Circulation

We are made up of trillions of cells and as such, most of them are a long way away from a direct source of food and oxygen. Unicellular organisms such as bacteria and yeast do not require a circulatory system since direct diffusion is sufficient to provide them with what they need to survive. However, for multicellular organisms like humans which have a small surface area to volume ratio, a transport system is essential. Just think about how far from your small intestines your fingers are, or how far your toes are from your lungs. The circulatory system transfers substances to and from the body cells.

This site is a really good resource for information about the circulatory system:
Components of the circulatory system

There are 3 components:
  • the heart - pumps the blood around the body
  • the blood - the liquid that carries substances around the body
  • the blood vessels - the tubes that carry the blood
This is a simplified representation of haemoglobin, the protein within red blood cells that allows them to carry oxygen.
Haemoglobin is an example of a quaternary protein - that is, it is a protein that is actually formed by 4 separate chains of amino acids folding together. Each molecular of haemoglobin contains 4 iron atoms, and it is the iron that allows the carriage of oxygen. When haemoglobin is bound to oxygen (oxyhaemoglobin), it has a red colour, but in its deoxygenated form it is blue-red. 

One red blood cell contains about 250 million molecules of haemoglobin which allow it to carry 1000 million molecules of oxygen at any one time. Clever huh?!
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BLOOD...
There are 4 components of the blood, each of which has important roles. 
Plasma
  • Liquid part of the blood
  • Yellow colour
  • Transports blood cells and other substances (CO2 from muscles to lungs, urea from liver to kidneys, soluble products of digestion from small intestine to all cells of the body)
Red blood cells
  • Carry O2 from lungs to rest of body
  • No nucleus and have biconcave disc structure
  • Packed full of haemoglobin - protein which allows them to carry O2
   Haemoglobin + O2 = Oxyhaemoglobin
White blood cells
  • Bigger than red blood cells but less dense
  • Part of the body's immune system
  • Common types are lymphocytes and phagocytes (see Defence topic)
Platelets
  • Fragments of cells, with no nucleus
  • Involved in blood clotting, a series of enzyme-controlled reactions
  • Platelets release a substance that turns soluble fibrinogen into fibrin
ABO Blood Groups
In 1900 an Austrian scientist called Karl Landsteiner identified the ABO blood grouping system. Prior to this point, it was very hit-and-miss as to whether you would survive a blood transfusion. The central principal of the blood groups is that antigens are present on the red blood cells, and it is what compliment of antigens you have (or don't have) that determines your blood group. 
  • There are 2 types of antigens: antigen A and antigen B

In addition, there are 2 types of antibodies:
  • Anti-A and anti-B

You will NOT have the antibodies against the antigens that you express on your red blood cells (so, if you are blood group A, you express antigen A on your red blood cells, and thus have anti-B antibodies in your plasma). 


Remember:
If you have blood type O, you do not express any antigens on your red blood cells.
If you have blood type AB, you express both antigen A and antigen B
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Knowing what blood group you are is critical in determining what blood can you receive during a transfusion. If you are given the wrong type of blood, the antibodies present in your plasma will bind to the antigens on the red blood cells in the donor blood, causing clumping, or agglutation. This can be lethal.

Here is a different way of looking at blood compatibility. DO NOT worry about learning or remembering this - you are better off learning the above table and knowing which antigens and antibodies each blood group has. This will allow you to work out compatibility. 
If you are still unsure, go to Blood.co.uk which is full of interesting information on blood
Blood clotting is a series of enzymatically-controlled reactions which result in the formation of a clot. Damage to a blood vessel (such as a cut) activates the platelets. Platelets cause fibrinogen (a soluble protein) to form a mesh of insoluble fibrin. This acts to trap red blood cells, forming a clot. When the clot dries it becomes a scab.

Inappropriate activation of platelets can cause an internal blood clot, called a thrombus, to form. These can be deadly as they can move around the body, potentially blocking blood vessels in the brain or heart.

For a fascinating insight into the history of the heart visit this site:

Anatomy of the heart

Arteries
  • carry blood AWAY from the heart
  • have thick muscular walls (helps push blood)
  • high pressure and flow rates
  • carry oxygenated blood
Veins
  • carry blood BACK to the heart
  • have thin walls
  • low pressure and flow rates
  • valves help to prevent fall-back of blood
  • contractions in the muscles surrounding the veins help keep blood flowing against gravity
  • carry waste-rich deoxygenated blood

Capillaries
  • connect arteries and veins
  • 1 cell thick walls to allow diffusion of substances into and out of them
  • carry both oxygenated and deoxygenated blood
The blood vessels that you need to remember are:
  • vena cava (vein)
  • pulmonary vein (unlike other veins, this carries oxygenated blood back to the heart)
  • aorta (artery)
  • pulmonary artery (unlike other arteries, this carries deoxygenated blood)
  • coronary arteries (supply the cardiac muscle of the heart itself)

The human circulatory system is a DOUBLE CIRCULATORY SYSTEM. This means that for every circuit of the body, the blood passes twice through the heart.
Pulmonary circulation - from heart to lungs and back again
Systemic circulation - from heart to body and back again



If you are still unsure about the circulation of blood, have a look at this You Tube video called Pump It Up!
Thanks Tolia!
Research Project
For your research project, I have asked you to look into how the circulatory system can go wrong, and what medical interventions there are available to treat diseases and conditions of the heart and blood vessels. Here are a few links to sites to get you started that will help you find the information you are looking for. Remember: this needs to be in your own words. DO NOT copy directly from any source, be it the internet, your text books or a journal article. That is plagiarism and as you know, this will not be tolerated.
Check out this excellent website on heart and circulation