Blood Cells You have three kinds of blood cells; they all develop throughout your life inside your big bones, in the bone marrow.


Red Blood cells (Erythrocytes) These are what make blood red; there are about 5,000,000,000 in a cubic centimetre of blood. They have an unusual shape and even more unusually, they do not have a nucleus.


They only live for about 120 days. One of the jobs of the blood is to carry oxygen from the lungs to the rest of the body. Oxygen gas combines with a protein containing iron called haemoglobin inside the red cells. The blood changes colour when this happens. Oxygenated blood is bright red in colour, but when it gives up its oxygen in the tissues, it goes dark red, not, as some think, purple or blue.


White Blood cells (Leucocytes) There aren’t so many of these and they are bigger than red cells. They defend against disease; some of them ‘eat’ bacteria! In photographs they are often seen as a bit purpley because they have been stained with a dye; you couldn’t see them otherwise.


How to take your pulse You can do this on your wrist. Place two fingers of your right hand on the inside of your left wrist and press down until you can feel the beat of the artery (called the radial artery). Use a watch to count the beats in 15 seconds and multiply by 4 to get beats per minute. Try measuring it before and after exercise. A child’s heart rate is faster


than that of a much older person.


Platelets (Thrombocytes) These are small and help to make blood clot. When you get a cut, the blood leaks out, of course, but it stops eventually. Platelets gather wherever there is damage and link up into a kind of network that slowly turns solid, making a clot, and plugging the hole. Clotting is normally good, but it can cause problems if blood clots inside a blood vessel, like in your brain.


Plasma


If you get fresh blood in a bottle or test tube and leave it to stand for an hour, all the cells will settle to the bottom as a dark red layer, leaving a clear straw-coloured liquid above. This is plasma. It contains water, salts and proteins, such as albumin and antibodies that help defend your body against disease.


Fresh blood Allow to settle = Plasma


Blood cells


There are lots of websites about blood, but have a look at these two: Blood transfusion service website www.blood.co.uk/pages/e17compn.html www.funsci.com/fun3_en/blood/blood.htm


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Did You Know?


An adult has about 9 pints of blood in their body. Children have less, of course.


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