Heart Structure and Cardiac Cycle

The heart is organised into four separate chambers - two atria and two ventricles. The atria and ventricles contract and relax in a coordinated rhythm to keep blood flowing around the body. It’s an impressive organ - over the course of your lifetime it will beat an astonishing 2.5 billion times.

Heart structure

The heart is made up of four chambers divided into two sides. Remember that the left side of the heart is always drawn on the right (so it is the left of the person who’s heart it is, rather than your left as you’re looking at the paper). The left side of the heart has a much thicker wall, as it needs to pump more strongly to deliver blood all around the body (whereas the right side just needs to send the blood to the lungs). The left side carries oxygenated blood whereas the right side carries deoxygenated blood.

The chambers at the top are called atria and these chambers receive the blood from the veins supplying the heart. Blood flows from the atria to the ventricles, which are separated from the atria by atrioventricular valves to prevent blood flowing in the opposite direction. There are another set of valves between the ventricles and the arteries which are called the semi-lunar valves as they look like little half-moons.

The main artery which takes oxygenated blood from the left side of the heart to the rest of the body is called the aorta whereas the artery which delivers deoxygenated blood between the right side of the heart and the lungs is called the pulmonary artery. The major vein which returns blood from the body to the right side of the heart is the vena cava and the vein which ferries blood from the lungs to the heart is called the pulmonary vein. Remember than the arteries are away from the heart and veins are always going to the heart. Whenever we use the word ‘pulmonary’ we are referring to the lungs. The heart muscle itself also needs its own blood supply, so that it can get plenty of oxygen and glucose to keep respiring and keep pumping - these are called the coronary arteries. It’s a blockage in these coronary arteries which leads to a heart attack.

Structure of blood vessels

Arteries carry blood away from the heart to the various organs of the body. They need to cope with the high pressure generated from the heart forcing out blood with each heart beat. This is why they have a really thick muscular wall containing lots of elastic tissue. The inner lining of the arteries, called the endothelium, is folded which allows the artery to expand (elastic recoil) which also helps it to withstand the high pressure. The small lumen ensures a high pressure is maintained.

Veins carry blood from the organs of the body towards the heart. Here, blood is flowing at a much lower pressure so veins have a large lumen and much thinner walls containing little elastic fibres or muscle tissue. Veins have valves which prevents the slow-moving blood from flowing backwards. The contraction of nearby body muscles helps blood to flow through veins.

Capillaries are the blood vessels which connect arteries and veins. It is in the capillaries where substances move out of the blood to the body tissues - things like oxygen, glucose and mineral ions. Any waste products, such as carbon dioxide and water, will move out of the body tissues and into the capillaries. The capillaries have small holes, called pores, in their walls to enable the exchange of substances. Their walls are just one cell thick which reduces the diffusion distance for these substances.

The Cardiac Cycle

The cardiac cycle is a coordinated sequence of contractions and relaxations by the heart muscle which causes blood to move from the atria, into the ventricles and then the arteries. Muscle contractions are referred to as systole and relaxation is referred to as diastole. The movement of blood through the different compartments of the heart rely on changes in volume and pressure as a result of the contraction of the heart muscle walls. It occurs in the three following stages:

1. Atrial systole

   The atria contract while the ventricles relax. This decreases the volume inside the atria which increases the pressure. This increased pressure forces the atrioventricular valves open and pushes blood into the ventricles.

2. Ventricular systole

   The ventricles contract while the atria are relaxed. This decreases the volume inside the ventricles which increases the pressure. The pressure is higher in the ventricles compared to the atria, which forces the atrioventricular valves closed and causes the semi-lunar valves to open. The closure of the AV valve is important because it prevents the back-flow of blood into the atria. Blood is forced out of the ventricles and into the arteries (the aorta and the pulmonary artery).

3. Diastole

   Both the atria and ventricles are relaxed so the pressure is low in both chambers. Since the pressure is higher in the arteries than in the heart chambers, the semi-lunar valves are forced closed which prevents blood flowing backwards into the ventricles. Blood is returned to the heart and the atria fill with blood.

Investigating heart rate in invertebrates

A common experiment when learning about the heart in A-Level is to investigate the effects of caffeine on heart rate in small invertebrates such as the water flea (Daphnia). The water flea is usually chosen because it has a transparent body, which comes in handy when we need to observe the heart, and it is such a simple organism with a less-developed nervous system compared to other animals that we assume it doesn’t feel stress or pain during the experiment. They’re also abundant (they can be bought as fish food) so using them in a school experiment isn’t going to affect population numbers.

Experimental method:

1. Make up a range of solutions containing different concentrations of caffeine. Make sure to include a control containing no caffeine at all.

2. Using a pipette, extract a single Daphnia and transfer into the dimple on a cavity slide.

3. Place the slide onto the microscope stage and observe until you can clearly see the beating heart.

4. Using a different pipette, add one drop of the caffeine solution to the slide.

5. Count the number of heartbeats in 20 seconds using a stopwatch. Multiply the data by tree to calculate the number of heartbeats in one minute.

6. Repeat the experiment using the same concentration of caffeine. Ensure that you use a different Daphnia for each repeat.

7. Repeat the experiment using the other caffeine solutions of varying concentrations and the control solution.

Results

Once we have collected the data we need to plot a graph of heat rate on the y-axis and caffeine concentration on the x-axis. If all goes to plan, you will see that as the concentration of caffeine increase, heart rate becomes faster. However, as caffeine concentration becomes too high we will start to see a drop in the heart rate as the concentration of caffeine has reached toxic levels.

Ethical issues

• Using invertebrates for research is seen as more ethical compared to using vertebrates, such as mammals, which are more likely to feel pain and stress during an experiment. This is because they have a much simpler nervous system compared to other, more complex organisms.

• However, since we can’t get inside their heads, it is difficult to say for certain that Daphnia don’t feel any stress during experiments. Some people believe that it is unethical to cause distress or suffering to any living organism, no matter how simple.

• The best way to minimise the impact on the invertebrate is to keep their conditions as similar to their natural environment as possible e.g. store the organisms in a similar temperatures and pH.

• You could also argue that since Daphnia are bred for fish food, they are going to die anyway (and their fate in a school experiment is no worse than being fed to a goldfish). Their abundance also means that their use won’t affect their population numbers or any other organisms in their food chain.

Next Page: Cardiovascular Disease