Chapter 42: water and electrolyte balance in animals
An electrolyte is a compound that dissociates into ions when dissolved in water.
In humans electrolyte imbalance lead to muscle spasms, confusions, irregular heartbeat, paralysis or
Animal gain water by 4 ways:
Absorbing it via osmosis
By product of cellular respiration
Lose water by:
42.1 Osmoregulation and osmotic stress
Diffusion is the movement of substances from regions of higher concentration to regions of lower
concentration, along their concentration gradient (Figure 42.1a).
Osmosis is the diffusion of water through a selectively permeable membrane from areas of higher
water concentration to areas of lower water concentration (Figure 42.1b).
The concentration of dissolved substances in a solution, measured in moles per liter, is referred to as
When solutes are randomly distributed throughout the solutions on both sides of a membrane,
an equilibrium is established in which molecules continue to move back and forth across the
membrane, but at equal rates.
Achieving homeostasis with respect to water and electrolytes is straightforward in marine invertebrates
such as sponges and jellyfish.
Seawater is a fairly constant ionic and osmotic environment and nearly matches the electrolyte
concentrations found within these animals.
Relative to seawater, their tissues are isotonic. Such animals are called osmoconformers.
Marine fish are osmoregulators—they actively regulate osmolarity inside their bodies to achieve
homeostasis (Figure 42.2a).
Osmoregulation is the process by which living organisms control the concentration of water and salt in
Osmoregulation is required in marine vertebrates because their tissues are hypotonic relative to salt
water—the solution inside the body contains fewer solutes than the solution outside.
Freshwater fish lose salt and gain water (Figure 42.2b).
Because the gill epithelium of freshwater fish is hypertonic relative to the surrounding water—meaning
that the solution inside the cells contains more solutes than the solution on the outside—the epithelial
cells gain water through osmosis.
Osmotic stress occurs when the concentration of dissolved substances in a cell or tissue is abnormal. Depending on environmental conditions, terrestrial animals may need to conserve or to excrete
electrolytes to maintain homeostasis (Figure 42.3).
42.2 Water and electrolyte balance in aquatic environments
Because sharks must excrete salt and take in water, they have been used as a model organism in
 of certain lectrolytes in their cells and extra cellular fluid specifically sodium ions, K ions and chloride
ions are much less than their  in seawater.
Shark blood still isotonic to seawater because it contains large quantities of soluble compounds such as
urea and TMAO. Combination  of urea and TMAO high enpugh tot match osmotic  of seawater and
prevent water loss from osmosis.
Even though shark tissues are isotonic to seawater, sodium ions diffuse into the gills along a
concentration gradient (Figure 42.4).
Proteins and processes responsible for transporting salt also occur in wide range of other marine
Salt excretions in sharks uncovered a general concept In animal and land physiology. In mnay cases,
cells do not transport ions against their electrochem gradient directly, instead cells move ions indirectly. Done by active transport to set up strong electrochem gradient NA in animals H in plants. Na or H
gradient used to transport array of substances w/out further expenditure of energy.
How do sharks osmoregulate?
The shark rectal gland secretes a concentrated salt solution.
Ions can be concentrated in this way only if they are actively transported against a concentration
Solutes move across membranes by passive or active transport.
Passive transport often occurs via channels or by transmembrane proteins that act as carriers.
Active transport requires membrane proteins that act as pumps and energy in the form of ATP.
Role of Na/K ATPase
Epithelial cells along the inner surface, or lumen, of the shark rectal gland contain sodium–potassium
pumps (Na /K –ATPase).
To test that sharks hav the pump biologists isnrted radioactive plant defence compound called ouabin,
which poisons the pump. Toxic to animals because binds to the pump and prevents it from fcning.
The location of the pumps is paradoxical because they are found along the basolateral membrane of
the cells and not on the apical membrane (Figure 42.5).
Paradox arose because sodium would be pumped opposite to the direction it is secreted. Molecular model for salt excretion
The current molecular model for salt excretion in the shark is as follows (Figure 42.6):
Na /K –ATPase pumps sodium ions out of epithelial cells across the basolateral surface and into the
surrounding extracellular fluid. The pump creates an electrochemical gradient favoring the diffusion of
Na into the cell and K out of the cell.
+ – + +
A Na /Cl /K cotransporter, powered by the gradient favoring Na diffusion into the cell, brings these
three ions from the extracellular fluid into epithelial cells across their basolateral surfaces.
Although Na /K –ATPase pumps sodium ions back out, K and Cl concentrations build up inside the
cell as a +esult of the cotransport process. A potassium channel located on the basolateral membrane
allows K to diffuse back across the basolateral surface after it has been pumped in. A chloride channel
located in the apical membrane of the epithelial cells allows Cl to diffuse down its concentration
gradient into the lumen of the gland.
Sodium ions also diffuse into the lumen of the gland, following their charge and concentration gradient.
But instead of passing through the epithelial cells as Cl does, Na diffuses out through spaces between
A common molecular mechanism underlies many instances of salt excretion
The same combination of membrane proteins is found in epithelial cells that transport sodium and
chloride ions in many animals.
Marine birds and reptiles drink saltwater and must excrete NaCl. They have salt–excreting glands in
their nostrils that function much like the shark rectal gland.
Because marine fish with bony skeletons are hypertonic to seawater, salt constantly diffuses in through
their gills, which contain specialized chloride cells configured precisely like the cells lining the shark
Cells with the same configuration of pumps, cotransporters, and channels are responsible for
transporting salt in the kidneys of mammals. How do salmon osmoregulate?
Many species of salmon are anadromus meaning young develop from eggs laid in freshwater, then
migrate to the ocean where they spend several years feeding and growing and return to freshwater to
There is a significant increase in Na /K ATPase activity in the gills of young chum salmon preparing to
migrate to salt water, as well as a dramatic increase in the number of chloride cells in the gills.
In contrast, most of the chloride cells observed in freshwater chum are located in the sheetlike lamellae
that extend from the base of gill filaments (Figure 42.7).
Chloride cells in the gill lamellae import electrolytes, and chloride cells at the base of the gills secrete
Both cells contain sodium potassium pumps, but they operate in opposite directions.
42.3 water and electrolyte balance in terrestrial insects
The desert locust has been used as a model organism to study the molecular mechanisms of water and
electrolyte balance in terrestrial insects.
Rarely drink water because little or no water is available in environments they live in.
How do insects minimize water loss from body surface?
Water loss is an inevitable by–product of respiration.
Insects have a relatively large surface area with which to lose water and a small volume in which to
retain it. Gas exchaine occurs across membrane of epithelial cells that link an extensive system of tubes
An insect’s trachea connects with the atmosphere through openings called spiracles.
Muscles just inside each spiracle close or open the pore.
Exoskeleton of the insect consist of tough layer of chitin and layers of protein called cuticle. Highly
hydrophobic and impermeable to water. Insects minimize evaporation from their body surfaces by covering the exoskeleton with a layer of wax,
which is highly impermeable to water.
Types of nitrogenous wastes : impact on water balance
Animal cells contain aa and na used to synthesize proteins, RNA and DNA. Aa and na are nitrogenous
Ammonia (NH3) is a by–product of catabolic reactions. Ammonia is a strong base; it readily gains a
proton to form an ammonium ion (NH4 ). Toxic to cells because at high  rasies pH of intarcellular
and extracellular fluids enough to poison enzymes.
Animals that excrete ammonia directly usually lose a lot of water. Fish detoxify ammonia by diluting it
to a low concentration in watery urine.
Humans convert ammonia to less toxic urea and excrete it in urine. Urea excretion requires water, but
not as much as ammonia excretion.
Birds, reptiles, and terrestrial arthropods convert ammonia to uric acid. Because uric acid is very
insoluble in water, it can be excreted as a dry paste.
Table 42.1 shows the nitrogenous wastes produced by animals.
Production of urea and uric acid is often inrepeted as an adaptation that allows animals to cope with dry
habitats. There is fitness tradeofss between energetic cost if excreting urea or uric acid and the benefit of
Maintaining Homeostasis: excretory system
To maintain homeostasis, insects must carefully regulate the composition of a bloodlike fluid
Heart pumps hemolympg through vessels and into body cavity. Hemolymph bathes tissue directly.
Nutrients pass from the hemolymph into cells.
Waste products such as ammonia diffuse out of cells and into the hemolymph.
Hemolymph also contains a wide variety of electrolytes.
Insects also rely on Malphighian tubules, an excretory organ, and the hindgut—the posterior portion
of their digestive tract. Preurine formation in malighian tubules
Pre–urine forms in the Malpighian tubules (Figur