Blood components

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Blood components


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Blood components

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USMLE® Step 1 style questions USMLE

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A patient is hospitalized for recurrent bacterial pneumonia. Several blood samples are taken from the patient, centrifuged, and subsequently analyzed. Which components will only be found in the buffy coat of this patient’s blood sample?  

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The word “blood” evokes lots of thoughts - from tiny paper cuts to major injuries - spilling the red liquid is almost never a good thing. That’s because

blood helps us move nutrients and waste around the body, regulate our pH level, and helps prevent infections.

In fact, some components of blood help prevent the loss of blood during an injury.

The components of blood can be separated out by simply spinning the blood in a centrifuge-- a machine that whips a vial of blood in a circle over and over, really quickly. A bit like what happens to clothes in a washing machine.

When blood is centrifuged, the heaviest blood components moving to the bottom, and the lightest ones moving to the top.

Overall, three distinct layers form: the erythrocytes or red blood cells at the bottom, the buffy coat--which contains platelets and immune cells in the middle, and plasma at the top.

So starting at the bottom of the tube, there’s the large layer that takes up approximately 45% of the total blood volume made up of erythrocytes. This value is called the hematocrit.

A decreased hematocrit means that there are too few erythrocytes, either because they’re not being made or because they are being destroyed.

On the other hand, an increased hematocrit can be due to dehydration, because if there’s less liquid in the blood, then the portion taken up by erythrocytes would rise.

Alternatively, there might simply be too many erythrocytes being made, which can happen in some diseases.

Now, the main function of erythrocytes is to carry oxygen to tissues and bring carbon dioxide to the lungs so it can be expired.

Erythrocytes are shaped liked thin biconcave discs--meaning they have a depressed center which makes them flexible enough to fit through even the smallest blood vessels.

This shape also increases their surface area which helps them conduct gas exchange efficiently.

Erythrocytes lack organelles like the nucleus, which creates even more room for hemoglobin proteins which carry oxygen.

While red blood cells are fantastic for gas exchange, the fact that they don’t have any organelles means that they only live for about 120 days. So red blood cells are always being regenerated in the bone marrow.

The thin white middle layer just above the erythrocytes is called the buffy coat, and it contains platelets and leukocytes, or white blood cells.


There are four main components of blood: plasma, red blood cells, white blood cells, and platelets. Plasma is the liquid component of blood that consists of 55 % of the blood volume. It contains water, electrolytes, nutrients, hormones, and proteins. Red blood cells (RBCs) carry oxygen from the lungs to the body tissues. Red blood cells also transport carbon dioxide from the body. White blood cells (WBCs) help fight infection and disease. Platelets help with clotting.


  1. "Medical Physiology" Elsevier (2016)
  2. "Physiology" Elsevier (2017)
  3. "Human Anatomy & Physiology" Pearson (2018)
  4. "Principles of Anatomy and Physiology" Wiley (2014)
  5. "Passive blood plasma separation at the microscale: a review of design principles and microdevices" Journal of Micromechanics and Microengineering (2015)
  6. "Greenish discoloration of plasma: Is it really a matter of concern?" Asian Journal of Transfusion Science (2019)
  7. "Red Blood Cells: Centerpiece in the Evolution of the Vertebrate Circulatory System" American Zoologist (1999)

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