11.3.3 Conserving Water

• Animals that live in dry environments such as deserts depend on their ability to conserve water for survival
• Such animals often have very long loops of Henle
  • The longer the loop of Henle, the greater an animal’s ability to conserve water
    • A longer loop of Henle is able to generate steeper concentration gradients and so more water can be reabsorbed by osmosis
  • The collecting ducts of such animals are also very long
• Animals that need to conserve water have thicker medulla regions in their kidneys to provide additional space for their long loops of Henle
• This means that both loop of Henle length and medulla thickness can be indicators of an animal’s ability to conserve water

The length of the loop of Henle differs depending on an organism’s need to conserve water; the longer the loop of Henle, the more water can be reabsorbed

The thicker the medulla region of the kidney, the more concentrated urine can become, meaning that more water is reabsorbed in animals with a thicker medulla. A thicker medulla is needed to allow space for a longer loop of Henle.

• If the balance of water and solutes in the body are not maintained correctly, the result may be dehydration or overhydration
• Dehydration results if water is lost from the body and not replaced
• e.g.
  • After excessive sweating
  • After diarrhoea
• In dehydration the osmolarity of the body fluids rises above that of the surrounding cells, leading to cell water loss and shrinkage as water moves out of the cells by osmosis
• Symptoms of dehydration include
  • Low volume of dark, concentrated urine
  • Not being able to sweat, leading to a reduced ability to regulate body temperature
  • A drop in blood pressure due to reduced blood volume
  • An elevated heart rate as the heart attempts to compensate for the drop in blood pressure
  • Feeling tired due to increased exposure of the tissues to metabolic waste
    • Metabolic waste is usually removed from the body by being dissolved in water and excreted in the urine; this cannot happen quickly enough if there is a lack of water available
• Overhydration can result if there is too much water in the body fluids in relation to solutes such as sugars and salts; this can result from
  • Overconsumption of water
  • Not replacing sugars and salts after excessive sweating
  • Kidney problems
  • The side effects of certain drugs
• In overhydration the osmolarity of the body fluids drops below that of the surrounding cells, causing water to move into the cells by osmosis
  • This leads to swelling of the body’s cells
• Overhydration can lead to symptoms such as
  • Excessive urination as the body produces large volumes of colourless, dilute urine
  • Headaches resulting from swelling of cells in the brain
  • High blood pressure due to increased blood volume
  • Low heart rate as the heart attempts to compensate for the increase in blood pressure
  • Neurological problems due to low concentrations of important ions such as sodium ions

NOS: Curiosity about particular phenomena; investigations were carried out to determine how desert animals prevent water loss in their wastes

• Scientists observe events, or phenomena, in the natural world, noticing when phenomena cannot be explained using existing understanding
• When a phenomenon cannot be explained using existing knowledge, scientists come up with hypotheses that might explain such phenomena, and design investigations to allow them to test these hypotheses
• When scientists studying desert rodents such as kangaroo rats (genus Dipodomys) noticed that they were able to survive on a diet of dry seeds with little or no separate water intake, they became curious as to how the rats’ physiology enabled them to do this
• Analysis of the kangaroo rats’ water intake from food and their water losses due to breathing, excretion, and egestion, showed that water intake and water losses were equal
• Studying kangaroo rat anatomy and behaviour, scientists were able to conclude that they balanced their water intake and losses by
  • Having a very long loop of Henle and a thick medulla region
  • Producing urine many times more concentrated than their body fluids
  • Producing very small quantities of urine
  • Producing very dry faeces which they then consume to reabsorb any remaining water
• Curiosity about the extent of kangaroo rat water conservation abilities led scientists to investigate whether or not they could survive when given seawater to drink; they found that the kangaroo rats were able to fully excrete the excess salt from seawater with very little increased loss of water!