Practical Biology

A collection of experiments that demonstrate biological concepts and processes.

Modelling effect of aspirin on blood clots

Class practical

The effect of warm water on margarine models the effect of aspirin on a blood clot. Aspirin stops the platelets sticking together. It effectively dissolves clots – this is how it helps prevent strokes and heart attacks. In this practical, the coloured water (modelling blood) is poured through straws (modelling blood vessels). Students observe the time taken for liquid to pass through straws, then decide which straw models which kind of blood vessel.

Lesson organisation

This is a fairly quick procedure. It would provide some activity during a theory lesson about the circulatory system, or about factors affecting our circulatory health.

Apparatus and Chemicals

For the class – set up by technician/ teacher:

Sets of 3 straws containing
- no blockage
- 5 mm plug of cotton wool, pushed near one end of the straw
- 5 mm plug of margarine, pushed near one end of the straw

For each group of students:

Measuring cylinders, 50 cm3 or 100 cm3, 2

Beaker, 50 cm3 or 100 cm3, 1

Water, dyed red with food colouring, at about 40 °C, 100 cm3 per group

Stopclock or stopwatch

Health & Safety and Technical notes

Read our standard health & safety guidance


SAFETY: There are no significant risks associated with this activity


a Set up the straws as listed above, 3 straws per set


b Observe the straws and note which are blocked or open.

c Measure 20 cm3 of warm 'blood' into a syringe whose barrel moves freely.

d Hold the first straw over a measuring cylinder in a tray to catch spills.

e Pour the 'blood' carefully and slowly into the first 'blood vessel'.

f Measure the time taken for at least 18 cm3 of 'blood' to pass through the 'blood vessel'.

g Do this four times. Note each transit time.

h Repeat steps c to g for the second and third 'blood vessels'.

Teaching notes

Ask your students to observe the time taken for liquid to pass through straws, then decide whether the straw is a model of:

  • a healthy blood vessel
  • a blood vessel with a blood clot which could be causing a stroke or coronary heart attack
  • a blood vessel containing a blood clot and being treated with aspirin.

Ask students to explain in their own words how aspirin is used to treat blood clots, and to evaluate the model used in this procedure.

Typical results

Straw 1 Four fast readings – this models a healthy blood vessel with nothing stuck to its sides
Straw 2 Four relatively slow readings – this models a diseased blood vessel stopping oxygenated blood getting to the tissues.
Straw 3 One slow reading, but then the readings get faster – this models a blocked blood vessel which has been treated with aspirin to help dissolve the clump and prevent further clumps.

Background information

Aspirin (chemical name, acetylsalicylic acid; brand names include Aspro Clear®, Nu-seals®, Anadin®, Disprin®) is a widely used medicine. It is produced from a plant extract and is known best as a painkiller. It is used to treat fever and inflammatory disease as well as pain, and when a low dose is taken long term it can help prevent strokes or heart attacks.

The pharmacologist Sir John Vane won a Nobel prize for his research into how aspirin works. Aspirin works because it is an enzyme inhibitor; the enzymes it inhibits are called cyclooxygenases. Cyclooxygenases speed up the production of chemicals which can cause pain and inflammation, and others which cause platelets in the blood to stick together. If the cyclooxygenase enzymes are inhibited, fewer of these chemicals are produced so there is less pain and inflammation; fewer platelets stick together so there are fewer blood clots and less chance of a stroke or heart attack.

Unfortunately, some of the chemicals produced naturally by cyclooxygenase protect the gastrointestinal tract (the gut) and kidney. This means that aspirin sometimes has side-effects such as stomach ache. Another rare but dangerous reaction called Reye’s syndrome has been linked to aspirin; children and adolescents should not take aspirin without doctor's permission.

Blood normally flows freely through blood vessels. Imagine platelets in blood sticking to the side of the blood vessel first, and then the sticky area growing to become a blood clot as more and more blood cells stick. Eventually the blood will not be able to pass through and will stop flowing. The part of the body supplied by the blocked blood vessel will not receive any blood (or oxygen and nutrients). This is what happens in the brain when someone has a stroke, and in the heart when someone has a coronary heart attack. Aspirin helps to unstick the clump of platelets, clearing the vessel, keeping the blood moving, and helping prevent strokes and heart attacks.

Health and safety checked, September 2008

Related experiments

Elastic recoil in arteries and veins. An investigation into the properties of artery and vein tissue that links to the function of arteries and veins in the circulatory system.

Web links
This reports research into the use of aspirin as a preventative treatment against pulmonary thrombosis for post-operative patients. Students could compare this report with others found from a web search and try to evaluate this report or the research on which it was based.
Science supremo: Clinical Trials is an interactive roleplay in which students take on the role of scientists working in modern drug development, taking a new drug through clinical trials to assess its effectiveness in treating tuberculosis. The game is structured to enable group work in the classroom, collaborative problem solving and discussion. The teacher has a central role in guiding the process and supplying relevant resources. This enables a variety of teaching methods and lesson plans to be used.

The software is available to use online at the above link (which leads you through the teaching scheme) or you can access the game directly at It is also possible to download the software to use offline. Working through the stages of drug development will take more than a lesson, but it raises and explain many important issues in the real-life science of drug development. This is all relevant to the How Science Works component of the current GCSE specifications.

Science Supremo: Clinical Trials is a collaboration between DESQ and the Centre for Science Education at Sheffield Hallam University, supported by the Wellcome Trust People’s Award, and The Digital Science Initiative, a Wellcome Trust/ Nesta funded project.

(Websites accessed October 2011)

BPSlogo Thin

Thanks to the British Pharmacological Society for help in developing this procedure.