Page 702 - Fundamentals of anatomy physiology

Chapter 19 Blood 689

Figure 19–5 Stages of RBC Maturation. Red blood cells are pro- Regulation of Erythropoiesis

duced in the red bone marrow. The color density in the cytoplasm indi- For erythropoiesis to proceed normally, the red bone marrow
cates the abundance of hemoglobin. Note the reductions in the sizes of
the cell and nucleus leading up to the formation of a reticulocyte. needs adequate supplies of amino acids, iron, and vitamins

RED BONE MARROW (including B12, B6, and folic acid) for protein synthesis. We
obtain vitamin B12 from dairy products and meat. In order to
Day 1: absorb vitamin B12, we need intrinsic factor, which is produced
Proerythroblast in the stomach. Without vitamin B12 from the diet, normal
stem cell divisions cannot occur and pernicious anemia results.

Thus, pernicious anemia is caused by a vitamin B12 deficiency,
a problem with the production of intrinsic factor, or a problem

with the absorption of vitamin B12 bound to intrinsic factor.
Erythropoiesis is stimulated directly by the peptide

Erythroblasts hormone erythropoietin ( p. 666) and indirectly by several
Day 2:
hormones, including thyroxine, androgens, and growth hor-
Basophilic
erythroblast mone. As noted earlier in Spotlight Figure 19–1 on pp. 680–681,

Day 3: estrogens do not stimulate erythropoiesis. This fact accounts for
Polychromatophilic
the differences in hematocrit values between males and females.
erythroblast
Erythropoietin (EPO), also called erythropoiesis-

stimulating hormone, is a glycoprotein. It is formed by the

kidneys and liver. EPO appears in the plasma when periph-

eral tissues, especially the kidneys, are exposed to low oxygen

concentrations. A low oxygen level in tissues is called hypoxia

(hı.-POKS-e. -uh; hypo-, below + oxy-, presence of oxygen). Eryth-

Day 4: ropoietin is released (1) during anemia; (2) when blood flow
Normoblast
to the kidneys declines; (3) when the oxygen content of air

in the lungs declines, due to disease or high altitude; and

(4) when the respiratory surfaces of the lungs are damaged.

Ejection of Once in the bloodstream, EPO travels to the red bone marrow, 19
nucleus
Days 5–7: where it stimulates stem cells and developing RBCs.
Reticulocyte
Erythropoietin has two major effects: (1) It stimulates cell

Enters bloodstream division rates in erythroblasts and in the stem cells that produce

erythroblasts, and (2) it speeds up the maturation of RBCs,

mainly by accelerating Hb synthesis. Under maximum EPO

Mature red stimulation, bone marrow can increase RBC formation tenfold,
blood cell
to about 30 million cells per second.

The ability to increase the rate of blood formation quickly

and dramatically is important to a person recovering from a

severe blood loss. But if EPO is administered to a healthy indi-

vidual, as in the case of the cyclists mentioned in Chapter 18, the

After roughly four days of differentiation, the erythro- hematocrit may rise to 65 or more. p. 672 Such an increase
blast, now called a normoblast, sheds its nucleus and becomes a
reticulocyte (re-TIK-u. -lo. -sı.t), which contains 80 percent of the can place an intolerable strain on the heart. Similar problems
Hb of a mature RBC. Hb synthesis then continues for two to
three more days. During this period, while the cell is synthesiz- can occur after blood doping, a practice in which athletes
ing hemoglobin and other proteins, its cytoplasm still contains
RNA, which can be seen under the microscope with certain elevate their hematocrits by reinfusing packed RBCs that were
stains. After two days in the red bone marrow, the reticulocyte
enters the bloodstream. At this time, reticulocytes normally ac- removed and stored at an earlier date. The goal is to improve
count for about 0.8 percent of the RBC population in the blood
and can still be detected by staining. After 24 hours in circula- oxygen delivery to muscles, thereby enhancing performance.
tion, the reticulocytes complete their maturation and become
indistinguishable from other mature RBCs. The strategy can be dangerous, however, because it makes blood

more viscous and increases the workload on the heart.

Blood tests provide information about the general health

of an individual, usually with a minimum of trouble and ex-

pense. Several common blood tests focus on RBCs. These RBC

tests assess the number, size, shape, and maturity of circulat-

ing RBCs, indicating the erythropoietic activities under way.