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==Introduction==

==Introduction==

It is best that the fluid surrounding the cells remains fairly constant.  If volume is varied dramatically then it has major knock on effects on the blood pressure and perfusion of the organs and tissue.

It is best that the fluid surrounding the cells remains fairly constant.  If volume is varied dramatically then it has major knock on effects on the blood pressure and perfusion of the organs and tissue.

==Inter-Animal Differences==

==Inter-Animal Differences==

==Water Gain and Loss==

==Water Gain and Loss==

Water volume varies very little from day to day suggesting intake is equal to loss.  Water is gained from three sources - drinking, food and metabolic water.  In the periods between water intake ADH is secreted because the osmolarity of the plasma increases.  Once the osmolarity reaches a certain threshold the thirst centre makes thr animal drink.  However intake is not under precise control.  Metabolic water is derived from the oxidation of hydrogen within cells and represents about 8-12% of an animals water supply.  However this can be quite extreme.  For example the kangaroo rat survives almost fully on this metabolic water.

Water volume varies very little from day to day suggesting intake is equal to loss.  Water is gained from three sources - drinking, food and metabolic water.  In the periods between water intake ADH is secreted because the osmolarity of the plasma increases.  Once the osmolarity reaches a certain threshold the thirst centre makes the animal drink.  However intake is not under precise control.  Metabolic water is derived from the oxidation of hydrogen within cells and represents about 8-12% of an animals water supply.  However this can be quite extreme.  For example the kangaroo rat survives almost fully on this metabolic water.

If water gain and loss is to be balanced then the loss needs to be adjusted to match the variable intake.  Water is lost from, 4/5 main areas - the body surface, the airways, faeces, urine and lactation _{(if applicable)}.

If water gain and loss is to be balanced then the loss needs to be adjusted to match the variable intake.  Water is lost from, 4/5 main areas - the body surface, the airways, faeces, urine and lactation _{(if applicable)}.

===Water Loss Through the Faeces===

===Water Loss Through the Faeces===

This is minimal in omnivores and carnivores but of major importantance to herbivorous species.  It is not really under physiological control with regards to water balance and as such is not a viable means of adjusting the output to match the intake.

This is minimal in omnivores and carnivores but of major importance to herbivorous species.  It is not really under physiological control with regards to water balance and as such is not a viable means of adjusting the output to match the intake.

===[[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology|Renal Regulation of Water Loss]]===

===[[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology|Renal Regulation of Water Loss]]===

Water intake during lactation increases significantly even over and above the requirement for milk.  To produce 31l of milk and cow needs to take in an extra 57l of water.  This is thought to be because of an increased loss from the airways, skin and faeces.

Water intake during lactation increases significantly even over and above the requirement for milk.  To produce 31l of milk and cow needs to take in an extra 57l of water.  This is thought to be because of an increased loss from the airways, skin and faeces.