The extra blood for blood transfusions is given by other people, called donors, from their own bloodstreams. Before they give blood, their health is checked to make sure they are not unwell, and they are tested to make sure they don’t have any infections. The blood taken from them is then stored safely in bags until someone else needs it.

You can only have a blood transfusion using blood from a donor whose blood is as similar to yours as possible. Making sure that your blood matches with the blood of a donor is called cross matching, and it is something that is done very carefully before you have a blood transfusion.

If you lose a small amount of blood, like when you cut yourself, your body makes new blood cells to replace the ones you lose. If you lose a LOT of blood though, like during an operation, or after a bad accident, a transfusion of red blood cells might be the only way to replace it quickly.

You might also need red blood cell transfusions if you have illnesses like haemophilia or thalassaemia.

If you have leukaemia, or have had high doses of chemotherapy or a bone marrow transplant, you will probably have a low amount of platelets in your blood, and will need a platelet transfusion.

If you are having surgery on your liver, heart or kidney, or are having orthopaedic surgery you might need a plasma transfusion to make sure your blood clots properly.

You will have your blood transfusion in hospital, and before you have it, a doctor or nurse will check your full name, date of birth and other details on your wristband. Make sure you tell your doctor about any medicines that you are taking, as some, like aspirin, can make you bleed more during surgery. Also, if you have been given a card that says you need to have a specific type of blood, show it to your doctor or nurse as soon as you can.

The blood for transfusions is stored in small plastic bags. Each of these holds about half a litre of blood, which is usually given over 1 or 2 hours. You may be given more than one bag of blood during your transfusion. The bag is connected to a drip, and the drip is usually connected to a tiny plastic tube, called a cannula. The cannula is put into a vein in your arm. The blood runs out of the bag, through the drip and into your arm through the cannula.

Blood transfusions are sometimes given through a central line in a vein in your chest or a PICC line in the crook of your elbow instead of through a cannula, though. The central line is good if you need a few blood transfusions, because it’s much easier for the blood to get into your veins this way. A PICC line lets you move your arm around more easily because they are flexible, so it is much less likely to come out of your vein by accident.

A doctor or nurse will keep an eye on your blood pressure, body temperature and pulse while you are having the transfusion, and will check that you feel okay.

You probably won’t feel a thing while you are having it! A doctor or nurse will keep an eye on you regularly though, and if you DO start to feel unwell during or just after the blood transfusion, let them know straightaway.

Blood transfusions are almost always safe. The biggest risk is being given the wrong blood by accident. This is why your identification details will be carefully checked on your wristband before you have the transfusion to make sure you get the right blood.

The chance of getting an infection from blood from a blood donor is very very low, because this blood is very carefully checked to make sure it's as safe as possible.

While you are having the transfusion, or just after it, you might get a temperature, a skin rash, a headache or chills. These side effects are not usually very bad at all though, and your doctor can easily give you some medicine to help with them.

If there is anything else you are worried about, just let your doctor or nurse know.

If you have any other questions, you can ask your doctor, or chat to other people about having a blood transfusion in the Medikidz forums and virtual world, Mediland.

Or you can have a look at the NHS Choices article on blood transfusion.