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biology 1002 memorial university


removal of nitrogenous wastes (containing N), principally breakdown products of proteins (amino acids) + nucleic acids (nitrogenous bases A,G,C,T,U), Nitrogenous waste may be ammonia, urea, uric acid


process of regulating salt and water balance in the body

Relationship between excretion and osmoregulation

Products of excretion are removed with water (flushed out), The amount of water required to flush out nitrogenous waste (excretory) products has a significant impact on osmoregulation

Basic Fact of osmoregulation/excretion

In the face of a variety of environmental challenges, it is a fact that over time, in order to survive, the rate of water uptake and water loss in an animal MUST BALANCE

physiological systems of animals

enzymes, cell membranes, tissue systems, function in an internal fluid environment that has a different composition to exterior,Internal conditions must be maintained for optimal functioning

Fresh Water environment

Hypotonic to body fluids, the main problem is osmosis, there is a constant battle against uncontrolled influx of water. Adaptations are required to reduce H2O uptake, or get rid of excess water: To conserve internal solutes (salts) which will tend to diffuse out and to absorb salts from surroundings to replace those that are lost

Deserts or Oceans

Problem - Dehydration (evaporative loss) or OSMOSTIC water loss in a hypertonic environment. Need adaptations for water conservation, plus, in the ocean, excess salts taken in while drinking must also be removed


Total solute concentration in moles of solute / litre of solution, In biological solutions - osmolarity is measured in milliOsmoles/L (mOsm/L)


(of solutions) can be compared by measuring the OSMOLARITY of each solution

Direction of osmosis

From solution with fewer salts in solution, lower osmotic pressure (hypoosmotic), To solution with more salts in solution, greater osmotic pressure (hyperosmotic)


Solutions with the same osmolarity, no net water movement


solution with more salts in solution, greater osmotic pressure


solution with fewer salts in solution, lower osmotic pressure


Very toxic, very soluble, diffuses readily across membranes, cheap to produce. Animals that Excrete Ammonia: Fish - ammonia diffuses as NH4 and is washed away by watery environment

Freshwater fish excretion

gill membrane system actively exchanges NH4, replenishes Na+ that keeps diffusing out while getting rid of toxic NH4,NH4 can be excreted across body surface - continuous release of ammonium ions, especially in aquatic invertebrates


100,000 times LESS toxic than ammonia, can be transported and stored at high concentrations and excreted in more concentrated forms, And save precious water for homeostasis of body fluids

Uric Acid

for animal that really need to conserve water, relatively non-toxic, relatively insoluble, can be excreted in semi-solid paste form. The best option if water conservation (insects, land snails, many reptiles), or water conservation AND lightness (birds) are issues

How to excrete a solid

Uric acid is formed in the kidneys - still in solution, Transported in solution to cloaca where it joins with ions to form a precipitate, Water is reclaimed and Uric acid is excreted as a white "sludge" along with feces (normal colour)

Hydra (nitrogenous waste)

ammonia, released from cells of epidermis and gastrodermis into water or GVC

Earthworm (nitrogenous waste)

produces urea in urine, terrestrial - lives in moist environments so water is only a concern, not a problem

Grasshopper (nitrogenous waste)

uric acid, inhabits more arid environments, must conserve water

Fish (nitrogenous waste)

ammonia, has access to unlimited water, produces ammonia at the gills that is flushed away rapidly

Mammal (nitrogenous waste)

produce urea in urine, Similar to earthworm: water is an issue but not a problem

Factors affecting energy required for osmoregulation

Osmotic gradient to be overcome, Permeability of the animal's surface, Energetics of pumping solutes across body membranes

Ways an animal can gain water

By drinking, By osmosis (in fresh water), in diet, Metabolic water production

Ways an animal can lose water

By osmosis (in salt water), excretion, elimination, evaporation

Ways an animal can gain salts

Diffusion (in salt water), drinking, diet

Ways an animal can lose salts

Diffusion (in fresh water), sweating, excretion


Iso osmotic to environment (Many marine invertebrates)


Actively maintain internal osmolarity at level different from the environment (All** vertebrates occupying marine, freshwater and terrestrial environments except hagfish)

Advantage of osmoconformation

minimum energy required to control water movement

disadvantages of osmoconformation

With no mechanism to control osmolarity, changes in environmental conditions (increase or decrease in osmolarity) will cause a water gain or loss), Osmoconformers are reliant on environmental stability, do not have the option of colonizing freshwater or terrrestrial habitats

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