THE LIFE OF THE RBC AND THE PLATELET CLINICAL PATHOLOGY/NURSING
Brandy Sprunger-Helewa, CVT, RVT, AAS, VTS (ECC)
Erythrocytes and platelets both begin their lives as hematocytoblasts, or stem cells, within the bone marrow. From
there and within the bone marrow, they become ever-maturing erythroblasts and platelets until they are released into
circulation. Once there, erythrocytes become denucleated, mature red blood cells. Platelets are actually fragments of
large nucleated megakaryocytes; anywhere from 1,000–3,000 platelets are formed from one megakaryocyte. Once
fragmented, platelets become capable of providing primary hemostasis when an injury occurs to the blood vessels,
and are also nonnucleated.
Because erythrocytes and platelets do not contain a nucleus, they cannot divide to create replacements of
themselves, nor can they create proteins to repair themselves when damaged. This is why erythrocytes only have a
lifespan of 120 days, while platelets only live for about ten days. Were erythrocytes to contain mitochondria, they
would use up all of the oxygen that they were designed to carry to peripheral tissues. For energy, erythrocytes
anaerobically metabolize glucose within the plasma, as do platelets. This is why red cells and plasma or serum must
be separated after being spun down in a centrifuge; blood glucose levels would be falsely decreased if we did not.
One drop of blood contains 260 million red blood cells and 7.5–15 million platelets. Erythrocytes are biconcave,
which allows them to stack, bend, and fold into small capillaries, carrying oxygen to the most peripheral parts of the
body. The total surface area of all the erythrocytes in the body is about 3,800 square meters; that’s almost as big as a
football field, and is 2,000 times the surface area of the body itself! It takes 20 seconds for a red cell to make a
complete circuit around the body, and it will do this 75,000 times during its life time. Platelets, on the other hand,
are biconvex, and become more spherical in shape when being used to produce a clot. Platelets also develop long
tendrils that allow them to hang on to each other, making the clot stronger. Humans create over 100 billion platelets
in just one day, making them the most proliferative blood cell type in the body.
Erythrocytes
Each erythrocyte contains about 280 million hemoglobin molecules, and each of these has four “corners” that
contain a heme molecule. This heme molecule is essentially a ring around an iron core, which is what facilitates the
binding of oxygen. Heme and iron is what gives red blood cells their “red” hue. Not all of the oxygen in the body is
attached to erythrocytes; only about 98% of it is. The rest is dissolved in plasma waiting to be picked up and
transported by the red blood cells. This is why the use of a pulse oximeter does not give an accurate indication of
total body oxygenation. Pulse oximeters are only telling you how many erythrocytes are transporting oxygen, not
how much oxygen is dissolved in plasma.
It is in the capillaries that the erythrocytes must stack, bend, and twist to be able to move along the capillary beds to
deliver the oxygen to the tissues. Once the oxygen is delivered, carbon dioxide quickly attaches to the empty
hemoglobin. Hypoxia and hypotension stimulate red cell production by sending messages to the nephrons in the
renal cortex of the kidneys to release erythropoietin. Erythropoeitin stimulates the bone marrow to increase
proerythroblast numbers while also decreasing the amount of time it takes for an erythrocyte to mature. As hypoxia
or hypotension improve, the nephron receives the negative feedback and reduces the amount of erythropoietin
released, slowing down production.
There are many things that can affect erythrocyte life span, either directly or indirectly. These can include parasites,
infectious diseases, illnesses or neoplasias, drugs, and toxins. Some affect the erythrocyte’s ability to carry oxygen
while others destroy the shape of the cell itself. Still others may demolish the erythrocyte in its entirety. A list of the
common causes for anemia are listed in Table 1.
Hemangiosarcomas of the heart, liver, or spleen as well as disseminated intravascular coagulation (DIC) and
vasculitis cause shearing injuries to the red blood cells seen as schistocytes and acanthocytes on blood smears.
Rough blood vessels damage the smooth surface of erythrocytes as they pass by, tearing off pieces of the
membranes. These pieces are unable to hold hemoglobin, and, thus, cannot carry oxygen. Immune mediated
hemolytic anemia (IMHA) is a very common cause of both intravascular and extravascular red cell hemolysis. Red
cells are inappropriately tagged with antibodies that the immune system deems as foreign and then sets about
destroying the cells.
