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Total Water Balance - Physiology (view source)
Revision as of 13:27, 16 August 2008
, 13:27, 16 August 2008no edit summary
Although 65% of water is found intracellulary there is a significant quantity in the digestive and urinary tracts. In ruminants this could equal the volume of the interstial fluid. This water is not easy to mobilise but can be if required to stabilise blood volume.
Although 65% of water is found intracellulary there is a significant quantity in the digestive and urinary tracts. In ruminants this could equal the volume of the interstial fluid. This water is not easy to mobilise but can be if required to stabilise blood volume.
==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 to 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 need 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 <sub>(if applicable)</sub>.
===Insensible Water Loss===
Insensible water loss is water loss which cannot be sensed by the individual and tend to be a response to thermoregulation rather than being a regulated process of water homeostasis. It includes evaporation from the skin and epithelial surfaces of the airways and varies with environmental temperature and humidity as well as air movement. The main losses being from sweating and increased respiration.
===Water Loss Through the Faeces===
This is minimal in omnivores and carnivores but of major important to herbivorous species. It is not really under physiological control with regards to water balance and as such is not a viable means to adjusting the output to match to intake.
===[[Aquaporins of the Kidney and Water Homeostasis - Anatomy & Physiology|Renal Regulation of Water Loss]]===
Only the kidenys are capable of precise adjustment of water loss to compensate for the variation in water intake. However a minimum amount must always be excreted to remove the waste. For example of 70kg human produced 600mosmol of solutes every 24 hours even without food. This is mainly Urea, Uric acid, phosphate, creatine, creatinine other waste products and ions. In humans the maximum urine osmolarity is 1200mosmol/l therefore their is a minimum water loss of 500ml even when no food or water is available. When this is added to another 500ml in the exhaled air and 400ml evaporated from the body surfaces a person loses 1400ml of water per 24 hours. This is the inevitable water loss. 300ml of metabolic water is produced per 24 hours in this person leaving an 1100ml deficit to be compensated for by drinking.
===Water Intake and Lactation===
Water intake during lactation increases significantly even over and aboce the requirement for milk. To produce 31l of milk and cow needs to take an extra 57l of water. This is thought to be because of an increased loss from the airways, skin and faeces.