Figure 142

Electron micrograph of erythrocytes.

© Bruce Iverson leave the bone marrow and enter the general circulation. In the presence of unusually rapid erythrocyte production, however, many reticulocytes do enter the blood, a fact of clinical diagnostic usefulness.

Because erythrocytes lack nuclei and organelles, they can neither reproduce themselves nor maintain their normal structure for very long. The average life span of an erythrocyte is approximately 120 days, which means that almost 1 percent of the body's erythrocytes are destroyed and must be replaced every day. This amounts to 250 billion cells per day! Erythrocyte destruction normally occurs in the spleen and the liver. As will be described below, most of the iron released in the process is conserved. The major breakdown product of hemoglobin is bilirubin, which, as noted above, gives plasma its color (the fate of this substance will be described in Chapter 17).

The production of erythrocytes requires the usual nutrients needed to synthesize any cell: amino acids, lipids, and carbohydrates. In addition, both iron and certain growth factors, including the vitamins folic acid and vitamin B12, are essential.

Iron As noted above, iron is the element to which oxygen binds on a hemoglobin molecule within an erythrocyte. Small amounts of iron are lost from the body via the urine, feces, sweat, and cells sloughed from the skin. In addition, women lose an additional amount via menstrual blood. In order to remain in iron balance, the amount of iron lost from the body must be replaced by ingestion of iron-containing foods; particularly rich sources are meat, liver, shellfish, egg yolk, beans, nuts, and cereals. A significant upset of iron balance can result either in iron deficiency, leading to inadequate hemoglobin production, or in an excess of iron in the body, with serious toxic effects (hemochromatosis).

The homeostatic control of iron balance resides primarily in the intestinal epithelium, which actively absorbs iron from ingested foods. Normally, only a small fraction of ingested iron is absorbed, but more importantly, this fraction is increased or decreased, in a negative feedback manner, depending upon the state of the body's iron balance—the more iron in the body, the less ingested iron is absorbed (the mechanism is given in Chapter 17).

The body has a considerable store of iron, mainly in the liver, bound up in a protein called ferritin. Ferritin serves as a buffer against iron deficiency. About 50 percent of the total body iron is in hemoglobin, 25 percent is in other heme-containing proteins (mainly the cytochromes) in the cells of the body, and 25 percent is in liver ferritin. Moreover, the recycling of iron is very efficient (Figure 14-3). As old erythrocytes are destroyed in the spleen (and liver), their iron is released into the plasma and bound to an iron-transport plasma protein called transferrin. Almost all of this iron is delivered by transferrin to the bone marrow to be incorporated into new erythrocytes. Recirculation of erythrocyte iron is very important because it involves 20 times more iron per day than is absorbed and excreted. On a much lesser scale, nonerythrocyte cells, some of which are continuously dying and being replaced, release iron from their cytochromes into the plasma and take up iron from it, transferrin serving as a carrier.

Folic Acid and Vitamin B12 Folic acid, a vitamin found in large amounts in leafy plants, yeast, and liver, is required for synthesis of the nucleotide base thymine. It is, therefore, essential for the formation of DNA and hence for normal cell division. When this vitamin is not present in adequate amounts, impairment of cell division occurs throughout the body but is most striking in rapidly proliferating cells, including eryth-rocyte precursors. Thus, fewer erythrocytes are produced when folic acid is deficient.

Production of normal erythrocyte numbers also requires extremely small quantities (one-millionth of a gram per day) of a cobalt-containing molecule, vitamin B12 (also called cobalamin), since this vitamin is required for the action of folic acid. Vitamin B12 is found only in animal products, and strictly vegetarian diets are deficient in it. Also, as described in Chapter 17, the absorption of vitamin B12 from the gastrointestinal tract requires a protein called intrinsic factor, which is secreted by the stomach; lack of this protein also causes vitamin B12 deficiency.

Regulation of Erythrocyte Production In a normal person, the total volume of circulating erythrocytes remains remarkably constant because of reflexes that regulate the bone marrow's production of these cells.

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Old erythrocyte removal

Spleen (and liver)

Iron recirculation

Dietary absorption

Erythrocyte hemoglobin

Erythrocyte hemoglobin

New erythrocyte formation

Old erythrocyte removal

New erythrocyte formation

Spleen (and liver)

Function Bone Marrow

Bone marrow

Iron recirculation

Iron recirculation

Dietary absorption

Bone marrow

Iron recirculation

Loss (urine, skin cells, sweat, menstrual blood)

Storage (mainly in liver)

Storage (mainly in liver)

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Essentials of Human Physiology

Essentials of Human Physiology

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