Kidney sorting mechanisms

Model how the kidney removes wastes from the blood for excretion: sorting by size, diffusion and sorting by shape.
Science content
Biology: Sensing, Organ Systems (4, 5, 6)
Lessons activity is in

Sorting by size
Make a mixture of marbles, gravel and sand.
Shake the mixture in a kitchen sieve and see what passes through the sieve.
The smallest items, which pass through the sieve (sand) represent the smaller blood molecules which pass through the kidney membrane into the kidney tubes (salts, water, urea, sugar, vitamins). The larger items held by the sieve (marbles and gravel) represent the blood cells and large protein molecules like albumin and antibodies, which stay in the blood.
Show image of kidney membrane that filters the blood, allowing smaller molecules into the kidney:……
Did you get small gravel stuck in the mesh? The kidney actively cleans its filter, to stop it clogging up.

Sorting by shape
Special molecules in the kidney tube walls only let some molecules through back into the blood.
If the small molecules have the right shape to fit, they can pass through the membrane.
Like a lock and key mechanism.
Students find the lock which opens with their key. Their key is numbered. The locks each have a letter.
Draw a string of numbered boxes on the board. Students add the letter on the lock they open in the box with the same number as their key. The boxes spell out the name of the molecule gates in the kidney tube walls: "transport protein" (also called "carrier protein").

Moving from high to low concentration by diffusion
Drip food dye onto wet tissue on a flat surface, and watch as it slowly spreads out away from the concentrated drip, and into areas where there is no food dye.
This models diffusion (although it is too fast to be actual diffusion). Diffusion is how molecules move across membranes the same way, from high to low concentration.
Water and salts are moved back into the blood by this mechanism.

The kidney sorts blood molecules using these three above mechanisms.
Show diagram of kidney ducts and blood image, to show where each happens:
First, in the rounded glomerulus, all the small molecules are filtered into the kidney, while large molecules and cells stay in the blood (sorting by size).
Then along the kidney tubes water, salts, sugars, vitamins and other nutrients are move through transport proteins back into the blood. Urea does not go back into the blood. (sorting by shape).
Many small molecules diffuse from high concentration to low concentration, which does not need any energy.
Sometimes energy is used to pump molecules up their concentration gradient e.g salt.
Urea mostly stays in the kidney tubes - it is not taken back into the blood, also other toxins. They flow to the bladder, to be later excreted.

As well as removing urea from our blood, the kidney regulates how much salt and water are returned to the blood, and how much is excreted. This keeps the blood chemistry exactly right, so that our organs can function properly.
The kidney filters an enormous amount of blood: every heartbeat, ¼ of your blood goes through the kidneys (1.2L of nearly 5L total)!

Grades taught
Gr 6