9.4 Water Balance

The body’s internal environment of extracellular fluid must:
- Maintain a constant volume.
- Maintain solute content.
- Maintain temperature.
Terrestrial vertebrates (including humans) require a stable aqueous environment to survive, which they carry inside their body while continuously replenishing it.
Aquatic organisms also need to maintain their internal environment in response to external changes in solute concentration and temperature.
Example: Atlantic Salmon (Salmo salar):
- Spends part of its life in freshwater and part in saltwater.
- In freshwater (solute concentration ~0.1%), the salmon's internal concentration (~1.0%) causes water to flow into its body, necessitating mechanisms to expel excess water.
- As it matures and moves to saltwater (solute concentration ~3.5%), it experiences continuous water loss and must replenish it.
Differences between terrestrial and aquatic environments:
- Terrestrial organisms have mechanisms to conserve water due to lower water availability compared to aquatic environments.

Osmosis:
- The movement of water molecules occurs from a high concentration area to a low concentration area across a selectively permeable membrane.
- This membrane permits water to pass but restricts most solute molecules.
Selectively Permeable Membrane:
- Maintains differences in solute concentration across its two sides, influencing water movement.
- Proteins are essential solutes that contribute to osmotic conditions.
Osmotic Pressure:
- Defined as the pressure resulting from differing water concentrations across a selectively permeable membrane.
- The greater the concentration gradient of water, the higher the osmotic pressure.
Solutions:
- Hyperosmotic (Hypertonic): A solution with higher solute concentration.
- Hypoosmotic (Hypotonic): A solution with lower solute concentration.
- Isoosmotic (Isotonic): Solutions with equal solute concentrations.
Water movement continues until solutions are isoosmotic, but even then, there is equal movement in both directions, resulting in no net movement.
Hydrostatic Pressure: Influences water flow; if hydrostatic pressure equals osmotic pressure, there is no net water flow despite potential higher water concentration outside.

In plant cells, the surrounding water is usually hypoosmotic, prompting water influx into root cells and leading to cell expansion against cell walls.
- This results in turgor pressure, which provides rigidity and allows the plant to remain upright.
If external fluid becomes hyperosmotic, turgor pressure drops, and plants will wilt.
In animal cells, hydrostatic pressure does not build like in plant cells:
- In a hypoosmotic environment, animal cells can swell and burst due to continuous water influx.
- In a hyperosmotic environment, cells lose water and shrink, both conditions being detrimental for survival.

Osmoregulation: The process of actively regulating osmotic pressure within bodily fluids and cells.
- Ensures intracellular and extracellular fluids remain isoosmotic.
- Requires continuous water movement by osmosis and solute movement by diffusion or active transport.
Some animals (e.g., marine animals like sponges, jellyfish) do not regulate the concentration of their extracellular fluid as it matches the surrounding environment, allowing free water flow.
Plants need to maintain a minimum osmotic and hydrostatic pressure for rigidity and nutrient transport.
Land animals (particularly vertebrates) require complex mechanisms to maintain constant intracellular and extracellular fluid concentrations differing from their external environments.

Excretion is critical in maintaining ionic and pH balance alongside osmotic concentration in cells.
It involves removing ions and toxic metabolites (especially nitrogenous compounds such as amino acids and nucleic acids) from the body:
- Osmoregulation Relation: Excretion is closely associated with osmoregulation, as eliminating waste helps maintain osmotic pressure and concentration within the body.
Excretory System: Functions to remove waste products and foreign materials.
- Main organs involved include the kidneys and bladder.
As proteins are broken down during metabolism, the liver converts waste molecules into soluble metabolites.
- Kidneys filter these and eliminate them with aqueous waste to maintain water and pH balance.

Different animal groups produce various forms of nitrogenous waste:
- Bony Fish: Produce ammonia directly as waste.
- Mammals and Cartilaginous Fish: Convert ammonia into urea, which is less toxic and soluble.
- Most Birds: Produce uric acid, characterized by low solubility and lower toxicity.
Ammonia ( ext{NH}_3): Highly toxic; even 0.005 mg/L can be lethal for humans.
- Can only be excreted in very dilute solutions, favoring marine animals that have abundant water.
- In mammals, urea serves as a less toxic alternative that requires less water to excrete.
Animals like many birds and terrestrial invertebrates produce uric acid, allowing them to conserve water more efficiently.

All organisms must balance their internal aqueous environment with the external environment.
Osmoregulation ensures that intracellular and extracellular fluids remain isoosmotic while maintaining different internal concentrations compared to the external environment.
Excretion is tightly linked to osmoregulation, as it helps expel metabolic waste.
The excretory system removes nitrogenous waste, excess water, and toxic compounds, principally involving the liver and kidneys.

(a) Describe the similarities and differences between diffusion and osmosis.
(b) Why is osmosis particularly important for biological functions?
Why do cells need to use osmoregulation?
What is the relationship between osmoregulation and excretion?
Explain the advantages and disadvantages for terrestrial animals of creating urea rather than other forms of nitrogenous waste.
What advantage do birds derive from producing uric acid as a nitrogenous waste compound?
Predict which animal group would produce more urea: carnivores with a high-protein diet or herbivores. Explain the water requirements.
Describe the outcome if a cell from a saline environment is placed in freshwater.
Explain excretion to a Grade 8 student in the context of osmoregulation.
Why can a walleye from Lake Ontario not survive in the Bay of Fundy? Include definitions of hypoosmoticity, hyperosmoticity, and isoosmoticity.
Differentiate between "hypertonic" and "hyperosmotic".
How do mineral salts, glucose, and urea participate in osmotic regulation?
Discuss the uses of sugar and salt in drying fruits and meats.