Total Water Balance - Physiology

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.

The Importance of Maintaining Adequate Levels of Water

Too Little Water

• Inadequate perfusion due to low blood volume
• A build up of waste products due to insufficient urine volume
• Increased solute concentrations affect cell function
• Inability to sweat and control body temperature
• Death

Too Much Water

• Oedema
• Increased blood volume and therefore pressure could damage the CVS
• A decrease in solute concentrations disrupt normal cell function

Inter-Animal Differences

There are differences in water content between the many tissues which make up the body. Tissues which have low metabolic activity such as adipose tissue and bone have relatively lower water content. This leads to a variation in water between individuals for example a very fat cow may only have a 40% body water content compared to 70% in a lean cow. There is also an age difference. Young animals tend to have a high body water content where as older well nourished animal have a lower.

Due to the differences in relative tissue distribution there is a sex related difference also. Males which tend to have more muscle have a higher water content where as female which tend to have more adipose tissue have a lower.

Water Locations

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 it is 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 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)}.

Insensible Water Loss

Insensible water loss is water loss which cannot be sensed by the individual and tends 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 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.

Renal Regulation of Water Loss

Only the kidneys 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 a 70kg human produces 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 there 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 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.

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