water & salt physiology

intracellular fluids

fluids inside cells

extracellular fluids and it subparts

fluids outside of cells; divided into: interstitial fluids found between cells in ordinary tissues and blood plasma, the part of the blood other than the blood cells themselves

fluid compartments of the body

intracellular fluids, interstitial fluids, and blood plasma

importance of inorganic ions (such as salts or electrolytes)

• ionic composition of body fluids affects the crucial three-dimensional molecular conformations of enzymes and other proteins

• maintain correct electrical gradients across cell membranes

• play key roles in nerve-impulse transmission and muscle excitation

importance of water in body fluids

• it is the matrix in which ions are dissolved

• affects the volumes of cells and tissues

• osmotic movements of water affect hydrostatic pressures

relationship in body fluids

• blood plasma is typically very similar to interstitial fluid in osmotic pressure

• intracellular fluid insides cells has about the same osmotic pressure as the interstia

osmotic regulation (osmoregulation)

maintenance of a constant or nearly constant osmotic pressure in blood plasma

osmotic conformity

blood osmotic pressure always equals the osmotic pressure of the environmental water; marine invertebrates such as crabs, shrimp, and mussels are good examples

ionic regulation

the maintenance of a constant or nearly constant concentration of an inorganic ion in the blood plasma

ionic conformity

concentration of the ion in its blood plasma matches the concentration in its external environment

volume regulation

regulation of the total amount of water in a body fluid; applies to interstitial and intracellular fluids as well to the blood plasma

volume conformity

completely passive changes of body-fluid volume (e.g., driven by osmosis); occurs only rarely in animals

example of how osmotic regulation, ionic regulation, and volume regulation are related

freshwater fish that takes on a quantity of H2O from its dilute environment by osmosis; the influx will have three simultaneous effects:

1.) it will tend to lower the osmotic pressure of the fish’s blood plasma (which challenges osmotic regulation)

2.) it will tend to dilute ions in the fish’s blood plasma (which challenges ionic regulation)

3.) it will tend to increase the volume of water in the fish’s blood plasma and other body fluids (which challenges volume regulation)

functions of kidney

regulate composition of blood plasma by removing water, salts, and other solutes from the plasma (“filters waste”); help regulate fluid volume;

osmotic U/P ratio

ratio of osmotic pressure of urine over osmotic pressure of blood plasma; an index of the action of the kidneys in osmotic regulation

relative osmoticity

urine be isomotic, hyperosmotic, or hyposmotic to blood plasma

isomotic: U/P = 1

hypersomotic: U/P >1

hyposmotic: U/P <1

U/P = 1; isosmotic urine significance

osmolarity or urine equal to blood plasma (around 300 mOsm/L in mammals); water and solutes excreted in same proportion; no net change in blood plasma osmolarity

U/P <1; hyposmotic urine significance

urine more dilute than blood plasma; excess water is excreted while salts and solutes are reabsorbed into the body; plasma osmolarity raised by eliminating excess water

U/P >1; hyperosmotic urine significance

urine is more concentrated than blood plasma; solutes are excreted in high concentrations while water is preferentially reabsorbed back into the body; lower plasma osmolarity and conserves body water

how to hyperosmotic regulators freshwater teleosts compensate for the passive movements of solutes and water across the gills

• produce a large volume of hyposmotic (dilute) urine to compensate for water gain

• use chloride cells to exchange cations, where Na+ is transported into the animal and H+ is transported out into the environment

• use chloride cells to exhange anions, where Cl- is transported into the animal and HCO3 is transported out into the environment

a freshwater teleost acclimated to hard water with a high soute concentration transfered to soft water with alowre solute concentraion would see elevated or reduced demand for actions of mitochondrich rich ionocytes? would you expect their density in the gill epithelium to increase or decrease?

elevated; increased

how to marine teleosts, as hyposmotic regulators, compensate for the passive movement of water across the gills

drink sea water to acquire water and later actively transport the Na+ and Cl- out of their gills

what would occur in a euryhaline teleost as it migrates from freshwater to seawater

drink more water; produce isosmotic urine; dump ions into the water using ionocytes; upregulate NKCC cotransporters in the gills; upregulate CFTR in the gills

why would burning fat contribute to fluid homeostasis in zebra finches without access to food or water

water liberated during fat catabolism, metabolic water, is a source of water gain

movement of water

always moves to where the is more salt; salt always wants to diffuse toward where there is less salt

what issue do freshwater fish struggle with? how do they solve it?

water constantly invades fish’s body through its gills while salts leak out

they never drink water; they pee out a massive volume of very watery/dilute (hyposmotic) urine to get rid of excess water; they use special pumps in their gills (ionocytes) to actively pump salts like Na+ and Cl- out of the freshwater and into their blood

what issue do marine fish struggle with? how do they solve it?

water constantly flushed out of fish into salty water, leaving it deyhydrated

they drink seawater non-stop to avoid drying out; they use gill ionocytes to actively pump excess salts like Na+ and Cl- back into the ocean; their kidneys produce only a tiny trickle of urine to save water

extrarenal salt excretion

animals with weak kidneys that can’t concentrate urine well must use glands outside the kidney

marine teleost use gill ionocytes

sharks use a specialized rectal gland

marine birds have nasal salt glands

marine turtles have lachrymal salt glands behind eye (produce salty tears)

metabolic water

a major byproduct made when fat is broken down for energy

fat + O2 → CO2 + H2O

burning body fat serves dual purposes: gives energy and creates interval water for hydration (such as in zebra finch study)

glomerular filtration rate (GFR)

the rate at which the kidneys squeeze liquid out of the blood to start making urine

desert animals (like chuckwallas) need a low gfr so they conserve water; freshwater animals need a high gfr because they need to dump excess water

what’s filtered from kidney

small things like glucose, salts, and urea pass easily through kidney filter; large things like albumin (a blood protein) and red blood cells are too big and remain in the blood

antidiuretic hormone (ADH) when high and low

the body’s “water saver” hormone

when high: body is dehydrated; ADH opens aquaporins in the kidney’s collecting duct, pulling water back into the body and marking urine concentrated

when low: lots of water has been consumed; aquaporins remain closed, so water stays in the pipe, creating dilute (hyposmotic) urine

atrial natriuretic peptide (ANP)

does opposite of ADH; released when blood pressure/volume is too high to force kidneys to dump water/salt; turned off in when animal is dehydrated

hydrostatic pressure in kidneys

high blood pressure from the heart physically squeezes/filters fluid through a mesh filter

active solute transport in insects

insects lack high-pressure vessels and instead use Malpighian tubules that actively pump salt ions (K+, Na+) into the tubule using cellular energy, and water follows salt by osmosis

marine elasmobranch’s method to deal with seawater

store high concentrations of urea (plus TMAO to stabilize proteins) in their blood until their interal solute level matches or exceeds the ocean

water diffuses into them naturally, so no need to drink seawater; excess salt is excreted via rectal gland

how terrestrial animals deal with risk of dehydration

evaporative water loss is reduced by lipid/wax skin layer and cooling exhaled air

water generated internally as a byproduct of aerobic cellular respiration (kangaroo rats survive strictly on metabolic water from seeds without drinking liquid water)

produce hyperosmotic urine via long nephron loops; birds and reptiles excrete uric acid (past that uses very little water)