Review Sheet Ch. 44 Osmoregulation and Excretion
1) Osmoregulation is homeostasis of body water content and total solute concentations. A solute is any
substance dissolved in water. In osmotic relations, it does not matter what the identity of the solutes
are, just the total concentration. For example, a red blood cell will shrivel if placed in a high sodium
chloride solution. It will shrivel in a similar way if placed in a concentrated solution of sucrose,
sodium acetate, etc.
2) Osmoregulation avoids changes to cell size and shape (e.g. avoid swelling or shrivelling due to
osmotic pressures). It also regulates the solute concentration in cells.
3) Among marine animals, most, (e.g. crustaceans), are osmoconformers and have body fluids with
solute concentrations just as high as sea water. Others, like vertebrate fish, are osmoregulators and
maintain lower solute concentrations in their body fluids than in the seawater. All freshwater animals
are osmoregulators and maintain higher solute concentrations than their environment. Apparently
body fluids as dilute as freshwater are not possible in a living organism.
4) Osmoregulation is normally achieved by active transport. Proteins in cell membranes pump solutes
using ATP energy. There is a unique transport protein for each type of particle. Curiously, no
organism has ever developed a transport protein for water.
5) Marine fish drink ocean water to get the water they need. To maintain an osmotic gradient between
their tissues and the ocean water, these fish pump salts out of their gills and into the ocean water.
Their urine is about isosmotic with their body fluids, and thus urination has primarily an excretory, not
osmoregulatory, function.
6) Freshwater fish and any other freshwater organisms like paramecium have a tendency to bloat because
osmotic pressure tends drive water into their cells and body fluids. In both fish and paramecium, the
solution is similar. Fish produce a copious, watery urine. In a day, a typical freshwater fish will
urinate a third of its body weight in urine. Likewise, a paramecium collects excess water in its
contractile vacuole and disposes of it.
7) Osmoregulation is concerned with relative amounts of water and solutes (of any kind) in the body.
Excretion is a related function, but is concerned with the movement of particular toxins out of the
body.
8) Transport epithelial cells are very important for both osmoregulation and excretion. Examples are in
the gills of marine fish, salt glands in albatross nostrils, secretory cells of kidneys.
9) Any chemical can be toxic, if its concentration in the body fluids is high enough. Chemicals described
as “toxins”are toxic in relatively low concentrations. One large source of toxins in most animals is the
breakdown of amino acids and nucleic acids, which releases nitrogenous wastes that are relatively
toxic, compared to other metabolic products. Getting rid of nitrogenous wastes is usually a high
priority for most animals. Most nitrogenous wastes are initially produced as ammonia.
10) Aquatic species have access to large amounts of water, allowing them to flush ammonia easily from
their bodies.
11) Terrestrial species usually have limited access to water. This has imposed selective pressure to expel
wates using less water. Many terrestrial animals and osmoregulating marine animals (mammals, adult
amphibians, sharks, some bony fish and turtles) usually convert ammonia to a urea, which can be
tolerated in higher concentrations. This allows the animal to expel the nitrogenous waste using a
smaller amount of water.
12) Other animals, such as reptiles (including birds) and insects, convert ammonia to uric acid. Like urea,
uric acid is less toxic than ammonia. It is not soluble in water, and is excreted as a white paste (e.g.
bird droppings).
13) Urine is basically an extract of the blood plasma. The formation of urine has four general steps:
Filtration, pressure-filtering of body fluids producing a filtrate. Reabsorption, reclaiming valuable
solutes from the filtrate. Secretion, addition of toxins and other solutes from the body fluids to the
filtrate. Excretion, the filtrate leaves the system.
14) Note that filtration is non-selective. This means that the “pre-urine” filtrate has the same relative
concentrations of solutes as the blood plasma, excepting molecules too large to be filtered out of the
plasma. This is why reabsorption and secretion are necessary.
15) The mammalian excretory system centers on paired kidneys. Is supplied with blood by a renal artery
and drained by a renal vein.
16) The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla.
17) The nephron, the functional unit of the vertebrate kidney. It consists of a single long tubule and a ball
of capillaries called the glomerulus.
18) Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of
Bowman’s capsule.
19) Filtration of small molecules is nonselective and the filtrate in Bowman’s capsule is a mixture that
mirrors the concentration of various solutes in the blood plasma.
20) From Bowman’s capsule, the filtrate passes through three regions of the nephron: The proximal
tubule, the loop of Henle, and the distal tubule. Secretion and reabsorption of various substances
occurs in these tubules.
21) Urine from several nephrons flow into a collecting duct. Urine exits each kidney through a duct called
the ureter. Both ureters. drain into a common urinary bladder.
22) The kidney function not just as excretory organs, but also are important for osmoregulation.
Countercurrent exchange of salts in the loop of Henle create a concentration gradient in the renal
medulla relative to the renal cortex. This high concentration of salts in the medulla allows the urine to
have a higher salt concentration than the blood. This allows the body to conserve water, even when
drinking water saltier than the body’s own interstitial fluid.
23) The ability of the kidneys to create a concentration gradient is greater when the loops of Henle are
longer. Kangaroo rats and dromedary camels have relatively longer loops of Henle than humans, for
example. These desert animals are capable producing urine more concentrated than seawater.
Although they don’t usually drink seawater, they could gain hydration by drinking seawater. The most
concentrated urine that a human can produce is less concentrated than seawater. Thus, when a human
drinks sea water, more water is used flushing out the salt than is contained in the ingested sea water.
This is why you will dehydrate more quickly drinking seawater than drinking nothing at all. If you are
a miner trapped in a cave-in and have no access to drinking water, will you survive longer if you drink
your urine? Why or why not?