Blood vessels and the Daphnia heart-rate practicalEdexcel A-Level Biology A: Revision notes

Blood flows in a closed circuit of vessels, and each type's structure suits its function.

• Arteries carry blood away from the heart at high pressure. The wall is thick, with elastic fibres that stretch and recoil to withstand the pressure and smooth the flow, smooth muscle and a narrow lumen. They branch into narrower arterioles.
• Capillaries are where exchange occurs. The wall is a single layer of endothelial cells (one cell thick), giving a very short diffusion distance. They form large networks with a huge surface area, and the lumen is so narrow that red blood cells pass slowly in single file.
• Veins carry blood back to the heart at low pressure. The wall is thin with little elastic tissue and muscle, the lumen is wide, and valves prevent backflow. Movement of skeletal muscles squeezes veins and helps push blood towards the heart.

Daphnia (water fleas) are transparent, so the heart is visible through a microscope. Caffeine is a stimulant, and the practical investigates whether it changes heart rate.

Method: prepare caffeine solutions of different concentrations (for example 0.0, 0.1, 0.2, 0.4 %) by diluting a stock solution. Place a Daphnia in a drop of solution on a cavity slide with cotton wool fibres to restrict movement. Leave for the same time (about 5 minutes) so the caffeine can act, then count the heartbeats in a set time (e.g. 20 s) and multiply to give beats per minute. Use a fresh Daphnia, or rinse and rest the animal, for each concentration.

Variables: independent = caffeine concentration; dependent = heart rate; control = temperature, exposure time, size and age of Daphnia, volume of solution. Repeat with several Daphnia and calculate the mean. Caffeine is expected to increase heart rate.

When describing results, state the trend and quote data: heart rate increases as caffeine concentration rises, for example from 186 to 255 beats per minute. Calculate percentage change as $\dfrac{\text{final} - \text{initial}}{\text{initial}} \times 100$.

Limitations and improvements:

• Individual Daphnia vary, so use more animals and a mean.
• Counting by eye is difficult at high rates; record a video and replay it slowly.
• Temperature changes under the microscope light affect heart rate, so use a water bath or heat filter.
• Make sure the animal has acclimatised and the exposure time is identical.
• Use more concentrations to show the relationship more fully.

Using animals in research raises ethical questions. Invertebrates such as Daphnia are often used because they are cheap, small, breed quickly and have a simpler nervous system than vertebrates, so they are thought less likely to feel pain. Using them can replace or reduce the use of vertebrates.

However, it is not certain that invertebrates cannot suffer, so researchers should still use refined methods: low concentrations and short exposure, gentle handling, suitable housing, returning animals to culture water afterwards, and using no more animals than needed for reliable results. A balanced answer weighs the possible harm against the scientific and medical benefit and reaches a justified conclusion.