Osmotic Regulation and
Excretion Chpt 44, all
4-12-06
Next lecture circulation Chpt 42, all
Environmental Extremes
• 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
Water, salt & metabolic wastes
• Osmoregulation (ions and water balance)
– Regulates solute concentrations and balances the
gain and loss of water. Water cannot be actively
transported—can only move from low osmotic
concentration (low salt) across a semipermable
membrane to a high osmotic concentration (high salt
or other solute).
• Excretion
– Gets rid of metabolic wastes (free amino groupsNH2), the end product of metabolism of proteins and
nucleic acids. Includes variable amounts of water to
carry away the waste. Carbon dioxide route of
excretion via the lungs.
• 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.
Most animals have blood that interfaces with the outside and
inside environments. Blood interfaces with the interstitial space
(space between cells and capillaries). Interstitial space interfaces
with cell membranes. Insects blood (hemolymph) directly bathes
the cells
– IMPORTANT!
– Animal cells must be in osmotic equilibrium
with their surrounding environments, because
if they swell or shrink their membranes will
rupture.
–Plants protected by a cellulose wall so can stand increased
pressure
Includes interstital space
and blood space
Extracellular
Na+ 145 mM
K+ 4 mM
Intracellular
Na+ 15 mM
K+ 150 mM
Cl+110mM
Na+ leak
Cl+ 20mM
3Na+
ATP
Free amino acids?
ADP + Pi
2K+
K+ leak
Osmosis
• Cells require a balance
– Between osmotic gain and loss of water
(thirst and heat stroke). Lose too much
water from sweating and upset salt balance
(K).
• 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 (have the same
osmotic pressure inside and outside). Wescor
osmometer would show what??? Same reading!
• Osmoregulators expend energy to control
water uptake and loss
– In a hyperosmotic or hypoosmotic environment
Extreme Regulators
• Most animals are said to be stenohaline
– And cannot tolerate substantial changes in
external osmolarity
• Euryhaline animals
– Can survive large fluctuations in external
osmolarity
Example: an African cichlid fish,
Tilapia mossambica. Can tolerate
fresh water and up to 200% seawater.
100% seawater= 3.4g salt/100mls.
Also killifish (minnow) with chloride
cells on inside of opercular flap.
How do they do it??
Figure 44.2
Marine Animals
•
•
•
•
•
•
Most marine invertebrates are osmoconformers. The horseshoe
crab.
Those of you who have had the lab this week have found out that its
hemolymph osmolarity varies with that of the seawater.
Generalizations:
Most marine vertebrates and some invertebrates are
osmoregulators.
Ex:
The Fiddler crab however is an osmoregulator from 10% seawater
to about 90% seawater and then it conforms to its seawater
concentration (However its osmolarity is about 100 milliosmoles less
than the seawater it has been acclimated in.
•
The Toad fish maintained a blood osmolarity of 300 in 100%
seawater. (Good osmoregulator)
•
What marine fish is a conformer? Clue –it lacks a back bone and a
mouth!
Hag Fish
Sharks!
Marine fish are hypoosmotic to Sea Water
• They lose water by osmosis and gain salt
by diffusion across the thin gill epithelium.
Gain salt from food if invertebrates. These
fishes balance water loss by drinking
seawater and excreting NaCl at the gills
leaving behind the H2O.
NaCl and some water in ingested
seawater absorbed by the
intestine. Most divalent ions left
behind.
Gain of water and
salt ions from food
and by drinking
seawater
Excretion of
salt ions
from gills
Figure 44.3a
Osmotic water loss
through gills and other parts
of body surface if no scales
Excretion of divalent salt ions
and small amounts of water in
scanty urine from kidneys
(a) Osmoregulation in a saltwater fish
Osmoregulation in Fresh H2O Fish
Water drawn in across the gill by osmosis. Some
salt lost from the blood at gill. Water removed
by excreting large amounts of very dilute urine.
Salt replaced from ingested food and active
uptake across the gills.
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
Excretion of
large amounts of
water in dilute
urine from kidneys. 3%
of body weight/hr
Figure 44.3b (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
100 µm
100 µm
Figure 44.4a, b
(a) Hydrated tardigrade
(b) Dehydrated tardigrade
Land Animals
• Land animals manage their water budgets
– By drinking and eating moist foods and by
using metabolic water
Water
balance in a
kangaroo rat
(2 mL/day
= 100%)
Water
balance in
a human
(2,500 mL/day
= 100%)
Ingested
in food (750)
Ingested
in food (0.2)
Ingested
in liquid
(1,500)
Water
gain
Derived from
metabolism (250)
Derived from
metabolism (1.8)
Feces (0.9)
Water
loss
Figure 44.5
Urine
(0.45)
Evaporation (1.46)
Feces (100)
Urine
(1,500)
Evaporation (900)
Transport Epithelia
• Transport epithelia (thin pavement cells with tight
junctions). No solutes move between cells.
– Are specialized cells that regulate solute movement
(by actively transporting solutes across their cell
membranes). Their membranes are permeable to
selected solutes and may or may not be permeable to
water.
– Generally arranged into complex tubular networks, like
salt glands and kidney tubules, but can be arranged in
sheets like in the intestine and killifish opercular flap.
Salt Gland in a Sea Gull
• An example of transport epithelia is found in the salt glands of
marine birds
–
Which remove excess sodium chloride from the blood
Nasal salt gland
(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.
Nostril
with salt
secretions
Lumen of
secretory tubule
Vein
Capillary
Secretory
tubule
(b) One of several thousand
secretory tubules in a saltexcreting gland. Each tubule
is lined by a transport
epithelium surrounded by
capillaries, and drains into
a central duct.
Figure 44.7a, b
Artery
NaCl
Transport
epithelium
Direction
of salt
movement
Blood Secretory cell
flow
of transport
epithelium
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.
Countercurrent flow ensures that
maximum salt is extracted from
the blood.
Nitrogen Excretion (NH3)
• Among the most important wastes
– Are the nitrogenous breakdown products of proteins
and nucleic acids
Nucleic acids
Proteins
Nitrogenous bases
Amino acids
–NH2
Amino groups
Many reptiles
Most aquatic
Mammals, most
(including
animals, including amphibians, sharks,
birds), insects,
most bony fishes some bony fishes
land snails
O
C
NH3
Figure 44.8
Ammonia
O
C
HN
NH2
NH2
Urea
O
C
H
N
C
C N
N
H
H
Uric acid
C O
An animal’s nitrogenous
wastes reflect its phylogeny
and habitat. The type and
quantity of an animal’s waste
products has a large impact
on its water balance.
Forms of Nitrogenous Wastes
• Different animals
– Excrete nitrogenous wastes in different forms
• 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
Ammonia
• Animals that excrete nitrogenous wastes as
ammonia (very toxic but very water soluble).
• Fishes and tadpoles living in water
Release it across the whole body surface or
through the gills (fishes and frog tadpoles)
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. Desert
rodents especially efficient at concentrating
urea.
• Estivating lung fish converts amino nitrogen to
urea when in the mud cocoon and when rain
reappears it converts urea to ammonia and
excretes it via the gills. Can do this because
urea less toxic.
Uric Acid-nontoxic and poorly
soluble
• Insects, land snails, and many reptiles,
including birds
• Excrete uric acid as their major nitrogenous
waste (white pasty cap on bird feces)
• Uric acid is largely insoluble in water
– And can be secreted as a paste with little
water loss. Water reabsorbed in cloaca of
birds and reptiles
Evolution and Environment influence
type of Nitrogenous Waste Excreted
• 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