Formation of RBCs

An erythrocyte (erythro = red) is a fully mature red cell found in the peripheral blood. Although highly specialized,
it is little more than a small bag – a membrane surrounding a solution of protein and electrolytes.

In appearance, an erythrocyte is a bi-concave, disc-shaped cell, somewhat like a doughnut that has no
hole. Small enough to pass easily through capillaries in single file, RBCs can change shape into almost any
configuration – a characteristic that gives them access to all tissue cells.

RBCs Formation

RBCs are formed continuously, but their number is precisely regulated. Too few cells will not oxygenate tissue;
too many cells will impede blood flow. Mature RBCs cannot reproduce themselves, so several million new
cells enter the blood daily from blood-forming centers in bone marrow. The term for red blood cell formation is

Life Span: When an RBC is about 120 days old, it is trapped and removed from the blood by the spleen or the
liver. Its iron atoms, however, are recycled; approximately 25 milligrams of iron become available daily from the
breakdown of old RBCs.

Bone Marrow: Virtually all bones in children up to the age of 5 are blood-forming centers. Bone marrow
becomes less productive as age increases. In adults (over age 20), RBCs are formed in the marrow of the
vertebra, sternum (breastbone), ribs, and the ends of the long bones.

Cell Generations: RBCs develop in a series of cell generations in the bone marrow (Figure 2). After several
generations, new cells called basophilic erythroblasts emerge. (A note about naming: WBCs are dyed or
stained for viewing microscopically. A cell with basophilic staining properties is a cell that stains specifically
with basic dyes; a cell with eosinophilic staining properties is a cell that stains with eosin, a red acidic dye.
Erythroblast literally means an immature red cell.)

Formation of RBCs

Formation of RBCs

In the next cell generations, hemoglobin begins to give the cells their typical red color and the nucleus is
extruded from the cell. The cell is now called a reticulocyte (reticula = network) because staining causes strands
of residual RNA cell content to clump into a network.

Reticulocytes pass into the capillaries by diapedesis (squeezing through the pores of the membrane) and
become mature RBCs within one or two days. These cells normally make up about 1% of circulating RBCs.

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