10.3Chapter 10: Excretion in Humans

Who made the best model? — Alhussan Biology - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 3 of 6 2 quizzes

📚 Alhussan Biology - Stage 9 Hub / Topical Revision

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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

Part 1 of 6Why use models

Why use models

Physical model

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

Strengths and limitations a model shows key features clearly but is simpler than reality.

Evaluating a model

Evaluating a model judge which features it represents well, which it leaves out and which ideas it gets wrong.

Part 2 of 6What the kidney does in filtration

Nephrons

Nephrons each kidney contains about a million tiny filtering units called nephrons.

Glomerulus

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

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

Urine what remains, urea with excess water and salts, forms urine.

Part 3 of 6Ejaz's funnel model

Ejaz's funnel model

Line a funnel with filter paper and balance it on a flask.

Mix water, salt, sand and red food colouring.

Pour the mixture through the filter paper.

Mixture

Mixture it models the blood, and salt represents urea because it is safer than toxic urea.

Funnel and filter paper

Funnel and filter paper they model the kidney.

Liquid passing through

Liquid passing through it models the urine.

Sand left on the paper

Sand left on the paper it models the blood cells and large, useful molecules that are not filtered out.

Part 4 of 6Fatima's sieve model

Materials

Materials a sieve used to strain vegetables and a mixture of dried peas and beans, salt crystals and sand.

Sieve

Sieve it models the kidney, and the holes are where substances leave the blood.

Peas and beans

Peas and beans they represent blood cells and large molecules the body needs.

Salt

Salt it represents urea.

Sand

Sand it represents water.

Result

Result salt and sand pass through the holes, like urea and water being filtered out, while the peas and beans stay in the blood.

Part 5 of 6Other filtration models

Starch and glucose

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

Dialysis tubing a bag of dialysis tubing in a beaker of water is another model.

Part of the model	Represents
Filter paper, sieve or tubing	Wall of glomerulus and capsule
Sand or large particles	Blood cells and large proteins
Water and dissolved small particles	Filtrate

Part 6 of 6Strengths and limits of models

Strength

Strength the models show that filtration depends on molecule size.

Limitations

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

Improvements add a second stage that returns useful substances, or use a pump to apply pressure.

Judging the best model

Judging the best model compare how many real features it represents and how accurately, not how neat it looks.

Key terms

Term	Meaning
Model	A simplified representation used to explain something hard to see.
Nephron	A tiny filtering unit of the kidney.
Glomerulus	A ball of capillaries where blood is filtered in the kidney.
Filtrate	The fluid that passes out of the blood into the nephron.
Ultrafiltration	Filtration of small molecules from blood under high pressure.
Limitation	A feature a model does not show or gets wrong.