Water and Salt Physiology - Notes

Mice water turnover: ~25% daily.
Humans water turnover: ~7% daily.
Water turnover impacts body weight.
Definitions:
- Intracellular fluids: Inside the cell.
- Extracellular fluids: Outside the cell.
- Interstitial fluids: Between cells.
- Blood plasma: Part of the blood that is not blood cells.
- In open circulatory systems (e.g., crustaceans), interstitial fluids and blood plasma are the same.
- Inorganic ions: Salts or electrolytes.
Water and Ion Exchange:
- Intracellular : Interstitial: Water, Ions
- Interstitial : Blood plasma: Fenestrations - pores

Three types of blood plasma regulation:
1. Osmotic regulation
2. Ionic regulation
3. Volume regulation
Osmotic regulation (osmoregulation): Maintenance of constant osmotic pressure in blood plasma.
- Osmotic pressure = amount of pressure needed to prevent a solvent from moving across a semipermeable membrane
- Humans maintain ~300mOsm.
Ionic regulation: Maintenance of constant inorganic ion concentration in plasma; ion-specific controls.
Ionic conformity: ion concentration in blood plasma matches external environment.
Volume regulation: Regulation of total water amount in a body fluid.
- Animals generally regulate blood volume, interstitial, and intracellular fluids.
Integration of systems:
- Regulation of blood plasma = regulated interstitial fluid = cells work in a constant environment.
- Costs energy!

Salt water:
- Salinity: grams of dissolved inorganic matter per Kg of water.
- Sea water salinity: 34-36g/kg.
- Sea water osmotic pressure: 1000mOsm.
Fresh water:
- Freshwater salinity: <0.5g/kg.
- Freshwater osmotic pressure: 0.5-15 mOsm.
Brackish waters: Ocean water mixes with freshwater, estuaries. Salinity changes in time and space.

Animals lose water to the air via evaporation; dehydration dominates water-salt physiology studies.
Deserts defined by dryness, not necessarily heat.
Water vapor = gaseous water
Humidity = water content of air.
Raising the concentration of dissolved entities in a solution lowers the water vapor pressure of the solution.
Rate of evaporation depends on:
- Environmental humidity
- Water vapor pressure of the body fluid
- Rate of air movement

Kidneys regulate blood plasma composition by removing water, salts, and solutes.
- Produce urine; compare urine/plasma (U/P) ratios.
Osmotic U/P ratio: osmotic pressure of urine / osmotic pressure of blood plasma.
- U/P = 1: urine is isosmotic to plasma.
- U/P < 1: urine is hyposmotic to plasma.
- U/P > 1: urine is hyperosmotic to plasma.

Salty drinking water may not provide H2O if the animal needs to use more water to excrete the salts than it gains from the water.
Air-dried foods contain moisture.
Carbohydrates and lipids:
- oxidation during metabolism results mostly in formation of $CO<em>2$ and $H</em>2O$
- the $H_2O$ adds to an animal’s body water
Protein-rich foods can be dehydrating for terrestrial animals

Aerobic catabolism of organic food molecules forms water:
- C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
To assess net impact on water balance, subtract obligatory water losses (respiratory, urinary, faecal) from gains of metabolic water.
Net Gain of Metabolic Water: The total of all the obligatory water losses during the catabolism of a gram of ingested barley is 0.47 g.
The net gain is therefore 0.07 g of water per gram of ingested barley
Metabolic water matters most in animals that conserve water effectively

Cell membranes permeable to water.
Changes to osmotic pressure threatens constant cell volume
Cell-volume regulation
- In a more saline environment: raise intracellular quantities of free amino acids
- In a more dilute environment: decrease intracellular quantities of free amino acids