Osmoregulation
• Osmoregulation – the active regulation of the
osmotic pressure of an organism’s fluids to
maintain homeostasis of the organism’s water
content
– Osmotic pressure (of a solution) – a measure of its
tendency to take in water by osmosis
– Osmosis – the diffusion of water across a semipermeable membrane
• Always occurs from a more dilute solution to a less dilute
solution
• Higher concentration of solutes  more osmotic pressure
Osmoregulation
• To maintain osmotic balance, the extracellular
compartment of an animal’s body must be
able to take water from, and excrete excess
water, into the environment
• Exchanges of water and inorganic ions
(electrolytes) between the body and the
external environment occurs across
specialized epithelial cells, and, in most
vertebrates, through a filtration process in the
kidneys
Osmoregulation
kidney
Osmoregulation
• Homeostasis maintains a constant level of ions
in extracellular fluids
– Na+ - major cation
– Cl- – major anion
– Ca+2, Mg+2, and K+ – also very important
Osmoregulation
– Hypertonic solution – cell loses water (and shrinks);
more solutes in solution relative to cell
– Isotonic solution - no net water movement; equal
number of solutes relative to cell
– Hypotonic solution - cell gains water (expands);
fewer solutes relative to cell
REMEMBER, HIGHER CONCENTRATION OF
SOLUTES INCREASES OSMOTIC PRESSURE AND
THE TENDENCY TO TAKE IN WATER BY OSMOSIS
Osmoregulation
Turgor pressure in plants!
Osmoconformers
• In most marine invertebrates, the osmolarity
of their blood fluids is the same as that of
their external environment, seawater
– That is, their extra-cellular fluids are isotonic to
their external environment
– NO tendency for water to leave or enter the body
– An organism in osmotic equilibrium with its
environment is called an osmoconformer
Osmoconformers
Sodium
Magnesium
Calcium
Potassium
Chloride
Seawater
478.3
54.5
10.5
10.1
558.4
Jellyfish
474
53
10
10.7
580
Polychaete
476
54.6
10.5
10.5
557
Sea urchin
474
53.5
10.6
10.1
557
Mussel
474
52.6
11.9
12
553
Squid
456
55.4
10.6
22.2
578
Isopod
566
20.2
34.9
13.3
629
Crab
488
44.1
13.6
12.4
554
Lobster
541
9.3
11.9
7.8
552
Hagfish
537
18
5.9
9.1
542
Adapted from Potts and Parry 1964 as cited by J. Levinton 2009
Osmoregulators
• An organism that actively discharges water (in
a hypotonic environment) or takes in water (in
a hypertonic environment) is called an
osmoregulator
• Osmoregulators can maintain a relatively
constant blood osmolarity despite differences
in concentration of solutes in the surrounding
environment
Osmoregulators
• Examples…
– Freshwater vertebrates have much higher solute
concentration in their body fluids than in the
surrounding water (increase solutes increases
osmotic pressure; increased tendency to take in
water)
– Freshwater vertebrates must prevent water from
entering their bodies as much as possible and
actively transport ions back into their body
Osmoregulators
• More examples…
– Most marine vertebrates are hypotonic to their
environment; these animals are in danger of
losing water by osmosis
– Marine vertebrates must retain water and
eliminate excess ions via their kidneys and gills
– They retain water by drinking seawater (yum!)
Osmoregulators
• Even more examples…
– Terrestrial vertebrates have a higher concentration
of water than in the air in their surrounding
environment; these animals tend to lose water to
the air by evaporation from the skin and lungs
– These animals evolved osmoregulatory/urinary
systems to retain water
Nitrogenous Wastes
• In many animals, the removal of water and
ions is coupled with the removal of wastes
generated by metabolism
• Amino acids (proteins) and nucleic acids are
nitrogen-containing molecules
• When proteins and nucleic acids are
metabolized (catabolized for energy or
converted into carbohydrates or lipids), they
become nitrogenous wastes, that must be
eliminated from the body
Nitrogenous Wastes: Ammonia
• Nitrogenous wastes are toxic to the body and
must be removed
• Ammonia is formed in the liver by the
metabolism of amino and nucleic acids
• Ammonia is very toxic to cells and must be
present only in very dilute concentrations
• Bony fish drink seawater (marine) or have
excess water to remove (freshwater) and
therefore have very dilute urine, and so
excrete ammonia safely
Nitrogenous Wastes: Urea
• Urea and uric acid are less toxic to the
organism, and are eliminated by cartilagenous
fish, adult amphibians and mammals
• Urea is water-soluable and so can be excreted
in large amounts in the urine
• Urea is also synthesized in the liver; takes
energy to convert from ammonia
Nitrogenous Wastes: Uric Acid
• Uric acid is also less toxic, but is NOT watersoluable
• Birds, reptiles and insects excrete uric acid
• Uric acid does not require large amounts of water
to be excreted
• Uric acid is that “special something” you find on
your windshield; forms a pasty, white material
• Costs more to generate uric acid than urea, but
conserves water!
Nitrogeous Wastes: Uric Acid
• The insoluability of uric acid is very important
to reptiles and birds, which lay eggs
– Uric acid accumulates inside egg with developing
embryo, but precipitates and crystalizes
– Does not interfere with developing embryo; does
not dissolve in yolk
– Worth the extra cost of production
– Gout in humans – deposits of uric acid crystals in
joints ; kidney stones; inability to properly make
uric acid water soluable
Nitrogenous Wastes
Osmoregulatory Organs
• Single-celled organisms use contractile
vacuoles to rid wastes (freshwater protozoans
are constantly gaining water by osmosis)
• Invertebrates use specialized cells and tubules
– Flatworms (Platyhelminthes) use protonephridia
which branch into bulblike flame cells throughout
the body
• Cilia inside the flame cells draw in fluids from the body;
substances then excreted through pores
Osmoregulatory Organs
• Earthworms have nephridia - a system of
tubules that open to the inside and outside of
the body
• Fluid enters internal openings called
nephrostomes by filtration
• Tubules are lined with epithelial cells that
reabsorb essential salts
Osmoregulatory Organs
• Insects use malpighian tubules
– Extensions of the digestive tract
– Waste products are transferred into the tubules by
active transport
• Secretion of K+ creates an osmotic gradient that causes
water to enter the tubules by osmosis from the body’s
open circulatory system
• Most of the water and K is then reabsorbed into the
circulatory system via the epithelium of the hindgut,
leaving behind only small molecules and waste
products to be excreted from the rectum
Mammalian Kidneys
• The vertebrate kidney consists of many
nephrons, the filtering unit of the kidney
• In humans, the kidneys consist of a pair of fistsized organs located in the lower back
• The kidney receives bloom from a renal artery
and produces urine
• Urine drains from each kidney through a
ureter towards the urinary bladder (and into
the urethra)
Mammalian Kidneys
• The kidney has three basic functions:
– Filtration – fluid in the blood is filtered into the
tubule system, leaving behind cells and large
proteins in the blood and a filtrate composed of
water and all of the solutes from the blood
– Reabsorption – important solutes, such as
glucose, amino acids and inorganic ions, and
water are selectively moved out of the filtrate
back to the blood (reabsorbed)
– Secretion – substances from the blood are moved
into the tubule system ‘permanently’ (e.g., toxins)
The Human Kidney
Mammalian Kidneys: Filtration
• Each nephron consists of a long tubule and
associated blood vessels
• Blood is carried by an affarent arteriole to a
tuft of capillaries in the renal cortex, the
glomerulus
• Blood is filtered in the glomerulus as the
blood pressure forces fluid through porous
capillary walls
– Blood cells and large proteins are too large to
enter
Mammalian Kidneys: Filtration
• Large amounts of blood plasma, containing water
and dissolved molecules enter the nephron
tubules in a region known as Bowman’s capsule
• Bowman’s capsule envelopes the glomerulus
• Blood components that were not filtered drain
into an efferent arteriole, which empties into a
second bed of capillaries
• Filtrate then flows through the loop of Henle
(birds and mammals), which dips into the
medulla before ascending back into the cortex
Mammalian Kidneys: Filtration
• After leaving the loop of Henle, the fluid
drains into a collecting duct
– Merges with other collecting ducts to empty its
contents, now called urine, into the renal pelvis
– Filtration is a nonselective process with regard to
small molecules
– Filtrate initially consists of water, urea, salts,
glucose, vitamins, etc.
Mammalian Kidneys: Reabsorption
and Secretion
• Most of the water and dissolved solutes that
enter the glomerular filtrate must be returned
to the blood by reabsorption
• Reabsorption is the selective transport from
the filtrate back to the blood
• Sugars, vitamins, organic nutrients, and water
are reabsorbed (don’t want to lose!)
• Secretion is a very selective process
Mammalian Kidneys: Reabsorption
and Secretion
• Reabsorption of glucose and amino acids is
driven by active transport carriers
• LOTS of capillaries surround the loop of Henle
to readjust composition of filtrate
• Approximately 25x the volume of your body is
filtered each day
• Aquatic animals have short loops of Henle;
desert organisms have longer loops (conserve
water!)
Mammalian Kidneys: Reabsorption
and Secretion
• The function of the loop of Henle is to create a
gradient of increasing osmolarity from the cortex
to the medulla; allows water to be reabsorbed via
osmosis in the collecting duct as it runs down into
the medulla past the loop of Henle
• The descending and ascending limbs of the loop
of Henle differ structurally in regards to their
permeability to water
– Ascending limb is impermeable to water; descending
limb is thin and permeable
Kidneys: Hormonal Control
• Kidneys maintain relatively constant levels of
blood volume, pressure, and osmolarity
• These homeostatic functions of kidneys are
coordinated primarily by hormones
• Antidiuretic hormone (ADH) is released by the
pituitary gland when the solute concentration
of the blood rises
– Makes the epithelium of the distal tubules and the
collecting ducts more permeable to water;
stimulates water reabsorption from the collecting
duct (no ADH, no reabsorption in collecting duct)
Kidneys: Hormonal Control
• Alcohol inhibits ADH, dehydrates the body!
• … what a hangover is; the headache results
from the dehydration of your organ systems
• …why they say to drink water before you go to
bed after drinking
Osmotic balance
• The kidney appeared first in freshwater fish
• Fish, amphibian and reptile kidneys can only
produce a hypotonic (or isotonic) urine (no
loops of Henle)
• Marine bony fish must excrete electolytes and
retain water; most monovalent ions are
actively transported across gill surfaces;
divalent ions excreted in urine (isotonic urine)
Osmoregulation in marine fish
Osmotic balance
• Freshwater fish must retain electrolytes and
keep water out; freshwater fish do NOT drink
water and excrete a large volume of dilute
urine (hypotonic urine); reabsorb ions across
the nephron tubules and actively transport
ions across gill surfaces from the water into
their blood
Osmoregulation in freshwater fish
Osmotic balance
• Marine reptiles eliminate excess salt through
salt glands located near the nose or the eye
– “crocodile tears”
• Marine birds drink seawater and then excrete
excess salt through salt glands, which dribble
down the beak
Marine Bird Salt Gland
Salt glands in action (Southern fulmar)!
Marine Mammal Osmoregulation
• Marine mammals osmoregulate by not
drinking seawater and excreting highly
concentrated urine
Harbor seal kidney