How the Process of Donating Blood Works

Woman Donating Blood
Woman Donating Blood. Jan Wolak/E+/Getty Images

Ever donated blood at a blood drive at your church, school, or community center?  Maybe you’ve been interested but you weren’t quite sure what happens during and after the donation.  Have you been worried about how much blood is removed?  Let's review the process.     

The amount of blood in your body (called blood volume) is dependent on your weight.  The average adult weighing 150 – 160 pounds has a blood volume around 5 liters (about 10 pints).

  During a blood donation approximately 500 mL (1 pint) is removed.  This small amount of blood is regenerated by the body over time.  

Donating blood is a relatively simple procedure.   Initially the blood center staff will have you answer a series of questions to ensure you are healthy enough to donate and that your blood will be safe to use.  It is very important to answer these honestly.  Then your temperature, heart rate and blood pressure will be taken.  Lastly a tiny drop of blood will be taken from a fingerstick to make sure you aren't anemic.  Sometimes people will be told they have low iron from this test; this isn’t a completely accurate statement.  This blood work measures the hemoglobin (a protein in the red blood cell).  If the hemoglobin is low, then you are anemic and therefore ineligible to donate blood.  The most common reason for low hemoglobin is iron deficiency so this is why this can be confusing.


After the screening process is complete, you will go to another area to donate the blood.  You will lay down and a tourniquet (a band that squeezes your arm) will be placed on your arm so that your vein fills with blood making it easier to place the needle.  Your skin will be cleaned thoroughly prior to inserting the needle into the vein.

  Your blood will flow through tubing into a bag for collection. You may be asked to squeeze a small ball to help the process.  The first part of blood collected is placed in tubes to undergo testing.  This testing will determine your blood type as well as screen your blood for HIV, hepatitis B & C, West Nile Virus,  and other infectious diseases.  All blood donated in the US undergoes this testing.  This usually takes about 10 minutes.   

After the blood is collected, the needle will be removed and your arm will be bandaged.  It is important to sit for a short amount of time before getting up and walking after the donation as you can get dizzy.  Please eat and drink the delicious snacks they have available for you to start the fluid rehydration process.  For the rest of the day, drink lots of fluid to replace the blood lost from the donation.  The fluid lost during the donation is replaced over the next 24 hours.  You can donate blood again in 8 weeks. 

After collection, your blood will go to the blood center for processing and testing.  The blood collected is refer to as whole blood.  At some point in the process (either at collection or during processing) the white blood cells will be removed from the blood.

  This process, called leukoreduction, is to prevent febrile (fever) reactions in people receiving blood transfusions and to prevent transmission of the CMV (cytomegalovirus) during blood transfusions.   

Then the bag of blood is divided into different products.  First the red blood cells are separated into a unit called packed red blood cells.  The platelets are removed from the bag, this is called a random donor unit of platelets.  There aren't a ton of platelets in a random donor unit, so for an adult, 6 – 10 random donor units of platelets are combined for platelet transfusion.   The liquid portion of the blood, plasma, can be separated and frozen until ready to use.

Once it is thawed this is called fresh frozen plasma (FFP).   FFP contains some of the coagulation factors and can be used to stop bleeding in patients with particular bleeding disorders.   

Although the above method is the most common method for donating blood, it’s not the only way.  There is another method called apheresis.  In this method, you will have a needle placed in a large vein in your arm which will be attached to an apheresis machine.  This machine separates the blood cells.  One of the advantages of this method is that the blood removed is replaced with fluids, causing you to be less likely to experience dizziness afterwards.  This process takes a longer amount of time, about 1.5 to 2 hours.  If you are donating red blood cells, only these are collected and your white blood cells and platelets are returned to you with some fluid.  During this collection, 2 units of red blood cells can be collected.  Because you can donate 2 units instead of just one, you can donate red blood cells by apheresis every 16 weeks.   Platelets can also be collected this way (plateletpheresis), with the red blood cells and white blood cells being returned to you with some fluid.   Because platelets are replaced more rapidly than red blood cells (usually within a day), they can be donated this way every 7 days up to 24 donations in a year.  Plasma can also be collected during pheresis.  If you are going to donate platelets via plateletpheresis it is extremely important NOT to take medications containing aspirin or ibuprofen (Advil, Motrin).  These medications decrease the function of the platelets.   

 You can get additional information from your local blood donor center or the American Red Cross.  

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