Advanced Level

Modelling effect of aspirin on blood clots

Introduction

The effect of warm water on margarine models the effect of aspirin on a blood clot. Aspirin stops the platelets sticking together and effectively dissolves clots which is how it helps to 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 that will provide an opportunity for some activity during a theory lesson about the circulatory system or factors affecting our circulatory health.

Apparatus And Chemicals

For the class – set up by technician/ teacher:

Sets of 3 straws containing:
• no blockage
• a 5 mm plug of cotton wool – pushed near one end of the straw
• a 5 mm plug of margarine – pushed near one end as above.

For each group of students:

Measuring cylinder, 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

Read our health and safety guidelines

Procedure

SAFETY: There are no significant risks associated with this activity

Preparation

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

Investigation

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

Aspirin (chemical name, acetylsalicylic acid; brand names 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 taken at a low dose long term, can help prevent people having 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 and the enzymes it inhibits are called cyclooxygenases. Cyclooxygenases speed up the production of certain chemicals that can cause pain and inflammation, and also other chemicals that 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, and fewer platelets stick together meaning that there will be fewer blood clots (so less chance of a stroke or heart attack).

Unfortunately, some of the chemicals that cyclooxygenase produces naturally actually protect the gastrointestinal tract (the gut) and kidney so this means that aspirin sometimes has side-effects eg stomach ache. Another (very rare but dangerous) reaction called Reye’s syndrome has been linked to aspirin so children and adolescents must not take aspirin unless their doctors say they can.

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, which clears the vessel, keeps the blood moving and so can help prevent strokes and heart attacks.

These are the typical results.

Straw 1 Four fast readings – this models a healthy blood vessel without anything stuck to its sides
Straw 2 Four relatively slow readings – this models a diseased blood vessel that is stopping oxygenated blood getting to the tissues.
Straw 3 One slow reading, but then the readings get faster – this models a blocked blood vessel that has been treated with aspirin to help dissolve the clump and prevent further clumps.

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

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

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

http://news.bbc.co.uk/1/hi/health/712483.stm
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.

(Websites accessed September 2008)

www.sciencesupremo.org
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 which allows for 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 www.desq.co.uk/science/flash/scienceSupremo.html. 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 of the important issues in the real-life science of drug development – 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. (Website accessed August 2009.)

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

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