The document discusses osmoregulation and excretion in animals, explaining how different organisms balance water and solute levels. It describes the mechanisms that freshwater and marine animals use to regulate osmolarity, such as drinking seawater or excreting dilute urine. The kidney is highlighted as a major organ of osmoregulation that adapts its function depending on an animal's aquatic or terrestrial habitat.

1. Osmoregulation
and Excretion
Dr. Kristen Walker

2. Osmoregulation balances the uptake and loss
of water and solutes
• Relative concentrations of water and solutes
must be maintained within fairly narrow limits
• Osmoregulation is based largely on controlled
movement of solutes between internal fluids
and the external environment
• Excretion gets rid of nitrogenous metabolites
and other waste products

3. Osmosis and Osmolarity
• Cells require a balance between uptake and
loss of water
• Osmolarity, the solute concentration of a
solution, determines movement of water
across a selectively permeable membrane
• If two solutions are isoosmotic, the movement
of water is equal in both directions
• If two solutions differ in osmolarity, the net
flow of water is from the hypoosmotic to the
hyperosmotic solution

4. Figure 44.2
Selectively permeable
membrane
Solutes Water
Hyperosmotic side: Hypoosmotic side:
• Higher solute • Lower solute
concentration concentration
• Lower free H2O • Higher free H2O
concentration concentration
Net water flow

5. Osmotic Challenges
• Osmoconformers, consisting only of some
marine animals, are isoosmotic with their
surroundings and do not regulate their
osmolarity
– marine invertebrates
• Osmoregulators expend energy to control
water uptake and loss in a hyperosmotic or
hypoosmotic environment
– marine vertebrates and some invertebrates

6. Marine Animals
• Marine bony fishes are hypoosmotic to
seawater
– lose water by osmosis and gain salt by
diffusion and from food
– balance water loss by drinking seawater and
excreting salts

7. Figure 44.3a
(a) Osmoregulation in a marine fish
Gain of water Excretion Osmotic water
and salt ions of salt ions loss through gills
from food from gills and other parts
of body surface
Gain of water Excretion of salt ions and Key
and salt ions small amounts of water in
Water
from drinking scanty urine from kidneys
seawater Salt

8. Freshwater Animals
• Freshwater animals constantly take in water by
osmosis from their hypoosmotic environment
• lose salts by diffusion and maintain water
balance by excreting large amounts of dilute
urine
• Salts lost by diffusion are replaced in foods and
by uptake across the gills

9. Figure 44.3b
(b) Osmoregulation in a freshwater fish
Gain of water Uptake of Osmotic water
and some ions salt ions gain through
in food by gills gills and other
parts of body
surface
Key Excretion of salt ions and
large amounts of water in
Water dilute urine from kidneys
Salt

10. Land Animals
• Body coverings of most terrestrial animals help
prevent dehydration
• Desert animals get major water savings from
simple anatomical features and behaviors such
as a nocturnal lifestyle
• Land animals maintain water balance by eating
moist food and producing water metabolically
through cellular respiration

11. Figure 44.6
Water balance in Water balance in
a kangaroo rat a human
(2 mL/day) (2,500 mL/day)
Ingested Ingested
in food (0.2) in food (750)
Ingested
in liquid
Water (1,500)
gain
(mL)
Derived from Derived from
metabolism (1.8) metabolism (250)
Feces (0.09) Feces (100)
Water Urine Urine
loss (0.45) (1,500)
(mL)
Evaporation (1.46) Evaporation (900)

12. Energetics of Osmoregulation
• Osmoregulators must expend energy to
maintain osmotic gradients
• The amount of energy differs based on
– How different the animal’s osmolarity is from
its surroundings
– How easily water and solutes move across
the animal’s surface
– The work required to pump solutes across the
membrane

13. Transport Epithelia in Osmoregulation
• Animals regulate the solute content of body
fluid that bathes their cells
• Transport epithelia are epithelial cells that
are specialized for moving solutes in specific
directions
• They are typically arranged in complex tubular
networks
– e.g., nasal glands of marine birds, which remove
excess sodium chloride from the blood

14. Figure 44.7
Secretory cell Lumen of
of transport secretory
Vein Artery
Nasal salt epithelium tubule
gland
Ducts
Nasal gland
Salt
Capillary ions
Secretory tubule
Transport
Nostril with salt
epithelium
secretions
(a) Location of nasal glands (b) Secretory
in a marine bird tubules
Blood flow Salt secretion
Key (c) Countercurrent
exchange
Salt movement
Blood flow
Central duct

15. An animal’s nitrogenous wastes reflect its
phylogeny and habitat
• Type and quantity of an animal’s waste
products may greatly affect its water balance
• Among the most significant wastes are
nitrogenous breakdown products of proteins
and nucleic acids
• Some animals convert toxic ammonia (NH3)
to less toxic compounds prior to excretion

16. Figure 44.8
Proteins Nucleic acids
Amino Nitrogenous
acids bases
—NH2
Amino groups
Most aquatic Mammals, most Many reptiles
animals, including amphibians, sharks, (including birds),
most bony fishes some bony fishes insects, land snails
Ammonia Urea Uric acid

17. Forms of Nitrogenous Wastes
• Animals excrete nitrogenous wastes in different
forms: ammonia, urea, or uric acid
• These differ in toxicity and the energy costs of
producing them

18. Ammonia
• Animals that excrete nitrogenous wastes as
ammonia need access to lots of water
• They release ammonia across the whole body
surface or through gills

19. Urea
• The liver of mammals and most adult
amphibians converts ammonia to the less toxic
urea
• The circulatory system carries urea to the
kidneys, where it is excreted
• Conversion of ammonia to urea is energetically
expensive; excretion of urea requires less
water than ammonia

20. Uric Acid
• Insects, land snails, and many reptiles, including
birds, mainly excrete uric acid
• Uric acid is relatively nontoxic and does not
dissolve readily in water
• It can be secreted as a paste with little water
loss
• Uric acid is more energetically expensive to
produce than urea

21. Excretory Processes
• Most excretory systems produce urine by
refining a filtrate derived from body fluids
• Key functions of most excretory systems
– Filtration: Filtering of body fluids
– Reabsorption: Reclaiming valuable solutes
– Secretion: Adding nonessential solutes and
wastes from the body fluids to the filtrate
– Excretion: Processed filtrate containing
nitrogenous wastes, released from the body

22. Figure 44.10
1 Filtration
Capillary
Filtrate
Excretory
tubule
2 Reabsorption
3 Secretion
Urine
4 Excretion

23. Malpighian Tubules
• In insects and other terrestrial arthropods,
Malpighian tubules remove nitrogenous
wastes from hemolymph and function in
osmoregulation
• Insects produce a relatively dry waste matter,
mainly uric acid, an important adaptation to
terrestrial life
• Some terrestrial insects can also take up water
from the air

24. Figure 44.13
Digestive tract
Rectum
Hindgut
Intestine
Midgut Malpighian
(stomach) tubules
Salt, water, and
Feces To anus
nitrogenous
and urine
wastes
Malpighian
tubule
Rectum
Reabsorption
HEMOLYMPH

25. Kidneys
• Kidneys, the excretory organs of vertebrates,
function in both excretion and osmoregulation

26. Figure 44.14-a
Excretory Organs Kidney Structure Nephron Types
Cortical Juxtamedullary
Renal nephron nephron
Posterior cortex
vena cava Renal
medulla
Renal artery
Renal Kidney
artery
and vein Renal vein
Renal
cortex
Aorta
Ureter Ureter
Renal
Urinary medulla
bladder
Urethra Renal pelvis

27. Figure 44.14-b
Nephron Organization
Afferent arteriole
from renal artery Glomerulus
Bowman’s
capsule
Proximal
tubule
Peritubular
Distal
capillaries
tubule
Efferent
arteriole
from
glomerulus
Branch of
renal vein
Descending
limb
Loop
Vasa
of
Collecting recta
Henle
200 µm
duct
Ascending
limb Blood vessels from a human
kidney. Arterioles and peritubular
capillaries appear pink; glomeruli
appear yellow.

28. The nephron is organized for stepwise
processing of blood filtrate
• The filtrate produced in Bowman’s capsule
contains salts, glucose, amino acids, vitamins,
nitrogenous wastes, and other small molecules

29. From Blood Filtrate to Urine: A Closer Look
Proximal Tubule
• Reabsorption of ions, water, and nutrients takes
place in the proximal tubule
• Molecules are transported actively and passively
from the filtrate into the interstitial fluid and then
capillaries
• Some toxic materials are actively secreted into the
filtrate
• As the filtrate passes through the proximal tubule,
materials to be excreted become concentrated

30. Figure 44.15
Proximal tubule Distal tubule
NaCl Nutrients H2O
HCO3− H2O K+ NaCl HCO3−
H+ NH3 K+ H+
Filtrate
CORTEX
Loop of
Henle
NaCl
H2O
OUTER NaCl
MEDULLA
Collecting
duct
Key Urea
Active transport NaCl H2O
Passive transport
INNER
MEDULLA

31. Descending Limb of the Loop of Henle
• Reabsorption of water continues through
channels formed by aquaporin proteins
• Movement is driven by the high osmolarity of
the interstitial fluid, which is hyperosmotic to the
filtrate
• The filtrate becomes increasingly concentrated

32. Ascending Limb of the Loop of Henle
• In the ascending limb of the loop of Henle, salt
but not water is able to diffuse from the tubule
into the interstitial fluid
• The filtrate becomes increasingly dilute

33. Distal Tubule
• The distal tubule regulates the K+ and NaCl
concentrations of body fluids
• The controlled movement of ions contributes
to pH regulation

34. Collecting Duct
• The collecting duct carries filtrate through the
medulla to the renal pelvis
• One of the most important tasks is reabsorption
of solutes and water
• Urine is hyperosmotic to body fluids

35. Solute Gradients and Water Conservation
• The mammalian kidney’s ability to conserve
water is a key terrestrial adaptation
• Hyperosmotic urine can be produced only
because considerable energy is expended to
transport solutes against concentration
gradients
• The two primary solutes affecting osmolarity are
NaCl and urea

36. Adaptations of the Vertebrate Kidney to
Diverse Environments
• The form and function of nephrons in various
vertebrate classes are related to requirements
for osmoregulation in the animal’s habitat

37. Mammals
• Mammals that inhabit dry environments have
long loops of Henle, while those in fresh water
have relatively short loops

38. Birds and Other Reptiles
• Birds have shorter loops of Henle but
conserve water by excreting uric acid instead
of urea
• Other reptiles have only cortical nephrons but
also excrete nitrogenous waste as uric acid

39. Freshwater Fishes and Amphibians
• Freshwater fishes conserve salt in their distal
tubules and excrete large volumes of dilute
urine
• Kidney function in amphibians is similar to
freshwater fishes
• Amphibians conserve water on land by
reabsorbing water from the urinary bladder

40. Marine Bony Fishes
• Marine bony fishes are hypoosmotic compared
with their environment
• Their kidneys have small glomeruli and some
lack glomeruli entirely
• Filtration rates are low, and very little urine is
excreted

41. Hormonal circuits link kidney function,
water balance, and blood pressure
• Mammals control the volume and osmolarity of
urine
• Osmolarity of the urine is regulated by nervous
and hormonal control
• Antidiuretic hormone (ADH) makes the
collecting duct epithelium more permeable to
water
• An increase in osmolarity triggers the release of
ADH, which helps to conserve water

42. • Mutation in ADH production causes severe
dehydration and results in diabetes insipidus
• Alcohol is a diuretic as it inhibits the release of
ADH

43. Figure 44.UN01a
Animal Inflow/Outflow Urine
Freshwater Does not drink water Large volume
fish. Lives in Salt in H2O in of urine
water less (active trans-
concentrated port by gills) Urine is less
than body concentrated
fluids; fish than body
tends to gain fluids
water, lose salt
Salt out

44. Figure 44.UN01b
Animal Inflow/Outflow Urine
Marine bony Drinks water Small volume
fish. Lives in Salt in H2O out of urine
water more
concentrated Urine is
than body slightly less
fluids; fish concentrated
tends to lose than body
water, gain salt fluids
Salt out (active
transport by gills)

45. Figure 44.UN01c
Animal Inflow/Outflow Urine
Terrestrial Drinks water Moderate
vertebrate. Salt in volume
Terrestrial (by mouth) of urine
environment;
tends to lose Urine is
body water more
to air concentrated
than body
H2O and fluids
salt out