Erythrocytes

• Myeloid stem cells—these make RBCs, myeloid cells (granulocytes, eosinophils,

basophils, monocytes), and megakaryocytes (that make platelets).

As mentioned, there are growth factors that play a role in the development of these cells. For

RBCs, the main growth factor is erythropoietin or EPO. This is a hormone secreted by fibroblast

cells in the kidneys when they sense low oxygen tension. EPO goes to the bone marrow and

triggers the division of myeloid stem cells to make cells along the RBC or “erythrocyte” line. This

hormone is used in many medical situations that lead to reduced RBC counts in the body.

Another growth factor is thrombopoietin, which is a glycoprotein hormone that makes megakaryocytes/platelets. It is produced by the kidneys and liver.

Cytokines are made by many cells of the body and can act locally to stimulate the proliferation

of cells to make more WBCs. The two subtypes of cytokines include colony-stimulating factors

and interleukins. Colony-stimulating factors act completely locally to trigger myeloblasts to

make neutrophils, basophils, and eosinophils. These are called “granulocyte CSFs.” They are

used medically for cancer patients to increase the WBC count. Interleukins are signaling

molecules made by several cell types to encourage the maturation of cells during inflammation

and the immune response. There are many different interleukin types.

ERYTHROCYTES

The erythrocyte, or RBC, is the most common formed element in the blood. These cells make

up one-fourth of the total number of cells in the body. These are very small cells because they

need to squeeze through tiny capillaries to give oxygen to the tissues. They pick up nearly all

the oxygen in the lungs, carrying it to tissues; they only pick up 24 percent of the carbon

dioxide, however. They do not leave the vessels (unlike WBCs, which extravasate out of the

vessels).

Erythrocytes mature in the bone marrow and lose their nucleus in the process. Immature RBCs

are called reticulocytes and account for 1-2 percent of the total RBC count. The cells cannot use

cellular respiration because they don’t have mitochondria and utilize anaerobic respiration in

order to function. They have a unique structure, called the biconcave disc, which defines these

cells. The main protein in the cell is oxygen-carrying hemoglobin. The shape gives it a greater

surface area for oxygenation of the tissues. They flex to fit in tiny capillaries. Figure 85 shows

the shape of the erythrocyte:

Hemoglobin is the major protein in the erythrocyte. There are four folded chains that make up

the molecule—each of which is bound to a molecule of iron and each of which takes up one

oxygen molecule. Because of the numbers of hemoglobin molecules per cell are large (300

million molecules per cell), about 1.2 billion oxygen molecules can be transported per cell.

Bright red blood contains oxyhemoglobin, while dark red blood contains deoxyhemoglobin (no

oxygen attached to the molecule). When CO2 attaches to hemoglobin, it is called

carbaminohemoglobin.

An excess of RBCs is called polycythemia, which is a problem because it causes blood that is too

viscous to circulate. A low number of RBCs leads to low hemoglobin levels, resulting anemia.

Doctors can also measure the percentage of hemoglobin molecules that carry oxygen—a

number called the “percent saturation” or “O2 sat” level. Low oxygen readings in the blood (a

low O2 sat level) is called hypoxemia. Conditions of hypoxemia lead the kidney fibroblasts to

secrete erythropoietin to make more RBCs.

There are several nutrients necessary for the production of 2 million RBCs per second. Iron is

the most important trace mineral that must be there for hemoglobin production. Ferritin and

hemosiderin are the storage molecules for iron. Copper is also necessary for hemoglobin