Excretory System Activity
Date: March 2007
I am a 7th grade science teacher and need a good
lab activity to go with the excretory system section of our
studies...any ideas? Please consider that we don't have any high
tech lab equipment....
One idea for an excretory activity demonstrates how the kidneys work as
filters. You can make it as complicated or as simple as you like to be
level-appropriate for your students.
The basic idea is to get some cheesecloth, or some other fine-mesh fabric
(if you can get actual filter paper, that's even better), 1 gal water, food
coloring, 1lb fine sand, and a tall, narrow glass jar. Mix the sand and
water to form a thin, loose mixture, and add some color to it. Fill the jar
half-way with just water, and drape the cheesecloth over the glass jar (fold
over the cheesecloth to make it many layers thick). Pour on the sand and
water to fill the rest of the jar, and watch the bottom layer turn color,
while none of the sand flows with the color -- the cheesecloth stops it. You
can pull up the cheesecloth, pour out the bottom water (=just like
urinating), and then add more water to the top. The top layer will gradually
lose its color after several changes. This is like the kidneys cleaning out
toxins, but retaining blood cells. You can substitute lots of other
materials that you have on hand, but the point is to have two easy-to-see
components, one that will move through the cheesecloth, and one that won't.
For more complicated experiments, you can have the students calculate how
much food coloring must be added to maintain the color -- this can be used
to describe how the body must maintain a salt balance.
A variation on the same theme demonstrates how the colon removes water from
the mix of digestive fluids and food leftovers. Start with the same
cheesecloth, some regular (not light) yogurt, a strainer, and a bowl. Be
advised: this experiment can't be done in one class period. Basically, you
put the cheesecloth in the strainer, put the yogurt in, and place it in the
fridge over a bowl to catch water that drains through the cheesecloth. Over
a couple days, the water will drain, and the yogurt will harden, like cream
cheese. If you compress it with a heavy can on top, it will get even firmer.
1 cup of yogurt will make about a half-cup of yogurt cheese (more or less,
based on how much time you leave it in there, and if you compress it). For
extra-grossness, you can add food coloring (or cocoa) to make it brown
(yuck!!! -- the kids will LOVE it), and the yogurt cheese is actually very
delicious -- I like mine with some cherry preserves on some baguette. Add
some fresh lemonade, and you have an extra-gross excretory-themed snack!
Any of these can be scaled up or down to fit your size needs. Hope these
Click here to return to the General Topics Archives
Update: June 2012