and produce dilute
urine for excretion

Digestive tract
Midgut
(stomach)
Malpighian
tubules
Rectum
Intestine
Hindgut
Salt, water, and
nitrogenous
wastes
Feces and urine
Anus
Malpighian
tubule
Rectum
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH
Malpighian Tubules
• In insects and other terrestrial arthropods,
malpighian tubules
– Remove nitrogenous wastes from hemolymph
and function in osmoregulation
Figure 44.12

• Insects produce a relatively dry waste matter
– An important adaptation to terrestrial life

Vertebrate Kidneys
• Kidneys, the excretory organs of vertebrates
– Function in both excretion and osmoregulation

• Concept 44.4: Nephrons and associated blood
vessels are the functional unit of the
mammalian kidney
• The mammalian excretory system centers on
paired kidneys
– Which are also the principal site of water
balance and salt regulation

• Each kidney
– Is supplied with blood by a renal artery and
drained by a renal vein
Figure 44.13a
Posterior vena cava
Renal artery and vein
Aorta
Ureter
Urinary bladder
Urethra
(a) Excretory organs and major
associated blood vessels
Kidney

• Urine exits each kidney
– Through a duct called the ureter
• Both ureters
– Drain into a common urinary bladder

(b) Kidney structure
Ureter
Section of kidney from a rat
Renal
medulla
Renal
cortex
Renal
pelvis
Figure 44.13b
Structure and Function of the Nephron and Associated
Structures
• The mammalian kidney has two distinct
regions
– An outer renal cortex and an inner renal
medulla

• The nephron, the functional unit of the vertebrate
kidney
– Consists of a single long tubule and a ball of
capillaries called the glomerulus
Figure 44.13c, d
Juxta-
medullary
nephron
Cortical
nephron
Collecting
duct
To
renal
pelvis
Renal
cortex
Renal
medulla
20 µm
Afferent
arteriole
from renal
artery
Glomerulus
Bowman’s capsule
Proximal tubule
Peritubular
capillaries
SEM
Efferent
arteriole from
glomerulus
Branch of
renal vein
Descending
limb
Ascending
limb
Loop
of
Henle
Distal
tubule
Collecting
duct
(c) Nephron
Vasa
recta(d) Filtrate and
blood flow

Filtration of the Blood
• Filtration occurs as blood pressure
– Forces fluid from the blood in the glomerulus
into the lumen of Bowman’s capsule

• Filtration of small molecules is nonselective
– And the filtrate in Bowman’s capsule is a
mixture that mirrors the concentration of
various solutes in the blood plasma

Pathway of the Filtrate
• From Bowman’s capsule, the filtrate passes
through three regions of the nephron
– The proximal tubule, the loop of Henle, and the
distal tubule
• Fluid from several nephrons
– Flows into a collecting duct

Blood Vessels Associated with the Nephrons
• Each nephron is supplied with blood by an afferent
arteriole
– A branch of the renal artery that subdivides into the
capillaries
• The capillaries converge as they leave the glomerulus
– Forming an efferent arteriole
• The vessels subdivide again
– Forming the peritubular capillaries, which surround the
proximal and distal tubules

Proximal tubule
Filtrate
H2O
Salts (NaCl and others)
HCO3
–
H+
Urea
Glucose; amino acids
Some drugs
Key
Active transport
Passive transport
CORTEX
OUTER
MEDULLA
INNER
MEDULLA
Descending limb
of loop of
Henle
Thick segment
of ascending
limb
Thin segment
of ascending
limb
Collecting
duct
NaCl
NaCl
NaCl
Distal tubule
NaCl Nutrients
Urea
H2O
NaCl
H2O
H2OHCO3
−
K+
H+
NH3
HCO3
−
K+
H+
H2O
1 4
32
3 5
From Blood Filtrate to Urine: A Closer Look
• Filtrate becomes urine
– As it flows through the mammalian nephron
and collecting duct
Figure 44.14

• Secretion and reabsorption in the proximal
tubule
– Substantially alter the volume and composition
of filtrate
• Reabsorption of water continues
– As the filtrate moves into the descending limb
of the loop of Henle

• As filtrate travels through the ascending limb of
the loop of Henle
– Salt diffuses out of the permeable tubule into the
interstitial fluid
• The distal tubule
– Plays a key role in regulating the K+
and NaCl
concentration of body fluids
• The collecting duct
– Carries the filtrate through the medulla to the renal
pelvis and reabsorbs NaCl

• Concept 44.5: The mammalian kidney’s ability
to conserve water is a key terrestrial adaptation
• The mammalian kidney
– Can produce urine much more concentrated
than body fluids, thus conserving water

Solute Gradients and Water Conservation
• In a mammalian kidney, the cooperative action
and precise arrangement of the loops of Henle
and the collecting ducts
– Are largely responsible for the osmotic
gradient that concentrates the urine

• Two solutes, NaCl and urea, contribute to the
osmolarity of the interstitial fluid
– Which causes the reabsorption of water in the
kidney and concentrates the urine
Figure 44.15
H2O
H2O
H2O
H2O
H2O
H2O
H2O
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
NaCl
300
300 100
400
600
900
1200
700
400
200
100
Active
transport
Passive
transport
OUTER
MEDULLA
INNER
MEDULLA
CORTEX
H2O
Urea
H2O
Urea
H2O
Urea
H2O
H2O
H2O
H2O
1200
1200
900
600
400
300
600
400
300
Osmolarity of
interstitial
fluid
(mosm/L)
300

• The countercurrent multiplier system involving
the loop of Henle
– Maintains a high salt concentration in the
interior of the kidney, which enables the kidney
to form concentrated urine

• The collecting duct, permeable to water but not
salt
– Conducts the filtrate through the kidney’s
osmolarity gradient, and more water exits the
filtrate by osmosis

• Urea diffuses out of the collecting duct
– As it traverses the inner medulla
• Urea and NaCl
– Form the osmotic gradient that enables the
kidney to produce urine that is hyperosmotic to
the blood

Regulation of Kidney Function
• The osmolarity of the urine
– Is regulated by nervous and hormonal control
of water and salt reabsorption in the kidneys

• Antidiuretic hormone (ADH)
– Increases water reabsorption in the distal
tubules and collecting ducts of the kidney
Figure 44.16a
Osmoreceptors
in hypothalamus
Drinking reduces
blood osmolarity
to set point
H2O reab-
sorption helps
prevent further
osmolarity
increase
STIMULUS:
The release of ADH is
triggered when osmo-
receptor cells in the
hypothalamus detect an
increase in the osmolarity
of the blood
Homeostasis:
Blood osmolarity
Hypothalamus
ADH
Pituitary
gland
Increased
permeability
Thirst
Collecting duct
Distal
tubule
(a) Antidiuretic hormone (ADH) enhances fluid retention by making
the kidneys reclaim more water.

• The renin-angiotensin-aldosterone system (RAAS)
– Is part of a complex feedback circuit that functions
in homeostasis
Figure 44.16b
Increased Na+
and H2O reab-
sorption in
distal tubules
Homeostasis:
Blood pressure,
volume
STIMULUS:
The juxtaglomerular
apparatus (JGA) responds
to low blood volume or
blood pressure (such as due
to dehydration or loss of
blood)
Aldosterone
Adrenal gland
Angiotensin II
Angiotensinogen
Renin
production
Renin
Arteriole
constriction
Distal
tubule
JGA
(b) The renin-angiotensin-aldosterone system (RAAS) leads to an increase
in blood volume and pressure.

• Another hormone, atrial natriuretic factor (ANF)
– Opposes the RAAS

• The South American vampire bat, which feeds
on blood
– Has a unique excretory system in which its
kidneys offload much of the water absorbed from
a meal by excreting large amounts of dilute urine
Figure 44.17

• Concept 44.6: Diverse adaptations of the
vertebrate kidney have evolved in different
environments
• The form and function of nephrons in various
vertebrate classes
– Are related primarily to the requirements for
osmoregulation in the animal’s habitat

• Exploring environmental adaptations of the
vertebrate kidney
Figure 44.18
MAMMALS
Bannertail Kangaroo rat
(Dipodomys spectabilis)
Beaver (Castor canadensis)
FRESHWATER FISHES AND AMPHIBIANS
Rainbow trout
(Oncorrhynchus mykiss)
Frog (Rana temporaria)
BIRDS AND OTHER REPTILES
Roadrunner
(Geococcyx californianus)
Desert iguana
(Dipsosaurus dorsalis)
MARINE BONY FISHES
Northern bluefin tuna (Thunnus thynnus)

Excretion

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