OSMOREGULATION &
EXCRETION
Chapter 44
OVERVIEW

Osmoregulation


Relative concentrations of water and solutes must be
maintained in a variety of environments (land,
freshwater, marine)
Excretion


Metabolism creates waste that must be expelled from
the body
Proteins and nucleic acids present a problem because
ammonia (primary waste product) is toxic
44.1 ~ OSMOREGULATION
Balancing the uptake and loss of water and
solutes over time. If they don’t osmosis will
cause animal cells to swell and burst or shrivel
and die.
 Isoosomotic = two solutions with the same
osmolarity
 Hyperosmotic = solution with the greater
concentration of solutes
 Hyposomotic = solution with the more dilute
concentration of solutes
 Water flows from a hyposomotic solution to a
hyperosmotic one.

TWO BASIC SOLUTIONS

#1 (only available to marine animals) is to be
isoosomotic with the environment.

Osmoconformer
Does not adjust its internal osmolarity
 Live in water that is fairly stable


#2 (available to any animal) is to control its
internal osmolarity because body fluids are NOT
isoosmotic with the outside environment.

Osmoregulator
Body fluids are not isoosomotic with surroundings
 Enables animals to live in diverse environments
 Has an energy cost (active transport of solutes)

COST OF OSMOREGULATION

Depends on:
How different osmolarity is from the surroundings
 How easily water & solutes move across the animal’s
surface
 How much work is required to pump solutes across
the membranes


Ranges from 5% to 30% of total resting metabolic
rate
STENOHALINE VS. EURYHALINE
Stenohaline – narrow salt; Most animals cannot
tolerate substantial changes in external
osmolarity
 Euryhaline – broad salt; Animals that can
survive large fluctuations in external osmolarity.


Examples: Salmon, Talapia
MARINE ANIMALS
Most marine invertebrates are osmoconformers
 Marine vertebrates and some invertebrates are
osmoregulators

Ocean is strongly dehydrating because it is much
saltier than than internal fluids and water is lost by
osmosis
 Balance water loss by drinking large amounts of
seawater
 Salt is actively pumped out of gills and passed
through urine

FRESHWATER ANIMALS
Problems are opposite those of marine animals
 Freshwater animals are constantly gaining water
by osmosis and losing salt by diffusion
 Maintain water balance by excreting large
amounts of very dilute urine and taking in salt
by the gills

TEMPORARY WATERS
Anhydrobiosis “life
without water” –
animals can survive in
a dormant state when
their habitats dry up
 Water bears
 Survive for a decade
or more in inactive
state

LAND ANIMALS
Desiccation “drying up” is a huge problem for
land animals
 Adaptations that help land animals avoid drying
up:







Waxy layers of insects exoskeleton
Shells of land snails
Layers of dead keratinized skin
Nocturnal
Drinking and eating moist foods
Using metabolic water (water produced during
cellular respiration)
44.2 ~ NITROGENOUS WASTES
When proteins and nucleic acids are broken down
nitrogenous wastes are produced
 When these macromolecules are broken apart for
energy ammonia (NH3) is produced which is very
toxic
 Three forms of ammonia that animals secrete

Ammonia
 Urea
 Uric acid

AMMONIA
Very soluble but only tolerated at low
concentrations so must be released in lots of
water
 Aquatic species (fish)

UREA
Substance produced in the vertebrate liver by
metabolic cycle that combines ammonia with
carbon dioxide
 System carries urea to kidneys where it is
excreted
 Mammals, adult amphibians, sharks, marine
bony fishes, turtles
 Advantage: low toxicity and requires less water
 Disadvantage: expend energy to produce it from
ammonia

URIC ACID
Largely insoluble in water and can be excreted as
a semi solid paste with very little water loss
 Insects, land snails, reptiles, birds
 Relatively non-toxic
 Requires considerable ATP to produce (more than
Urea)

EVOLUTION & ENVIRONMENT ON
WASTE
Uric acid can be stored within the reptilian egg
as a harmless solid left behind when the animal
hatches
 Type of waste produced by vertebrates depends
on habitat

44.3 ~ STEPS OF URINE
PRODUCTION
1. Filtration – body fluids are filtered to keep the
good stuff in the body fluids and put the bad stuff
in the filtrate
 2. Reabsorption – filtering the filtrate to make
sure none of the “good stuff” is kept in the filtrate
(active transport to reclaim valuable substances
in body fluids)
 3. Secretion – filtering the body fluid to make
sure all of the “bad stuff” is in the filtrate
 4. Excretion – filtrate leaves the body (urine)



Good stuff: cells, proteins, large molecules,
valuable solutes such as glucose,
Bad stuff: water, small solutes such as salts,
sugars, amino acids, and nitrogenous wastes.
Nonessential solutes and wastes
SURVEY OF EXCRETORY SYSTEMS

Protonephridia:
Flame-bulb system
Flatworms
 Functions in
osmoregulation
(wastes diffuse out
through body surface)

Metanephridia
 Annelids

Malpighian Tubules
 Insects and terrestrial
arthropods

Vertebrate Kidneys
 Function in osmoregulation and excretion

44.4 ~ MAMMALIAN KIDNEY
Site of water balance, salt regulation and
excretion
 Pair of kidneys
 Each 10 cm long (kidney bean shaped)
 Supplied with blood by a renal artery and
drained by a renal vein
 Urine exits through ureter and drains into
urinary bladder
 Urine is excreted from urinary bladder through
the urethra

STRUCTURE & FUNCTION OF
NEPHRON

2 regions to the kidney
Outer renal cortex
 Inner renal medulla

Functional unit of the kidney is the nephron –
consists of single long tubule and a ball of
capillaries called the glomerulus
 Bowman’s capsule – cup shaped swelling that
surrounds the glomerulus
 Each human kidney contains a million nephrons

Filtration happens as blood pressure forces fluid
from the blood in the glomerulus into the
Bowman’s capsule.
 Filtration is nonselective and filtrate contains:
salts, glucose, amino acids, vitamins, nitrogenous
wastes and other small molecules
 Pathway of filtrate – see figure 44.14

Between 1,100 to 2,000 L of blood flows through a
pair of human kidney’s each day.
 Nephrons process about 180 L of initial filtrate
 Nearly all sugar, vitamins, other organic
nutrients and about 99% of the water are
reabsorbed into the blood leaving only about 1.5
L of urine to be voided per day
 4 steps are completed in:






Proximal tubule
Descending limbo of the loop of Henle
Ascending limb of the loop of Henle
Distal tubule
Collecting duct
44.5 ~ WATER CONSERVATION IS A
KEY TO TERRESTRIAL ADAPTATION

As the filtrate flows in
the collecting duct
past interstitial fluid
of increasing
osmolarity, more
water moves out of the
duct by osmois,
thereby concentrating
the solutes, including
urea, that are left
behind in the filtrate.
REGULATION OF KIDNEY
FUNCTION
If a lot of salt is brought in with low water
availability the mammal can excrete urea and
salt with little water loss in hyperosmotic urine.
 If salt is scarce and fluid intake is high the
kidney can get rid of the excess water with little
salt loss by producing large volumes of
hypoosmotic urine
 Regulated through nervous and hormonal
controls

ADH – antidiuretic hormone
 RAAS – renin-angiotensin-aldosterone system
 ANF – atrial natriuretic factor

ADH is a response to an increase in the
osmolarity of the blood – when the body is
dehydrated.
 RAAS – responds when a situation that causes
an excessive loss of both salt and body fluids
(injury, severe diarrhea)

44.6 ~ DIVERSE ADAPTATIONS