Water Balance

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•  Consequences of dehydration and overhydration

Maintaining an appropriate water balance within the body’s tissues and cells is critical to the survival of an organism

  • Homeostasis cannot be maintained if water levels drop (dehydration) or are raised (overhydration) without regulation


  • Dehydration is a loss of water from the body such that body fluids become hypertonic
  • Individuals will experience thirst and excrete small quantities of heavily concentrated urine (to minimise water loss)
  • Blood pressure will drop (less water in plasma) and the heart rate will increase to compensate for this
  • The individual will become lethargic and experience an inability to lower body temperature (due to lack of sweat)
  • Severe cases of dehydration may cause seizures, brain damage and eventual death


  • Overhydration is a less common occurrence that results when an over-consumption of water makes body fluids hypotonic
  • Individuals will produce excessive quantities of clear urine in an effort to remove water from the body
  • The hypotonic body fluids will cause cells to swell (due to osmotic movement), which can lead to cell lysis and tissue damage
  • Overhydration can lead to headaches and disrupted nerve functions in mild cases (due to swelling of cells)
  • In severe cases, overhydration may lead to blurred vision, delirium, seizures, coma and eventual death

Effect of Water Balance on Erythrocyte Structure

water balance

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•  The length of the loop of Henle is positively correlated with the need for water conservation in animals

All animals need to maintain an appropriate water balance, however the need for water conservation will depend on habitat

  • Animals in arid, desert environments will need more efficient water conservation than animals in moist, mesic environments

Water conservation can be improved by having a longer loop of Henle, which increase the salt gradient in the medulla

  • A greater the salt gradient in the medulla means more water is reabsorbed by the collecting ducts and urine is concentrated

Hence, the length of the loop of Henle is positively correlated with the degree of water conservation in animals

  • Animals living in moist environments have short loops of Henle that don’t descend deeply into the medulla (cortical nephrons)
  • Animals living in arid environments have long loops of Henle that descend deeply into the medulla (juxtamedullary nephrons)

Comparison of Nephrons in Mesic and Desert Animals

water conservation

Relationship between Environmental Conditions and Medullary Thickness

medullary thickness