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BIOLOGY · Stage 9 · Chapter 10: Excretion in Humans

Biology 10.3 Who made the best model? — Modelling Kidney Filtration

5 min read 6 parts Lesson 3 of 6

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Lesson summary

10.3 Who made the best model?

  • Physical model
  • Strengths and limitations
  • Evaluating a model
  • Glomerulus
  • Funnel and filter paper
  • Liquid passing through

Section 1 of 6Why use models

  • Physical model = a way to help predict or explain something that cannot easily be seen.
    • Scientists cannot easily watch a nephron working, so they build models.
  • Strengths and limitations
    • a model shows key features clearly but is simpler than reality.
  • Evaluating a model
    • judge which features it represents well, which it leaves out and which ideas it gets wrong.

Section 2 of 6What the kidney does in filtration

  • Nephrons
    • each kidney contains about a million tiny filtering units called nephrons.
  • Glomerulus
    • blood enters a ball of capillaries inside a cup-shaped capsule.
    • Blood is under high pressure because the vessel leaving is narrower than the one entering.
    • Small molecules (water, glucose, salts, urea) are forced out into the capsule by ultrafiltration.
    • Large molecules such as proteins and the blood cells stay in the blood.
  • Filtrate
    • the filtered fluid flows along the tubule, where all the glucose and as much water and salt as needed are reabsorbed into the blood.
  • Urine
    • what remains, urea with excess water and salts, forms urine.

Section 3 of 6Ejaz's funnel model

  1. Line a funnel with filter paper and balance it on a flask.
  2. Mix water, salt, sand and red food colouring.
  3. Pour the mixture through the filter paper.
  • Mixture
    • it models the blood, and salt represents urea because it is safer than toxic urea.
  • Funnel and filter paper
    • they model the kidney.
  • Liquid passing through
    • it models the urine.
  • Sand left on the paper
    • it models the blood cells and large, useful molecules that are not filtered out.

Section 4 of 6Fatima's sieve model

  • Materials
    • a sieve used to strain vegetables and a mixture of dried peas and beans, salt crystals and sand.
  • Sieve
    • it models the kidney, and the holes are where substances leave the blood.
  • Peas and beans
    • they represent blood cells and large molecules the body needs.
  • Salt
    • it represents urea.
  • Sand
    • it represents water.
  • Result
    • salt and sand pass through the holes, like urea and water being filtered out, while the peas and beans stay in the blood.

Section 5 of 6Other filtration models

  • Starch and glucose
    • pour a mixture of sand, starch and a glucose and salt solution through filter paper.
    • Sand and large molecules stay on the paper and small molecules pass into the beaker.
    • Test the filtrate with iodine solution for starch and Benedict's reagent for glucose.
  • Dialysis tubing
    • a bag of dialysis tubing in a beaker of water is another model.
Part of the modelRepresents
Filter paper, sieve or tubingWall of glomerulus and capsule
Sand or large particlesBlood cells and large proteins
Water and dissolved small particlesFiltrate

Section 6 of 6Strengths and limits of models

  • Strength
    • the models show that filtration depends on molecule size.
  • Limitations
    • they do not show the high pressure that forces fluid out.
    • They do not show reabsorption, so useful glucose is left in the filtrate instead of being taken back by active transport.
    • They are much bigger and simpler than a real nephron and have no blood flow.
  • Improvements
    • add a second stage that returns useful substances, or use a pump to apply pressure.
  • Judging the best model
    • compare how many real features it represents and how accurately, not how neat it looks.

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