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Osmoregulation

The solute and water content of the internal environment must be regulated.

Known as osmoregulation.

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What is osmotic concentration?

• This is the total concentration of all the osmotically active particles in a liquid

• Osmotically active particles include Sodium and potassium. But basically any molecule of a solute in solution can be ‘osmotically acitve’

• Eg: a solution of water with 5 molecules of sodium chloride (NaCl) would have a 10 osmotically active particles. 5 sodium and 5 chlorines.

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• Isoosmotic: Two solutions separated by a membrane that have the same osmlarity, there is equal water movement b/w solutions

• Hypoosmotic: The solution with the lower concentration of solutes

• Hyperosmotic: The solution with the higher concentration of solutes

• Water will move from the hypoosmotic solution to the hyperosmotic one via OSMOSIS.

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Osmoregulation

Mechanisms for obtaining, retaining and eliminating water and solutes vary considerably depending on whether the organism is marine, freshwater or terrestrial.

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How can water be lost from an organism?

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Water loss

• By evaporation from skin/exoskeleton

• In faeces and urine

• From airways (exhalation of moist air from respiratory surfaces)

• Sweating

• Panting

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Terrestrial animals

The primary water balance problem for terrestrial animals is water loss.

Mechanisms to retard water loss:

• Behavioural adaptations

• Dry, scaly skin of reptiles

• Production of concentrated wastes

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Marine fish Marine fish have body fluids that are more

dilute than the surrounding water and so tend to lose water by osmosis.To achieve water balance marine fish:

• Drink sea water (to balance salt levels and reduce water loss)

• Produce small volumes of concentrated urine (to reduce water loss)

• Actively excrete salts at the gills

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Freshwater fishWater tends to enter tissues of freshwater fish by osmosis and must be expelled to avoid flooding the body.

To achieve water balance, freshwater fish:• Rarely drink (they get heaps of water diffusing in

from outside)• Produce large volumes of dilute urine (to get rid

of water)• Actively take up salts at the gills (to maintain

stable osmotic concentration)

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Excretion of Nitrogenous Wastes

• Formed from protein breakdown in cells of animals

• All amino acids contain nitrogen• When broken down for energy the nitrogen is

split off and forms ammonia (NH3)• Ammonia is toxic to cells and must be excreted• Ammonia can then be converted into Urea or

Uric acid which are much less toxic to the cells

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• Ammonia: Highly toxic and needs a lot of water to excrete. Generally used only by aquatic animals

• Urea: Much less toxic but also requires a fair bit of water to excrete and energy to make. Used by mammals and cartilaginous fishes

• Uric Acid: Large molecule with low toxicity. Takes a lot of energy to make but requires very little water to excrete. Used in reptiles and birds.

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Osmoregulation in Humans

The hypothalamus is involved in osmoregulation.

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What if there is not enough water?

When the hypothalamus detects too little water in the blood:

1. Sends message to pituitary gland to release ADH.

2. Collecting duct becomes permeable to water. Water moves by osmosis into nearby capillaries.

3. Less water is lost in urine, urine is concentrated.

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What if there is too much water?

When the hypothalamus detects too much water in the blood:

1. It stops signalling pituitary gland to make ADH.

2. In the absence of the hormone, the collecting duct becomes impermeable to water, urine is dilute.

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The concentrating ability of desert rodents depends on a thick renal medulla and long loops of henle

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