Aims of this Episode:

  • Identify the different parts of the Circulatory System;
  • Describe the differences between each blood vessel;
  • Identify the function of the different parts of the blood.

The Heart

Human hearts beat twice in each cycle, meaning our Circulatory System is known as a ‘double circulatory system’.

Firstly, the blood is received from the body through the Vena Cava and pumped to the lungs via the Pulmonary Artery, so that gas exchange can happen. The now oxygen rich blood re-enters the heart via the Pulmonary Vein and is then pumped at high pressure around the rest of the body, via the Aorta.

Not all organisms have a double circulatory system. Smaller animals, like fish, don’t need the high blood pressures seen in double circulatory systems, so their hearts only beat once per cycle.

If you ever get chance to dissect a heart, take some time to notice how one side is more muscular than the other.

The Blood Vessels

Artery The arteries usually carry oxygenated blood away from the heart and towards the cells and tissues around the body. They have a very thick muscular wall to cope with the high pressure. They also deform to allow the surges of blood to pass through. It is this deformation that you can feel when you take your pulse.

Vein The veins usually carry de-oxygenated blood towards the heart and away from the cells and tissues. They don’t have a very thick wall because the pressure is much lower. However, veins have valves to prevent the blood from flowing the wrong way down the body.

Capillary The capillaries are tiny blood vessels between the main arteries and veins. Imagine them as the small roads around your house that you travel on after you have come off the motorway. They have a very thin wall (one cell thick in some cases), which allows diffusion of the important compounds to happen very quickly – it creates a short diffusion pathway.

The Blood

Plasma – The plasma is the liquid, which carries all of the other parts of the blood around the body.

Red Blood Cells – The most popular type of blood cell, the Red Blood Cell carries the oxygen and carbon dioxide around the body. They are red because of the haemoglobin, which the oxygen and carbon dioxide attach to. Red Blood Cells also have a dip in the middle of the cell, which increases the surface area and allows diffusion to happen quicker.

White Blood Cells – These are much bigger than the Red Blood Cells and their job is to target an destroy any diseases or germs in the body. They do this by consuming the germ cell into themselves and destroying them.

Platelets – Platelets are key for clotting blood (binding the cells together to stop bleeding). When they arrive at a cut they form a spaghetti like mesh, which holds the blood cells together. This is more commonly known as a scab.

S@S 011: The Circulatory System
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