Chapter 44
Osmoregulation and Excretion.
Salvin’s Albatross
Concept 44.1


The ability to regulate the intake and loss
of water and solutes is called
Osmoregulation.
Based on the controlled movement of
solutes between the internal fluids and
external environment.
Concept 44.1


Osmoregulation is a problem that all
animals, including humans, face.
Water is one of the most important
compounds in an organism and must be
balanced in order for the organism to
survive.
Concept 44.1


Osmoregulation is how animals regulate
solute concentrations and balance the
gain/loss of water.
Since animals don’t have cell walls, the
cells may: swell and burst, if there’s a
constant net gain, or may shrivel and die,
if there’s a significant amount of water
loss.
Osmosis


Osmosis- the movement of aqueous
substances across a semi permeable
membrane.
This is the basis of Osmoregulation!!!
Types of osmosis


Osmolarity- a difference in osmotic
pressure.
The total solute concentration expressed
as molarity, or moles of solute per liter of
solution;
Osmosis and organisms

If an organism is:



Isoosmotic, it has the same solute
concentration as the surrounding
environment.
Hypoosmotic, it has a more diluted solution
concentration than the surrounding
environment.
Hyperosmotic, it has a greater
concentration of solutes than its surrounding
environment.
Osmotic challenges.




Two basic solutions to water balancing
problem!
One is to be isoosmotic with the
surroundings.
This is only available for marine animals.
Since it can’t adjust its osmolarity it would
be consider an Osmoconformer.
Osmoconformers





Are marine animals
Have stable composition
Are isoosmotic and do not need to
regulate their osmolarity
Often have a very constant internal
osmolarity
Therefore they must be……

Simple organisms!!!
Osmotic challenges



The other way an organism can regulate
their osmotic problem is to control the
osmolarity of its bodily fluids.
An osmoregulator must discharge
excess water if it lives in a hypoosmotic
environment.
It must take in the water if it lives in a
hyperosmotic environment.
Osmotic challenges



Osmoregulation allows an animal to live
in an environment that would otherwise
be uninhabitable for osmoconformers.
This would be freshwater and terrestrial
habitats.
Allows many marine animals to maintain
internal osmolarities different than that of
saltwater.
The costs of being complex




ENERGY!!!!
In order to maintain the osmolarity difference
between the body and the external environment,
the organism must extend energy.
They use active transportation to move the
solute concentrations in their bodily fluids.
The amount of energy used depends on the
gradient of the organism’s solute to the
evironment’s solute concentration.
Environmental changes



Most organisms, both osmoconformers
and osmoregulators, cannot survive
substantial changes in the environment.
These organisms are called
Stenohalines.
Derived from the Greek words stenos,
meaning narrow, and haline referring to
salt.
Environmental changes.





Some animals, however, can survive large
fluctuations in solution concentration.
These are referred to as Euryhalines.
Which is derived from the Greek words Eurys,
meaning broad and of course haline.
Examples are some species of Salmon and
Tilapia.
Tilapia can adjust to any salt concentration
between fresh water and 2,000 mosm/L, twice
that of saltwater.
Adaptations

Most marine invertebrates are isoosmotic



Means their solute concentration is the same
as their surrounding environment.
However they differ considerably from
seawater in their concentrations of most
specific solutes.
Thus even an osmoconformer has to
regulate its concentrations.
Adaptations



Sharks have a slighter higher osmolarity
than that of the sea water because they
retain urea.
Marine bony fishes are hypoosmotic to the
seawater and therefore must obtain
solutes through diffuse and their food.
They (marine bony fishes) excrete their
excess salt through rectal glands, gills,
salt-excreting glands, or kidneys.
Adaptations






Unlike bony fish and despite having a relatively low
internal salt concentration, marine sharks do not
experience a large and continuous osmotic water loss.
Why?
Sharks maintain high concentrations of urea.
An organic solute called trimethylamine oxide or TMAO
protects proteins from damage by the urea.
The sharks total osmolaric concentration would add up
to just slightly above that of saltwater so therefore a
shark is actually hyperosmotic to the saltwater.
Consequently, water slowly enters a shark and is
disposed of in the urine produced by the kidneys.
LE 44-3a
Gain of water and
salt ions from food
and by drinking
seawater
Excretion of
salt ions
from gills
Osmotic water loss
through gills and other parts
of body surface
Excretion of salt ions
and small amounts
of water in scanty
urine from kidneys
Osmoregulation in a saltwater fish
Adaptations



Would freshwater fish have the same problem as
marine fish?
No. Freshwater fish have the opposite problem
than marine fish.
Since the seawater is hyperosmotic to the
marine fish, then they are constantly losing
water and gaining salt, conversely since fresh
water is hypoosmotic to freshwater fish then
they are constantly gaining water and losing
salt.
LE 44-3b
Osmotic water gain
through gills and other parts
of body surface
Uptake of
water and some
ions in food
Uptake of
salt ions
by gills
Osmoregulation in a freshwater fish
Excretion of
large amounts of
water in dilute
urine from kidneys
Figure 44-2
Adaptations



One way to prevent this salt loss is to
excrete diluted urine.
The salt that is lost by diffusion and in the
urine are replaced by salts in food and by
picking up chloride cells in the gills.
Some fish, like salmon will exhibit traits of
both marine and freshwater fish
depending on the environment that they
are in.
Life without water.




Most animals cannot survive dehydration.
However some animals can survive the
dehydrated conditions by undergoing a dormant
phase.
This adaptation is called anhydrobiosis (life
without water)
Researchers believe that these animals have a
sugar called trehalose that in the absence of
water will provide the cell with membranes and
proteins.
LE 44-4
100 µm
100 µm
Hydrated tardigrade
Dehydrated
tardigrade
Adaptations



Did you know that a human dies if they lose only
12% of their body water.
The threat of dehydration for terrestrial animals
is great and offers a challenge to get around.
Some animals develop waxy coverings called…


CUTICLE
Some desert dwelling animals have become
nocturnal to avoid the dehydrating heat.
LE 44-5
Water
balance in a
kangaroo rat
(2 mL/day)
Ingested
in food (0.2 mL)
Water
balance in
a human
(2,500 mL/day)
Ingested
in liquid
(1,500 mL)
Ingested
in food
(750 mL)
Water
gain
Derived from
metabolism (1.8 mL)
Feces (0.09 mL)
Urine
(0.45 mL)
Derived from
metabolism (250 mL)
Feces (100 mL)
Urine
(1,500 mL)
Water
loss
Evaporation (1.46 mL)
Evaporation (900 mL)
Land adaptations


Even with these adaptations animals still
lose water through their gas exchange
organs, in their urine and feces, and
across their skin.
Land animals replenish their water balance
by drinking water and obtaining it in their
food.
Transport Epithelia




In most animals there are one or more different
kinds of transport epithelium or a layer/layers of
specialized epithelial cells that regulate solute
movements.
These are essential components of osmotic
regulation and metabolic waste disposal.
They move specific solutes in controlled
amounts in specific directions.
Some epithelia face outward towards the
environment while others line channels
connected to the outside by an opening on the
body’s surface.
Water lost per day
(L/100 kg body mass)
LE 44-6
4
3
2
1
0
Control group
(Unclipped fur)
Experimental group
(Clipped fur)
Epithelia



Are joined by impermeable tight junctions
Form a barrier at the tissue-environment
boundary
Are arranged into complex tubular
networks with extensive surface areas.
LE 44-7b
Lumen of
secretory tubule
Vein
Capillary
Artery
Secretory
tubule
NaCl
Transport
epithelium
Direction
of salt
movement
Blood
flow Secretory cell
of transport
epithelium
Central
duct
Nitrogenous waste and
phylogeny/habitat.




The type and quantity of an animal’s waste may
have a large impact on its water balance.
Metabolic processes of an organism produces a
nitrogenous waste ammonia.
Most mammals convert ammonia to urea, which
is less toxic to the body.
The urea is carried to the kidneys and
concentrated it is excreted with a minimal loss of
water.
Ammonia



In fish the ammonia is lost as ammonium
ions, while the kidneys only excrete a
minor amount of nitrogenous waste.
The gill epithelium takes up Na+ in
exchange for the ammonium ions
This helps to maintain a higher amount of
Na in the body fluids than in the
surrounding waters.
Urea


Transferring through the skin is good for
fish but not so beneficial for land animals
because ammonia has to be transported in
large amounts of diluted solutions.
The reason why land dwelling organisms
don’t use this type of excretion is that it
requires a huge amount of water and
most land animals simply do not have a
sufficient amount of water.
LE 44-8
Proteins
Nucleic acids
Amino acids
Nitrogenous bases
—NH2
Amino groups
Most aquatic
animals,
including most
bony fishes
Ammonia
Mammals, most
amphibians,
sharks, some bony
fishes
Urea
Many reptiles
(including
birds), insects,
land snails
Uric acid
Urea

Mammals, sharks and some amphibians
produce urea by a metabolic process in
the liver that combines ammonia with
carbon dioxide.
Main advantages



Less toxic; about 100,000 times less toxic
than ammonia
This allows urea to be transported safely
at high concentrations.
Little water is lost in the excretion
Main disadvantages



ENERGY!!!
It takes energy to convert the ammonia to
urea.
Many amphibians produce ammonia in
their aqueous stage and then produce
urea in their terrestrial phase.
Uric Acid




Insects, land snails, and many reptiles including
birds excrete uric acid.
Similarly uric acid is less toxic than ammonia
Unlike urea and ammonia, uric acid is insoluble
in water and can be excreted in a semi-solid
form.
High energy cost!!! More so than making urea.
Requires a considerable amount of ATP for
synthesis from ammonia
Nitrogenous wastes




In general the kind of nitrogenous waste depends on the
organism’s evolutionary history, habitat and availability of
water.
In some species individuals can change forms of
nitrogenous wastes when environmental conditions
change.
Over time evolution determines the limits of
physiological responses for a species, but during their
lives, individual organisms make physiological
adjustments within these evolutionary constraints.
Finally the amount of waste produced is coupled with
the amount and what the animal consumes.
Diverse excretory
systems are variations
on a tubular theme.
Excretory systems


Excretory systems produce urine by
refining a filtrate derived from body fluids.
The key functions of excretory systems
are filtration and the production of urine
from the filtrate by selective reabsorption,
and secretion.
Key functions



Filtration- pressure-filtering of body fluids,
producing a filtrate.
Production of urine form the filtrate by selective
reabsorption- reclaiming valuable solutes from
the filtrate
Secretion- addition of toxins and other solutes
from the body fluids to the filtrate.
LE 44-9
Capillary
Filtrate
Excretory
tubule
Filtration
Reabsorption
Secretion
Urine
Excretion
Excretory varieties of Flatworms



A Protenephridium is a network of dead end
tubules lacking internal openings.
The tubules branch throughout the body and the
smallest branches are capped by a cellular unit
called a flame bulb.
The flame bulb has cilia that draw water and
solutes from the interstitial fluid through the
flame bulb and then moves the urine outward
through the tubules until they empty into the
environment through pores called
nephridiopores.
Flame bulbs



The tubules reabsorb most solutes before
the urine exits the body.
Function mainly in osmoregulation
Most metabolic wastes diffuse out of the
animal through the body surface or are
excreted into the gastrovascular cavity and
eliminated through the mouth.
LE 44-10
Nucleus
of cap cell
Cilia
Interstitial fluid
filters through
membrane where
cap cell and tubule
cell interdigitate
(interlock)
Tubule cell
Flame
bulb
Protonephridia
(tubules)
Tubule
Nephridiopore
in body wall
Metaphridia




Another type of excretory system is called
Metanephridia.
Found in most annelid
Each segment of an earthworm contains a
pair of metaphridia, which are emmersed
in coelomic fluid and enveloped by a
capillary network.
The internal opening is surrounded by a
ciliated funnel called the nephrostome.
LE 44-11
Coelom
Capillary
network
Bladder
Collecting
tubule
Nephridiopore
Nephrostome
Metanephridium
Malipighian tubules





Malpighian tubules- organs in insects and
other terrestrial arthropods.
Malpighian tubules open into the digestive
tract and dead end at tips that are
immersed in hemolymph (circulatory fluid)
Water -> tubules-> rectum
Some of the solutes are transferred back
into the hemolymph.
Highly effective in conserving water.
LE 44-12
Digestive tract
Rectum
Hindgut
Intestine
Midgut
(stomach)
Malpighian
tubules
Feces and urine
Salt, water, and
nitrogenous
wastes
Anus
Malpighian
tubule
Rectum
Reabsorption of H2O,
ions, and valuable
organic molecules
HEMOLYMPH
Vertebrate kidneys




Built out of tubules in an organized
network way
Function in both osmoregulation and
excretory
Compacted and nonsegmentary organs
Ducts and other structures carry urine out
of the body.
Kidneys




Blood enters the kidneys by
the Renal artery.
It’d drained by the Renal vein.
Urine leaves the kidneys
through a duct called the
ureter and both drain into a
urinary bladder.
During urination, the urine
leaves through a canal called
the urethra.



Two distinct region, an outer
renal cortex and an inner
cortex.
The nephron, the functional
unit of the vertebrate kidney,
consists of a single long tubule
and a ball of capillaries called
the glomerulus.
The blind end of the tubule
froms a cup shaped swelling
called the Bowman’s capsule
which surrounds the
glomerulus.
LE 44-13
Posterior vena cava
Renal artery and vein
Kidney
Renal
medulla
Renal
cortex
Renal
pelvis
Aorta
Ureter
Urinary bladder
Urethra
Ureter
Excretory organs and
major associated blood
vessels
JuxtaCortical
medullary nephron
nephron
Afferent
arteriole
Glomerulus
from renal
Bowman’s capsule
artery
Proximal tubule
Peritubular capillaries
Renal
cortex
Collecting
duct
20 µm
Renal
medulla
To
renal
pelvis
Nephron
Section of kidney from a rat
Kidney structure
SEM
Efferent
arteriole from
glomerulus
Distal
tubule
Collecting
duct
Branch of
renal vein
Descending
Loop limb
of
Henle Ascending
limb
Vasa
recta
Filtrate and blood flow
Pathway to Filtration





From the bowman’s capsule the urine moves to the
proximal tubule the loop of henle and the distal tubule.
the distal tubule empties into a collecting duct which is
processed and filtered from many nephrons.
This filtrate flows from the many collecting ducts of the
kidney into the renal pelvis.
80% of the nephrons the cortical nephrons have reduced
loops of Henle and are almost entirely confined to the
renal cortex.
The other 20% have well developed loops that extend
deeply into the renal medulla.
LE 44-16b
Homeostasis:
Blood pressure,
volume
Increased Na+
and H2O reabsorption in
distal tubules
STIMULUS:
The juxtaglomerular
apparatus (JGA) responds
to low blood volume or
blood pressure (such as
due to dehydration or
loss of blood)
Aldosterone
Arteriole
constriction
Adrenal gland
Angiotensin II
Distal
tubule
Angiotensinogen
JGA
Renin
production
Renin
LE 44-16a
Osmoreceptors
in hypothalamus
Thirst
Hypothalamus
Drinking reduces
blood osmolarity
to set point
ADH
Increased
permeability
Pituitary
gland
Distal
tubule
STIMULUS
The release of ADH is
triggered when osmoreceptor cells in the
hypothalamus detect an
increase in the osmolarity
of the blood
H2O reabsorption helps
prevent further
osmolarity
increase
Collecting duct
Homeostasis:
Blood osmolarity
The kidney


The maintenance of osmotic differences
and the production of hyperosmotic urine
are possible only because considerable
energy is expended for the active
transport of solutes against concentration
gradients
The functions of the nephron can be
thought of as small machines producing a
region of high osmolarity in the kidney.
Urea


Urea which diffuses out of the collecting duct as
it traverses the inner medulla forms along with
NaCl the osmotic gradient that enables the
kidney to produce urine that is hyperosmotic to
the blood.
The osmolarity of the urine is regulated by
nervous system and hormonal control of water
and salt reabsorption in the kidneys. This
regulation involves the actions of antidiuretic
hormone (adh) the renin-aldosterone system
and atrial natriuretic factor.
Adaptations



The form and function of nephrons in various
vertebrates are related primarily to the
requirements for osmoregulation in the animal’s
habitat
The more hyperosmotic the excretion the longer
the loops of Henle.
In conclusion the evolution of the organism and
the habitat of the organism plays a huge part in
the organism’s excretion.