Osmoregulation and Urinary System.
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Osmoregulation is balancing of water and solute concentrations in
body fluids.
Basis is controlled movement of solutes between internal fluids and
external environment. This process also indirectly regulates water
movement as water follows solutes via osmosis. This process also
allows an organism to remove harmful waste products derived from
metabolic processes.
Osmosis:
 All animals have to balance rate of water uptake and loss as they have
no cell walls and will burst or shrink if they take up or lose too much
water respectively.
 Osmosis is the movement of water across a selectively permeable
membrane from regions of high concentration to regions of lower
concentration.
 Osmolarity is the solute concentration expressed as the number of
moles of solute per liter of solution. (mosm/L).
 Human blood = 300 mosm/l and seawater = 1,000mosm/L.
 Higher solute concentration = hyperosmotic solution.
 Lower solute concentration = hypoosmotic solution.
Two ways to balance water gain with water loss:
a) Osmoconformers
-Mainly marine invertebrates (lacking a notochord. In order to protect
themselves, they may have evolved a shell or a hard exoskeleton, but this is
not always the case). They have an internal osmolarity that is the same as
their external environment. The ions vary but OVERALL the osmolarity is
the same inside and outside the organism.
Examples of Osmoconformers:
Bryozoa, also known as moss animals or sea mats;
Cnidaria, such as jellyfish, sea anemones and corals;
Crustaceans, such a such as lobsters, crabs, shrimp, crayfish and barnacles;
Ctenophora: sea worms including flatworms, ribbon worms, annelids,
Sipuncula, Echiura, Chaetognatha, and the phoronids;
Echinoderms, including starfish, brittle stars, sea urchins, sand dollars, sea
cucumbers, and crinoids;
Mollusca, including shellfish, squid, octopus;
Sponges;
Tunicates, also known as sea squirts.
b) Osmoregulators
- Must control the internal osmolarity as internal fluids are NOT isotonic
with the external environment. If in a hypoosmotic environment, then needs
to expel excess water and if in a hyperosmotic environment, then needs to
take in water to offset water loss. Allows animals to live in environments
uninhabitable for osmoconformers such as freshwater and terrestrial
habitats. BUT high energy cost as diffusion tends to equalize
concentrations, so need to spend energy to maintain concentration gradients
to allow water to move in or out. Use ACTIVE TRANSPORT to manipulate
solute concentrations in body fluids. Ex. Brine shrimp in very salty lake 30%
of resting metabolic rate spent on osmoregulation.
Whether an osmoconformer or osmoregulator if the animal can handle large
fluctuations in external environment = Euryhaline (broad, salt) and if NOT
then is a Stenohaline (narrow, salt).
Euryhaline – Salmon, Tilapia, Eels, European shore crab and others that live
in estuaries etc or their life cycles involve movement between freshwater
and seawater environments.
Stenohaline – Most freshwater (Goldfish) and most marine organisms
(Haddock).
Adaptations of Osmoregulation in Marine, Freshwater and Terrestrial
animals.
a) Marine animals:
- Most of the animal phyla in the sea.
- Most marine invertebrates are osmoconformers. SUM of dissolved solutes
equals that of the seawater. So even these animals regulate their internal
composition of solutes.
- Most marine vertebrates and some invertebrates are osmoregulators.
Ocean is very dehydrating environment as water is lost and gain solutes via
diffusion.
- Marine boney fish (Cod), balance water loss by drinking lots of water and
their gills dispose of the excess NaCl. The Cl- ions actively transported out
and Na+ passively follows. The kidneys dispose of excess Ca, Mg and sulfate
ions with little water.
-Cartilagenous fish (Shark), kidneys remove mos of salt load and via rectal
gland but large water loss avoided by maintaining high concentrations of
urea. TMAO protects proteins from damage from urea. So, sharks have a
slightly hyperosmotic internal environment so water slowly enters via
osmosis ( NO DRINKING), and this is disposed via kidneys.
b) Freshwater animals:
- Opposite problem to marine organisms. Constantly gain water and lose
solutes by diffusion. However, they do maintain a lower DIFFERENCE in
osmolarity between internal and external environments cf marine organisms
so less energy needed to perform osmoregulation ( 1,000mosm/L cf
40mosm/L).
- Many maintain water balance by excreting large amounts of dilute urine.
-Salts lost in urine are replaced by foods and uptake across gills as Cl- ions
are actively transported into animal and Na+ follows passively.
- Euryhalines (Ex. Salmon), able to behave as needed in either environment.
c) Terrestrial animals:
- Biggest threat of desiccation. Is a big problem on land.
- Humans die if lose 12% body water! So adaptations to reduce water loss
key to success on land.
- Exs. Waxy exoskeletons, shells of land snails, layers of dead keratinized
skin cells, as well as being nocturnal.
- Still lot of water lost through moist surfaces of gas exchange organs,
urine, feces and skin. Balanced by eating and drinking and using metabolic
water.
- Ex. Kangaroo rat recovers 90% of loss by using metabolic water!!!
- Fig. 44.5.
Transport Epithelium:
- Need to maintain the composition of the cells cytoplasm and this is
done by bathing them in fluid of certain composition.
- Insects have hemolymph, vertebrates have interstitial fluid.
- This movement of solutes is maintained by transport epithelium.
Common to all are tight junctions that form an impermeable barrier at
the tissue-environment boundary ensuring that solutes pass through a
selectively permeable membrane.
- In most animals transport epithelia are arranged into complex tubular
networks with large surface areas ex. Salt glands in Albatross, gills of
freshwater fish, excretory organs ( often do osmoregulation and
excretion of waste).
Type
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of nitrogenous waste:
Reflects phylogeny and habitat.
Fig. 44.8.
Three types: ammonia, urea and uric acid.
a) Ammonia:
- V. soluble, highly toxic so needs to be diluted so need access to lots of
water.
- Excreted by aquatic species.
- Ammonia passes through membranes easily and diffuses to
surrounding water readily.
- In invertebrates occurs over whole body surface.
- Fishes lose as NH4+ across gills.
- In freshwater fish gills take up Na+ in exchange for NH4+. Helps
maintain higher Na+ concentration in body fluids than surrounding
water.
b) Urea:
- Ammonia too toxic for land animals as transport an issue!
- Excrete urea (made in liver as byproduct of metabolism of proteins
and nucleic acids by combining ammonia with CO2).
- Carried by circulatory system to excretory organs – kidneys.
- Low toxicity (100,000x less than ammonia!) so can transport and store
safely at high concentrations.
- Less water required.
- Disadvantage is that energy is required to make urea from ammonia.
- Animals that go between water and land switch between the two waste
products. Ex. Tadpoles excrete ammonia and when they are frogs they
excrete urea.
c) Uric acid:
d) - Unlike ammonia and urea is insoluble in water.
- Relatively non-toxic, excreted as a semi-solid paste, so very little
water required.
- More ATP required than to make urea.
- Many reptiles, birds insects and land snails excrete uric acid.
Excretion: Fig. 44.9.
-All produce urine.
- First body fluid is collected ( blood, coelomic fluid, hemolymph).
- Initial fluid is filtered through a selectively permeable membranes made up
of single layers of transport epithelium. These retain cells, proteins, large
molecules in body fluid.
Hydrostatic pressure (BP) forces water and small molecules (salts, sugars,
amino acids and nitrogenous wastes) into excretory system = filtrate.
- Selective reabsorption uses active transport to reabsorb valuable solutes
and waste and nonessential solutes are left in the filtrate or added to by
secretion (also uses active transport).
- The pumping of solutes also adjusts the osmotic movement of water into or
out of the filtrate.
- The processed filtrate is excreted from the system as urine.
Excretory systems:
a) Protonephridia (Flatworms). Fig. 44.10.
-Network of dead-end tubules with no internal openings.
- Tubules have capped off ends = flame bulb.
- Flame bulb has cilia projecting into tubule. This movement of cilia draws
interstitial fluid into the tubule where solutes reabsorbed and urine very
dilute. Exit throught nephridopores. Flame cells mainly for osmoregulation,
Excretion through mouth and across body surface.
In some isoosmotic parasitic flatworms protonephridia excrete nitrogenous
waste.
b) Metanephridia (Annelids). Fig. 44.11.
- Metanephridia immersed in coelomic fluid and opening leads into a
nephrostome.
- Fluid passes through nephrostome into coiled collecting tubule and
eventually into a storage bladder that opens to the outside through a
nephridiopore.
- Most of the solutes are reabsorbed in the tubule and returned to the
blood via capillaries.
- Nitrogenous waste remains in tubules and excreted outside.
c) Malpighian Tubules (Insects and other terrestrial Arthropods). Fig.
44.12.
- Open into digestive tract and dead end at tips immersed in the
hemolymph.
- The transport epithelia secrete solutes and nitrogenous waste into
the tubules from hemolymph.
- Water follows solutes into tubules and into rectum.
- Most solutes are pumped back into hemolymph and water again follows.
- Nitrogenous waste (uric acid) is eliminated as nearly dry matter along
with feces.
d)
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Vertebrate kidneys (Us). Fig. 44.13 and 44.14 and 44.15.
Blood supplied by renal artery and drained by renal vein.
Urine exits renal pelvis through ureter and drain into the bladder.
Urine expelled through urethra.
Kidney has an outer renal cortex, inner renal medulla.
Both regions are packed with excretory tubules and associated blood
vessels = nephrons.
Nephron made up of Bowman’s capsule, proximal tubule, loop of Henle,
distal tubule and collecting duct.
In Humans 80% are cortical nephrons and 20% are juxtamedullary
nephrons (these allow us to produce hyperosmotic urine – very
important for conservation of water).
Human kidney excretes 1L of urine out of 2,000L blood flow /pair of
kidneys/day.
Nearly all sugars, vitamins and other organic nutrients reabsorbed.
Blood
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vessels and nephron tubules: Fig. 44.13.
Afferent, Efferent arterioles, peritubular capillaries and vasa recta.
Tubules and vessels do NOT exchange materials directly.
Both immersed in interstitial fluid through which materials diffuse
between plasma and filtrate.
- This exchange facilitated by relative direction of blood and filtrate
flow in nephrons.
How the kidney works: Cells of transport epithelium (podocytes).
- Fig. 44.14.
1) Proximal tubule:
- Transport epithelial cells control pH by H+ ions, secrete ammonia to
neutralize acid.
- Reabsorb 90% of bicarbonate buffer.
- Drugs and other products of liver from peritubular capillaries to interstital
fluid and secreted into tubule.
- Valuable nutrients actively or passively transported from filtrate to
interstitial fluid and into capillaries.
- Most important in reabsorption of NaCl and water from large filtrate
volume. Transport epithelia actively transport Na+ into interstitial fluid and
water follows.
- Exterior of transport epithelium smaller surface area so minimize leaking
back of Na+ or water.
- Water and Na+ then diffuse into capillaries.
2) Descending Loop of Henle:
- Permeable to water but not to salts.
- Interstitial fluid bathing tubule is increasingly hyperosmotic from cortex
to medulla so water moves out of tubule.
- This loss of water increases the osmolarity of the filtrate.
3) Ascending Loop of Henle:
- Transport epithelium of ascending loop of Henle is permeable to salt BUT
not to water.
- There is a thin region then a thick region of the tubule as it ascends.
- As the concentrated filtrate ascends the Na+ diffuses out into the
interstitial fluid.
- In the thick part Na+ is actively pumped out an adds to the
hyperosmolarity of the interstitial fluid.
- The urine is now getting more diluted.
4) Distal tubule:
- Key in regulation of K+ and NaCl concentrations in body fluids.
- Varies amount of K+ secreted into filtrate and amount of NaCl reabsorbed
from filtrate.
- Contributes to pH regulation by controlled secretion of H+ and
reabsorption of bicarbonate.
5) Collecting Duct:
- Carries filtrate through medulla to renal pelvis
- Transport epithelium reabsorbs NaCl and so controls how much salt is
excreted.
- Not permeable to salt or urea in the cortex.
- Permeable to water but under hormonal control.
- Loses more and more water as it moves into the medulla.
- In the inner medulla it is permeable to urea so some diffuses out into the
interstitial fluid.
- This urea and NaCl contribute to high osmolarity of the interstitial fluid of
the inner medulla which in turn, allows kidney to conserve water and excrete
hyperosmotic urine.
NaCl and Urea maintain the osmolarity of the interstitial fluid.
Water moves out in the descending loop and this concentrates the salt.
As the filtrate moves up the ascending loop Na+ diffuses out and is later
actively pumped out into the interstitial fluid creating the concentration
gradient in the medulla.
Collecting duct impermeable to salt and urea but is permeable to water. So,
urine is hyperosmotic to interstitial fluid as it leaves kidney through renal
pelvis and ureter to bladder.
Kidney has a countercurrent multiplier system.
Capillaries flow in opposite direction to filtrate so also countercurrent
system. This makes sure that NaCl in interstitial fluid in medulla not carried
away.
Hormone regulation of Kidneys:
Fig. 44.16.
a) ADH (antidiuretic hormone) = enhances fluid retention by making kidneys
retain more water. Stimulated in cases of dehydration leading to an increase
in osmolarity of blood.
If blood osmolarity becomes greater than 300mosm/L, ADH is secreted by
hypothalamus and stored in posterior pituitary and secreted into blood
stream. ADH stimulates kidney’s distal tubules and collecting ducts to be
more permeable to water. This reduces urine volume and decreases blood
osmolarity until it falls below 300mosmo/L. Then ADH not secreted
(negative feedback).
Note: Alcohol inhibits release of ADH from hypothalamus, which leads to
loss of water in the urine and dehydration, one of the symptoms of a
hangover.
b) The Juxtaglomerular apparatus (JGA) located in the afferent arteriole
which supplies blood to the glomerulus, and RAAS (rennin-angiotensinaldosterone system) = Leads to an increase in blood volume and pressure.
Stimulated in cases where loss of BOTH salt and body fluids such as
diarrhea or injury results in loss of blood volume but NO increase in
osmolarity.
When BP or blood volume falls Renin is released and this converts
angiotensiogen to angiotensinogen II, which acts oni) Constriction of arterioles resulting in decreased blood flow.
ii) Stimulates adrenal glands to secrete aldosterone, which stimulates the
distal tubules to reabsorb Na+ and water. This results in increased BP and
blood volume.
iii) Stimulates proximal tubules to reabsorb NaCl and water, which results in
less salt and water in urine. This increases BP and blood volume.
c) ANF ( atrial natriuretic factor), opposes RAAS. Walls of atria in heart
release ANF if increase in blood volume and pressure. Inhibits release of
renin, inhibits NaCl reabsorption and reduces aldosterone release. So, lowers
BP and blood volume.