Bio 112 Unit 4
Water and Electrolyte Balance in Animals
● Crucial for animal Homeostasis: maintaining water balance, electrolyte balance, and
excreting waste products
● Electrolyte: a compound that dissociates into ions when dissolved in water - cells
require precise concentrations
○ Most abundant in animals: Na+ , Cl-, K+, CA2+
● REVIEW DIFFUSION AND OSMOSIS ON YOUR OWN AS NEEDED - CHAPTER 6
● Tonicity: comparing two things - cells have a selectively permeable membrane
○ Movement of water -
○ Hypertonic: lots of high solute concentration, water leaver cells
○ Isotonic: equal; cell isotonic
○ Hypotonic: low; water enters the cell
● Animals gain water in four ways:
○ (1) absorbing via osmosis
○ (2) drinking
○ (3) eating
○ (4) as a by-product of cellular respiration
● Animals lose water via
○ Osmotic loss - a big deal if you’re an aquatic animal
○ Urine feces
○ Evaporation - a big deal if you’re a land organism
● These must be constantly balanced
Osmoregulation and Osmotic Stress
Osmoregulation
Osmoregulation- maintaining salt (ions) and water balance of body fluids
○ Osmoregulation Strategies (P 859)
■ Osmoconformers (some marine invertebrates like sponges,
jellyfish, flatworms, AND sharks!)
● Isotonic with their surroundings and do not regulate,
because seawater is consistent
● Sharks (marine vertebrates) use TMAO and rectal gland
■ Osmoregulators (marine, freshwater, or terrestrial - fish, most
animals incl. humans)
● Are hypotonic or hypertonic to their environment
● Must expend energy to control water uptake and loss
● Will focus on the details of these!
Osmoregulators - Marine
● They face potentially desiccating environments
, ○ With the potential to quickly deplete the body of water
● Seawater is hypertonic compared to its tissues, so they are constantly
losing water
● (and their tissues are hypotonic compared to seawater)- water wants to
come out of the fish - have to drink seawater in order to compensate for
what they are losing and the body will work hard to remove it
● Marine fishes (seawater) lose some electrolytes and water in urine
formation - very concentrated
● Gain some electrolytes by eating - replace water by drinking SW
● Lose water by osmosis across gills
● Fish gills are exposed to water constantly so much osmoregulation occurs
here (also shrimp cleaning fish gills)
● Most marine regulators are said to be stenohaline
○ Can only handle a narrow range of changes in salinity (so they
cannot go from SW to FW)
● Only a few marine osmoregulators are euryhaline
○ Can survive large fluctuations in salinity
○ Tilapia fish, local stingrays
Osmoregulators - Freshwater (gain to much water - have to lose it)
● Freshwater animals are in danger of gaining too much water
○ Lose salts by diffusion
● Freshwater is hypotonic compared to the fish tissues, so they are
constantly gaining water
● (and their tissues are hypertonic compared to freshwater)
● Freshwater fish gain lots of water by osmosis across gills
● Lose some electrolytes in the urine
● Lose water in urine formation - very dilute!
● Gain some electrolytes in food
● NO DRINKING
Osmoregulators - Terrestrial
● Constantly lose water to environment, just like the saltwater fish.
● Not by osmosis, but by evaporation - just like land plants
● Land animals must manage their water budgets by:
○ Drinking
○ Eating moist foods
○ Using metabolic water
● Lose water from other parts of the to evaporation
● Lose water from lungs during breathing
● Lose electrolytes in uring (some)
● Gain water by drinking and eating
● Gain electrolytes in foo
*metabolic water comes from cell respiration
Water and Electrolyte Balance in Animals
● Crucial for animal Homeostasis: maintaining water balance, electrolyte balance, and
excreting waste products
● Electrolyte: a compound that dissociates into ions when dissolved in water - cells
require precise concentrations
○ Most abundant in animals: Na+ , Cl-, K+, CA2+
● REVIEW DIFFUSION AND OSMOSIS ON YOUR OWN AS NEEDED - CHAPTER 6
● Tonicity: comparing two things - cells have a selectively permeable membrane
○ Movement of water -
○ Hypertonic: lots of high solute concentration, water leaver cells
○ Isotonic: equal; cell isotonic
○ Hypotonic: low; water enters the cell
● Animals gain water in four ways:
○ (1) absorbing via osmosis
○ (2) drinking
○ (3) eating
○ (4) as a by-product of cellular respiration
● Animals lose water via
○ Osmotic loss - a big deal if you’re an aquatic animal
○ Urine feces
○ Evaporation - a big deal if you’re a land organism
● These must be constantly balanced
Osmoregulation and Osmotic Stress
Osmoregulation
Osmoregulation- maintaining salt (ions) and water balance of body fluids
○ Osmoregulation Strategies (P 859)
■ Osmoconformers (some marine invertebrates like sponges,
jellyfish, flatworms, AND sharks!)
● Isotonic with their surroundings and do not regulate,
because seawater is consistent
● Sharks (marine vertebrates) use TMAO and rectal gland
■ Osmoregulators (marine, freshwater, or terrestrial - fish, most
animals incl. humans)
● Are hypotonic or hypertonic to their environment
● Must expend energy to control water uptake and loss
● Will focus on the details of these!
Osmoregulators - Marine
● They face potentially desiccating environments
, ○ With the potential to quickly deplete the body of water
● Seawater is hypertonic compared to its tissues, so they are constantly
losing water
● (and their tissues are hypotonic compared to seawater)- water wants to
come out of the fish - have to drink seawater in order to compensate for
what they are losing and the body will work hard to remove it
● Marine fishes (seawater) lose some electrolytes and water in urine
formation - very concentrated
● Gain some electrolytes by eating - replace water by drinking SW
● Lose water by osmosis across gills
● Fish gills are exposed to water constantly so much osmoregulation occurs
here (also shrimp cleaning fish gills)
● Most marine regulators are said to be stenohaline
○ Can only handle a narrow range of changes in salinity (so they
cannot go from SW to FW)
● Only a few marine osmoregulators are euryhaline
○ Can survive large fluctuations in salinity
○ Tilapia fish, local stingrays
Osmoregulators - Freshwater (gain to much water - have to lose it)
● Freshwater animals are in danger of gaining too much water
○ Lose salts by diffusion
● Freshwater is hypotonic compared to the fish tissues, so they are
constantly gaining water
● (and their tissues are hypertonic compared to freshwater)
● Freshwater fish gain lots of water by osmosis across gills
● Lose some electrolytes in the urine
● Lose water in urine formation - very dilute!
● Gain some electrolytes in food
● NO DRINKING
Osmoregulators - Terrestrial
● Constantly lose water to environment, just like the saltwater fish.
● Not by osmosis, but by evaporation - just like land plants
● Land animals must manage their water budgets by:
○ Drinking
○ Eating moist foods
○ Using metabolic water
● Lose water from other parts of the to evaporation
● Lose water from lungs during breathing
● Lose electrolytes in uring (some)
● Gain water by drinking and eating
● Gain electrolytes in foo
*metabolic water comes from cell respiration
