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What are you donating when you donate blood?

Updated on April 1, 2013

Did you know that humans have a blood volume of about 5 liters? That’s five of the 1-liter bottles of pop that you bought for the March Madness games this week! In fact, that’s almost 10% of your total body weight, so you can afford to give some every once in awhile!

If blood is allowed to separate in the presence of an anticoagulant, it separates into red blood cells (42-47%), which is what you normally think of when you’re donating blood, the buffy coat consisting of leukocytes and platelets (1%) and plasma, containing, well, everything else.

So, when you donate blood, you’re giving more than red blood cells…

A Recipe for Plasma

Plasma is 90% water, and 10% solutes. The solutes consist of:

  • 7% plasma proteins
    • Albumin – a transport protein made in the liver, which not only transports hormones, but also helps to maintain osmotic pressure on blood vessel walls.
    • Globulins, such as immunoglobulin, also known as antibody.
    • Fibrinogen, a structural protein used in clotting.
    • Complement proteins, which are necessary for inflammation and a cascade of reactions leading to phagocytocis of microorganisms.
    • Lipoproteins, or fat.
  • 2.1% organic molecules, such as vitamins and amino acids.
  • 0.9% inorganic salts, such as electrolytes (the stuff in Gatorade, which help to keep your muscles and nerves functioning properly).

What are the cell types in blood?

  1. Erythrocytes – this is what you think of when you picture a red blood cell, and is by far the most abundant cell in blood (4-5 x 106/uL of blood, 7-8 micrometers in diameter).

These cells have no nuclei, or rather, they extrude their nuclei as they develop, and live about 120 days.

The image above shows the biconcave disc that is maintained by an actin-binding protein called alpha and beta spectrin. This protein forms a network with other proteins (ankyrin and protein 4.1) under the cell membrane, which enables the red blood cell to withstand the pressure of being forced through capillaries.

Defects in spectrin cause these cells to form spheres, and consequently, they cannot form stacks in capillaries. The stacking is called rouleaux formation, and allows for the maximum exchange of O2 and CO2 in peripheral tissues.

Red blood cells contain 33% hemoglobin, which is composed of four subunits and an iron-containing O2 binding site. Hemoglobin is what gives red blood cells their color, and is absolutely necessary for O2 and CO2 transport.

Some people may not be able to give blood, due to abnormalities in these cells.

Happy RBCs v. Unhappy RBCs

This is what’s called a sickle cell.

Sickle cell anemia is a molecular disease (the first to be discovered) that results from a single amino acid substitution (a hydrophobic valine instead of a hydrophilic glutamine) in the gene that enocodes one of the hemoglobin subunits.

The cells lose their biconcave shape and become comma shaped. As a result, the cells are fragile and capillaries are injured.

Believe it or not, 8% of African Americans are carry one allele with this mutation, so that some of their cells, about 40%, are sickle cells.

  1. Erythrocytes
  2. Leukocytes – these cells are just passers-by in the blood, and tend to perform most of their functions outside the blood. They are present at a concentration of 4-10 x103cells/uL of blood, and are 6-20 micrometers in diameter; larger than white blood cells, but hard to find. There are two subcategories here, with members of their own:

Granulocytes – a general term describing cells that contain antimicrobial “granules” in their cytoplasm. Thus, these cells are important in the first line of defense against microorganisms.

For example:

  • Myeloperoxidase – generates hypoclorous acid from hydrogen peroxide and chloride ions.

This is important; neutrophils have receptors on their surface, which react with bacteria and viral antigens, leading to the production of proinflammatory molecules. They also phagocytose, or ingest, the bug, and the granules are left to finish it off by the reaction shown here:

NADPH oxidase + O2à O2-à H2O2à myeloperoxidase + CL-à HOCL = …

Both the bacteria or virus, as well as the neutrophil, die in this reaction.

  • Of the granulocytes, the neutrophil is by far the most common (60-70%). These are immediately available if an infection were to occur, and live 6-7 hours in blood, and 1-5 days in tissues.

In fact, during acute inflammation induced by bacteria, neutrophils use nonspecific collagenase and type IV collagenase to digest their way through the basal lamina to the site of infection.

These cells marginate in blood vessels, meaning that they remain close to the blood vessel wall, waiting for the signal to move into the nearby tissue, a process called neutrophil diapedesis.

You may hear the term “polymorph” in association with this cell, because it has a lobed nucleus, thus it is dubbed polymorphonuclear.

FUN FACT: How can you tell if a blood sample is from a male or female? Sometimes, an inactive X chromosome present in females, called a Barr body, protrudes from one of the lobes of the nucleus in a female neutrophil.

Neutrophils have receptors on their surface, which react with bacteria and viral antigens, leading to the production of proinflammatory molecules. They also phagocytose, or ingest, the bug, and the granules are left to finish it off by the reaction shown above.

  • Eosinophils (2-4%) – these granulocytes are important in allergic reactions, as well as parasitic infections.
  • Basophils are next, but are very rare (0-1%) in blood.

Agranulocytes – these are leukocytes without specific granules.

  • Lymphocytes – Ah, these are the famous T- and B-cells that we hear so much about. These cells are important for the immune system to “remember” the pathogens that it encounters, and can live for many years. (Natural Killer cells are also included in this group, though they do have specific granules).
  • Monocytes – these become macrophages upon leaving the blood and functioning in peripheral tissues, and live less than one week. They primarily function to phagocytose and degrade foreign particles, and then display a piece of the particle on their surface, thereby alerting the other cells of the immune system to the presence of a foreigner.

Watch for another post on T- and B-cells and their interaction with monocytes, and see this one for an overview of the immune system!


  1. Erythrocytes
  2. Leukocytes
  3. Platelets are present at a concentration of 2-4 x 105/uL of blood, and are small; 2-4 micrometers in diameter. These are cell fragments from cells of the bone marrow called megakaryoctyes.

The megakaryocyte does not divide, but becomes larger, and fragments to generate platelets.

FUN FACT: If someone is missing a spleen, their blood is likely full of platelets, since the spleen "cleans" the blood, making that person prone to blood clots. Cardio exercise can fix that, by strengthening the ability of the heart to pump blood!

Phew! Next time you give blood, you might just know more about what you’re really giving than the person taking it!


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      Andrew M 5 years ago

      Hahaha, I like the spleen comment at the end....