Disclaimer: Due to unforeseen difficulties, we have had to take down the images on this notes page. They will be replaced shortly. We apologise for the inconvenience, but hope that the new images will provide you with an even better learning experience.

  1. Describe the circulatory system as a system of blood vessels with a pump and valves to ensure one-way flow of blood

The circulatory system is a system of blood vessels with a pump, and it has valves to ensure the one-way flow of blood.


  1. Describe the double circulation in terms of circulation to the lungs and circulation to the body tissue in mammals

Double circulation means that the blood flows through two circuits – one low-pressure circuit and one high-pressure circuit.

The low-pressure circuit is when the blood travels from the heart, to the lungs, and back. This is also known as the pulmonary circuit.

The high-pressure circuit, or systemic circuit, is when blood flows from the heart to the rest of the body, and back. This is higher pressure because the blood has to travel further, so the heart applies a greater force on this blood.


  1. Explain the advantages of double circulation

Double circulation is important because it maintains a high blood pressure to all the main organs of the body:

  • The right side of the heart collects blood from the body, builds up pressure, and pumps the blood to the lungs to be oxygenated. As the blood travels, the pressure drops.
  • The left side of the heart collects the oxygenated blood from the lungs, builds up pressure, then pumps the blood to the rest of the body. As blood travels, the pressure drops, until it collects in the right side of the heart again.

Double circulation also helps keep the oxygenated and deoxygenated blood separate and prevents their mixing – this allows a highly efficient supply of oxygen to the body.


  1. Name and identify the structures of the mammalian heart, limited to the muscular wall, the septum, the left and right ventricles and atria, one-way valves and coronary arteries

In the left diagram, the heart has been flattened and splayed out so it’s easier for you to see and understand how the structures connect together. The right one shows how the structures actually look




Note: your left is the heart’s right, which is why the lefts and rights are swapped in the diagram.


The heart is made of four chambers: the left and right atrium at the top, and the left and right ventricles at the bottom.

The right side of the heart is split from the left side by the muscular septum.

The heart has ‘atrioventricular’ valves between the atria and ventricles on both sides of the heart.

The atrioventricular valve on the right is called the tricuspid valve because it has three ‘cusps’.

The atrioventricular valve on the left is called the bicuspid valve because it has two cusps. It is also called the mitral valve.


The wall of the heart is made of a special type of muscle called ‘cardiac muscle’ or ‘myocardium’. The wall of the ventricles is thicker than the atrial walls. The left ventricle’s walls are slightly thicker than the right ventricle’s walls.

The heart needs a very high amount of energy to pump blood fast enough around our bodies, so it has a very high rate of respiration. This means the heart uses up a lot of oxygen. The blood inside the heart chamber isn’t enough to supply the heart with oxygen, so the heart has special arteries called ‘coronary arteries’ that supply the muscular wall of the heart with blood.



  1. State that blood is pumped away from the heart into arteries and returns to the heart in veins

Blood is pumped away from the heart into arteries and returns to the heart in veins.


  1. Describe the functioning of the heart in terms of the contraction of muscles of the atria and ventricles and the action of the valves

When the entire heart is relaxed, the heart is in diastole. During diastole, the pulmonary and aortic semilunar valves, and the atrioventricular valves are open. During diastole, the atria are filled with blood.

Next is atrial systole:

In this stage, the muscular walls of the atria contract, squeezing blood into the ventricles. The atrioventricular valves are forced open, but the semilunar valves are pushed shut due to the pressure of the blood.

Finally, in ventricular systole, the atria relax and the ventricular walls contract. This causes the atrioventricular valves to close and the semilunar valves to open, so blood is pushed out into the aorta and pulmonary artery.

Then, ventricular diastole occurs again, and so the cycle restarts.


  1. Name the main blood vessels to and from the:
    • Heart, limited to vena cava, aorta, pulmonary artery and pulmonary vein

The superior vena cava collects blood from the head and upper parts of the body and empties it in to the right atrium. The inferior vena cava collects blood from the rest of the body and empties it into the right atrium.

The pulmonary artery carries blood from the right ventricle to the lungs

The pulmonary veins collect blood from the lungs and empties it into the left atrium.

The aorta takes blood from the left ventricle to the rest of the body.

  • Lungs, limited to the pulmonary artery and pulmonary vein

The pulmonary artery carries deoxygenated blood from the right ventricle of the heart to the lungs.

The pulmonary veins carry oxygenated blood from the lungs to the left atrium of the heart.

  • Kidney, limited to the renal artery and the renal vein

The renal artery carries blood to the kidney.

The renal vein carries blood away from the kidney.









  1. Describe coronary heart disease in terms of the blockage of coronary arteries and state the possible risk factors as diet, stress, smoking, genetic predisposition, age and gender

Coronary heart disease (CHD) is caused by atherosclerosis.

This is when plaque builds up in your arteries, thereby narrowing or blocking them up. This plaque is made of cholesterol, fatty substances, cellular waste products, calcium and fibrin.

This build-up of plaque usually takes several years.

Overtime, the plaque may harden, reducing the flow of oxygen rich blood to the heart muscles. Sometimes, this plaque might rupture (break apart), causing the formation of blood clots, or the broken piece of plaque to travel down to a narrow arteriole and block it up. Both of these completely cut off the supply of oxygen to the heart muscles. This could cause heart failure.

This is coronary heart disease.

CHD can be caused by a diet high in fat, especially saturated fats. It can be caused by stress, smoking, and sometimes, it is hereditary (you may have a genetic predisposition towards CHD). There is also plenty of evidence that shows that females, while menstruating, are much less likely to develop CHD than males. Once females stop menstruating, they seem to have a similar risk of developing CHD as males.


  1. Investigate and state the effect of physical activity on pulse rate

Measure the pulse rate of a person at rest.

This can be done by pressing two fingers down on the inside of their wrist, between the bone and the tendon on their thumb side. You should be able to feel pulses. This is because, during ventricular systole, blood is forced down the arteries, so they expand slightly. Using a stopwatch, count the number of pulses in 30 seconds, and multiply by two to gain the pulse rate per minute.

That person should then undergo exercise, e.g. jogging for 1 – 5 minutes.

Measure their pulse rate again and compare.

Physical activity increases pulse rate.


  1. Explain the effect of physical activity on the heart rate

Physical activity increases heart rate. This is because physical activity increases the body’s need for energy, increasing the rate of respiration and thus the demand for oxygen. This means the heart needs to pump blood around the body faster, to deliver oxygen faster to the needy tissues, so the heart rate increases. If the heart rate increases, the number of ‘pulses’ of blood pushed out by the ventricles per minute also increases, so the pulse rate increases.

Note: pulse rate is usually equal to heart rate.


  1. Describe the structure and functions of arteries, veins and capillaries

Arteries carry blood away from the heart. They divide into smaller and smaller vessels until they become capillaries. It is in capillaries that materials such as oxygen, carbon dioxide, nutrients and waste are exchange between the blood and tissues. Capillaries join up to form larger vessels, until they eventually form veins. Veins carry blood back to the heart.

Arteries have the thickest walls. The elastin in their walls allow them to expand to accommodate the blood during each pump of the ventricle (instead of bursting), and it allows them to recoil to push the blood and maintain the high pressure.

Veins have much thinner walls, so their tunica media and tunica externa are relatively thin. As opposed to arteries, veins have valves to prevent the back flow of blood, especially when blood has to flow against gravity. The muscles around veins contract and relax, providing a force to push blood along the veins.

Capillaries are the smallest blood vessel, and carries the lowest pressure blood. Usually, only one RBC can fit through the diameter of the capillary (Capillaries usually have a diameter of approximately 7 micrometres, and RBCs have a diameter that is approximately equal to that.). This means they only have one layer, instead of three.

All three blood vessels have a single layer of endothelial cells (squamous epithelium). Arteries and veins have a tunica media (smooth muscle) and a tunica externa (elastin and collagen). Arteries have more elastin than veins in their tunica externa.

Arteries have a relatively small lumen (space inside the vessel), and veins have a larger lumen. Capillaries have the smallest lumen.

  1. Explain how the structures of arteries, veins and capillaries are adapted for their function

Arteries have the highest-pressure blood flowing through them, so they have the thickest walls. They don’t need valves.

Veins have blood at a lower pressure flowing through them, so their walls don’t have to be as thick. They also have a much larger lumen, allowing blood to flow much more easily through them. Veins need valves to prevent the back flow of blood.

Capillaries are very small in diameter, allowing them to bring blood closer to the needy tissues. This reduces the diffusion distance of oxygen from RBCs to tissue, and carbon dioxide from the tissue to the RBCs. They have a single cell thick wall, again reducing diffusion distance, and blood flows relatively slowly through arteries, allowing more time for diffusion.

Capillaries form an extremely extensive network of blood vessels, so despite individual capillaries having a small cross-sectional area, capillaries have the largest total cross-sectional area of all three blood vessel types.


  1. List the components of blood as red blood cells, white blood cells, platelets and plasma

Blood is made up of red blood cells (this is what makes blood red), white blood cells, platelets and plasma.





  1. Identify red and white blood cells, as seen under the light microscope, on prepared slides and in diagrams and photomicrographs

Red Blood Cells:


Red Blood Cell light microscope
Segmentet neutrophil” by Magdalena Wiklund is licensed under CC BY-ND 2.0

The image to the left is taken from a light microscope. The vast majority of cells in the image are red blood cells – notice how they are darker around the edges and lighter towards the centre. This is because RBCs are thinnest at the centre.

Note: this image has a white blood cell in it. Specifically, it has a neutrophil. There is more information about white blood cells below.





White Blood Cells:

White Blood Cells, also known as leukocytes, are a little bit more complicated, as there are many types of WBCs.

There are 5 main types of WBCs –

  • Monocytes, which later mature into macrophages – They have a long lifespan and help kill bacteria.
  • Lymphocytes – These create antibodies that play an important role in the battle against foreign bodies like bacteria and viruses.
  • Neutrophils – These are the most common WBC (50 – 70% WBCs in circulation are neutrophils). They digest and kill bacteria by phagocytosis.
  • Basophils – they ‘sound an alarm’ by secreting chemicals, to alert other WBCs of infectious pathogens.
  • Eosinophils – They attack and kill parasites, destroy cancer cells, and help with allergic responses.

These WBCs are often sorted into two groups called granulocytes (the WBCs with granules in their cytoplasm) and agranulocytes (the WBCs without granules).

The granulocytes are neutrophils, eosinophils and basophils.

The agranulocytes are monocytes and lymphocytes.

They are also sorted into two different groups – lymphocytes (the cells that produce antibodies) and phagocytes (the cells that perform phagocytosis).


Neutrophils are characterised by multiple lobes on their nuclei, and granules in their cytoplasm.

Eosinophils have two lobes on their nuclei and granules.

Basophils have two lobes on their nuclei again, are usually stained purplish-black, and have granules

Lymphocytes are the smallest of the WBCs and have a large spherical nucleus that takes up most of the cytoplasm.

Monocytes have a kidney-shaped nucleus and plenty of cytoplasm.


  1. State the functions of the following components of blood:
    • Red blood cells in transporting oxygen, including the role of haemoglobin

Red Blood Cells contain many thousands of haemoglobin, each of which are made of four polypeptides. Each polypeptide has one iron ion (Fe2+) attached to it. This is where an oxygen molecule binds. This means, each haemoglobin molecule can carry up to 4 oxygen molecules and hence, 8 oxygen atoms.

When the RBCs are in the lungs, they are surrounded by a high concentration of oxygen, leading to more and more oxygen binding with the haemoglobin. This leads to the blood becoming saturated with oxygen – it is carrying its maximum amount of oxygen. As the RBCs leave the lungs, and are transported in blood to respiring tissue, they are transporting oxygen to these tissues. As these tissues use up oxygen, the RBCs are surrounded by less oxygen, resulting in the release of oxygen from the haemoglobin. This is possible because the bond between the oxygen molecule and the haemoglobin isn’t strong enough to be permanent.

  • White blood cells in phagocytosis and antibody production

Neutrophils, macrophages, eosinophils and basophils can perform phagocytosis, and so, they are called phagocytes.

However, if they ask which WBC performs phagocytosis in a paper, the expected answer is usually neutrophils.

Phagocytosis is the ingestion and digestion of bacteria by white blood cells. This successfully breaks down the pathogen into its harmless components.

The stages of phagocytosis:

  1. ingestion: the bacteria/ food particles are engulfed by the WBC. This results in the formation of a food vacuole (also called a phagosome)
  2. Vesicles, called lysosomes, containing digestive enzymes and other toxic chemicals, fuse with the food vacuole, dumping the enzymes into said vacuole. This forms a phagolysosome.
  3. The bacteria are digested.
  4. The components of the bacteria are often egested (dumped outside the WBC).

Antibodies are formed by lymphocytes.

On the left is a diagram of an antibody. Different lymphocytes can develop to produce antibodies that are different to each other.

The Fv is the ‘variable region’ of the antibody and the Fc is the ‘constant region’ of the antibody. The shape of the Fv is specific to the bacteria that it binds to – its shape means that it can only bind to one specific area of one specific type of cell. Lymphocytes can develop to produce many different types of antibodies, ensuring that each different bacterium can be binded to by an antibody. The Fc is the same in every antibody.

The functions of antibodies include:

  • Act as a label: Cells with antibodies binded to them can be identified as target cells by phagocytes.
  • Neutralisation: the binding of an antibody to a harmful toxin can neutralise the toxin, making it useless.
  • Agglutination: They may cause several pathogens to stick together, preventing them from dividing and multiplying, causing them to die out. It also makes them easier for phagocytes to find and phagocytose.


  • Platelets in clotting (details are not required)

Platelets are small disc shaped cell fragments – they are created by cell fragments breaking off of a very large type of white blood cell called ‘megakaryocyte’. They are involved in the formation of blood clots.

  • Plasma in the transport of blood cells, ions, soluble nutrients, hormones and carbon dioxide

About 55% of blood is plasma. This is the solution that carries blood cells, and other solutes around the body.

Plasma is a pale-yellow sticky liquid. It is 92% water, 8% dissolved protein, soluble nutrients, hormones and carbon dioxide.

It takes RBCs close to respiring tissue to supply them with oxygen, it takes carbon dioxide from the respiring tissue and to the lungs, it carries nutrients from their sites of production to their sites of usage or storage, and transports hormones to their target organs.

The main protein in plasma is albumin.



Notes submitted by Sarah

Click here to go to the previous topic.

Click here to go to the next topic.

Click here to go back to the Science menu.