Erythrocyte Production

Erythrocyte production is called erythropoiesis (eh-RITH-ro-poy-EE-sis). It normally generates about 2.5 million RBCs per second (20 mL/day). The sequence of cell transformations leading to an erythrocyte is hemocytoblast ^ proerythroblast ^ erythroblast ^ normoblast ^ reticulocyte ^ erythrocyte. The proerythroblast is the first committed cell, having receptors for the hormone erythropoi-

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etin (EPO). Once EPO receptors are in place, the cell is committed exclusively to producing RBCs. EPO is secreted by the kidneys and liver and stimulates proerythroblasts to differentiate into erythroblasts. Erythro-blasts multiply and synthesize hemoglobin (the red oxygen-transport protein), then discard their nucleus, which shrinks and is lost from the cell. With the nucleus gone, the cell is called a reticulocyte—named for a fine network of endoplasmic reticulum (ER) that persists for another day or two. The overall transformation from hemocytoblast to reticulocytes takes 3 to 5 days and involves four major developments—a reduction in cell size, an increase in cell number, the synthesis of hemoglobin, and the loss of the nucleus.

Reticulocytes leave the bone marrow and enter the bloodstream. When the last of the ER disappears, the cell is a mature erythrocyte. About 0.5% to 1.5% of the circulating RBCs are reticulocytes, but this percentage increases under some circumstances. Blood loss, for example, stimulates accelerated erythropoiesis and leads to an increasing number of reticulocytes in circulation—as if the bone marrow were in such a hurry to replenish the lost RBCs that it lets many developing RBCs into circulation a little early.

Erythrocyte Homeostasis

The RBC count is maintained in a classic negative feedback manner (fig. 18.5). If the RBC count should drop (for example, because of hemorrhaging), then the blood will carry less oxygen—a state of hypoxemia7 (oxygen deficiency in the blood) will exist. The kidneys detect this and increase their EPO output. Three or 4 days later, the RBC count begins to rise and reverses the hypoxemia that started the process.

Hypoxemia has many causes other than blood loss. Another cause is a low level of oxygen in the atmosphere. If you were to move from Miami to Denver, for example, the lower O2 level at the high altitude of Denver would produce temporary hypoxemia and stimulate EPO secretion and erythropoiesis. The blood of an average adult has about 5 million RBCs/ ^L, but people who live at high altitudes may have counts of 7 to 8 million RBCs/ ^L. Another cause of hypoxemia is an abrupt increase in the body's oxygen consumption. If a lethargic person suddenly takes up tennis or aerobics, for example, the muscles consume oxygen more rapidly and create a state of hypox-emia that stimulates erythropoiesis. Endurance-trained athletes commonly have RBC counts as high as 6.5 million RBCs/^L.

4hemo = blood + poiesis = formation of

5myel = bone marrow

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18. The Circulatory System: Blood

Text

Stem cell

Committed cells

Precursor cells

Formed elements of circulating blood

Formed Blood Elements

Basophilic myelocyte

Basophil

Granulocyte-macrophage colony-forming unit

Granulocyte-macrophage colony-forming unit

Macrophage With Rbcs

B progenitor

Monoblast

Basophilic myelocyte

Monoblast

T cell precursor (lymphoblast)

T cell precursor (lymphoblast)

B progenitor

B cell precursor (lymphoblast)

B cell precursor (lymphoblast)

Basophil

Monocyte

Monocyte

T lymphocyte

T lymphocyte

B lymphocyte

B lymphocyte

Figure 18.4 Hemopoiesis. Stages in the development of all the formed elements of blood.

Saladin: Anatomy & I 18. The Circulatory System: I Text I © The McGraw-Hill

Physiology: The Unity of Blood Companies, 2003 Form and Function, Third Edition

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Hypoxemia (inadequate O2 transport)

Increased O2 transport

Sensed by liver and kidneys

Sensed by liver and kidneys

Erythropoiesis Feed Back Loop
Secretion of erythropoietin

Increased RBC count t

Accelerated erythropoiesis

Stimulation of red bone marrow

Figure 18.5 The Correction of Hypoxemia Through a Negative Feedback Loop.

Not all hypoxemia can be corrected by increasing erythropoiesis. In emphysema, for example, there is less lung tissue available to oxygenate the blood. Raising the RBC count cannot correct this, but the kidneys and bone marrow have no way of knowing this. The RBC count continues to rise in a futile attempt to restore homeostasis, resulting in a dangerous excess called polycythemia, discussed shortly.

Iron Metabolism

Iron is a critical part of the hemoglobin molecule and therefore one of the key nutritional requirements for erythropoiesis. Men lose about 0.9 mg of iron per day through the urine, feces, and bleeding, and women of reproductive age lose an average of 1.7 mg/day because of the added factor of menstruation. Since we absorb only a fraction of the iron in our food, we must consume 5 to 20 mg/day to replace our losses. Pregnant women need 20 to 48 mg/day, especially in the last 3 months, to meet not only their own need but also that of the fetus.

Dietary iron exists in two forms: ferric (Fe3+) and ferrous (Fe2+) ions. Stomach acid converts most Fe3+ to Fe2+, the only form that can be absorbed by the small intestine (fig. 18.6). A protein called gastroferritin, produced by the stomach, then binds Fe2+ and transports it to the small intestine. Here, it is absorbed into the blood, binds to a plasma protein called transferrin, and travels to the bone marrow, liver, and other tissues. Bone marrow uses Fe2+ for

I Remaining transferrin is distributed to other organs where Fe2+ is used to make hemoglobin, myoglobin, etc.

Gastroferritin

Saladin: Anatomy & I 18. The Circulatory System: I Text I © The McGraw-Hill

Physiology: The Unity of Blood Companies, 2003 Form and Function, Third Edition

688 Part Four Regulation and Maintenance hemoglobin synthesis; muscle uses it to make the oxygen-storage protein myoglobin; and nearly all cells use iron to make electron-transport molecules called cytochromes in their mitochondria. The liver binds surplus iron to a protein called apoferritin, forming an iron-storage complex called ferritin. It releases Fe2+ into circulation when needed.

Some other nutritional requirements for erythro-poiesis are vitamin B12 and folic acid, required for the rapid cell division and DNA synthesis that occurs in erythropoiesis, and vitamin C and copper, which are cofactors for some of the enzymes that synthesize hemoglobin. Copper is transported in the blood by an a globulin called ceruloplasmin.8

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