Red Blood Cells Erythrocytes

The major function of red blood cells, also known as erythrocytes, is to transport hemoglobin, which in turn carries oxygen from the lungs to the tissues. In some lower animals, hemoglobin circulates as free protein in the plasma, not enclosed in red blood cells. When it is free in the plasma of the human being, about 3 per cent of it leaks through the capillary membrane into the tissue spaces or through the glomerular membrane of the kidney into the glomerular filtrate each time the blood passes through the capillaries. Therefore, for hemoglobin to remain in the human blood stream, it must exist inside red blood cells.

The red blood cells have other functions besides transport of hemoglobin. For instance, they contain a large quantity of carbonic anhydrase, an enzyme that catalyzes the reversible reaction between carbon dioxide (CO2) and water to form carbonic acid (H2CO3), increasing the rate of this reaction several thousandfold. The rapidity of this reaction makes it possible for the water of the blood to transport enormous quantities of CO2 in the form of bicarbonate ion (HCO3) from the tissues to the lungs, where it is reconverted to CO2 and expelled into the atmosphere as a body waste product. The hemoglobin in the cells is an excellent acid-base buffer (as is true of most proteins), so that the red blood cells are responsible for most of the acid-base buffering power of whole blood.

Shape and Size of Red Blood Cells. Normal red blood cells, shown in Figure 32-3, are biconcave discs having a mean diameter of about 7.8 micrometers and a thickness of 2.5 micrometers at the thickest point and 1 micrometer or less in the center. The average volume of the red blood cell is 90 to 95 cubic micrometers.

The shapes of red blood cells can change remarkably as the cells squeeze through capillaries. Actually, the red blood cell is a "bag" that can be deformed into almost any shape. Furthermore, because the normal cell has a great excess of cell membrane for the quantity of material inside, deformation does not stretch the membrane greatly and, consequently, does not rupture the cell, as would be the case with many other cells.

Concentration of Red Blood Cells in the Blood. In normal men, the average number of red blood cells per cubic millimeter is 5,200,000 (±300,000); in normal women, it is 4,700,000 (±300,000). Persons living at high altitudes have greater numbers of red blood cells. This is discussed later.

Quantity of Hemoglobin in the Cells. Red blood cells have the ability to concentrate hemoglobin in the cell fluid up to about 34 grams in each 100 milliliters of cells.

The concentration does not rise above this value, because this is the metabolic limit of the cell's hemoglobin-forming mechanism. Furthermore, in normal people, the percentage of hemoglobin is almost always near the maximum in each cell. However, when hemoglobin formation is deficient, the percentage of hemoglobin in the cells may fall considerably below this value, and the volume of the red cell may also decrease because of diminished hemoglobin to fill the cell.

When the hematocrit (the percentage of blood that is cells—normally, 40 to 45 per cent) and the quantity of hemoglobin in each respective cell are normal, the whole blood of men contains an average of 15 grams of hemoglobin per 100 milliliters of cells; for women, it contains an average of 14 grams per 100 milliliters.

As discussed in connection with blood transport of oxygen in Chapter 40, each gram of pure hemoglobin is capable of combining with 1.34 milliliters of oxygen. Therefore, in a normal man, a maximum of about 20 milliliters of oxygen can be carried in combination with hemoglobin in each 100 milliliters of blood, and in a normal woman, 19 milliliters of oxygen can be carried.

Production of Red Blood Cells

Areas of the Body That Produce Red Blood Cells. In the early weeks of embryonic life, primitive, nucleated red blood cells are produced in the yolk sac. During the middle trimester of gestation, the liver is the main organ for production of red blood cells, but reasonable numbers are also produced in the spleen and lymph nodes. Then, during the last month or so of gestation and after birth, red blood cells are produced exclusively in the bone marrow.

As demonstrated in Figure 32-1, the bone marrow of essentially all bones produces red blood cells until a person is 5 years old. The marrow of the long bones, except for the proximal portions of the humeri and tibiae, becomes quite fatty and produces no more red blood cells after about age 20 years. Beyond this age,

Physiology Rbc Production

Figure 32-1

Relative rates of red blood cell production in the bone marrow of different bones at different ages.

Figure 32-1

Relative rates of red blood cell production in the bone marrow of different bones at different ages.

most red cells continue to be produced in the marrow of the membranous bones, such as the vertebrae, sternum, ribs, and ilia. Even in these bones, the marrow becomes less productive as age increases.

Genesis of Blood Cells

Pluripotential Hematopoietic Stem Cells, Growth Inducers, and Differentiation Inducers. The blood cells begin their lives in the bone marrow from a single type of cell called the pluripotential hematopoietic stem cell, from which all the cells of the circulating blood are eventually derived. Figure 32-2 shows the successive divisions of the pluripotential cells to form the different circulating blood cells. As these cells reproduce, a small portion of them remains exactly like the original pluripotential cells and is retained in the bone marrow to maintain a supply of these, although their numbers diminish with age. Most of the reproduced cells, however, differentiate to form the other cell types shown to the right in Figure 32-2. The intermediate-stage cells are very much like the pluripotential stem cells, even though they have already become committed to a particular line of cells and are called committed stem cells.

The different committed stem cells, when grown in culture, will produce colonies of specific types of blood cells. A committed stem cell that produces erythro-cytes is called a colony-forming unit-erythrocyte, and the abbreviation CFU-E is used to designate this type of stem cell. Likewise, colony-forming units that form granulocytes and monocytes have the designation CFU-GM, and so forth.

Growth and reproduction of the different stem cells are controlled by multiple proteins called growth inducers. Four major growth inducers have been described, each having different characteristics. One of these, interleukin-3, promotes growth and reproduction of virtually all the different types of committed stem cells, whereas the others induce growth of only specific types of cells.

The growth inducers promote growth but not differentiation of the cells. This is the function of another set of proteins called differentiation inducers. Each of these causes one type of committed stem cell to differentiate one or more steps toward a final adult blood cell.

Formation of the growth inducers and differentiation inducers is itself controlled by factors outside the bone marrow. For instance, in the case of erythrocytes (red blood cells), exposure of the blood to low oxygen for a long time results in growth induction, differentiation, and production of greatly increased numbers of erythrocytes, as discussed later in the chapter. In the case of some of the white blood cells, infectious diseases cause growth, differentiation, and eventual formation of specific types of white blood cells that are needed to combat each infection.

Stages of Differentiation of Red Blood Cells

The first cell that can be identified as belonging to the red blood cell series is the proerythroblast, shown at the starting point in Figure 32-3. Under appropriate

PHSC (Pluripotent hematopoietic stem cell)

Figure 32-2

Formation of the multiple different blood cells from the original pluripotent hematopoietic stem cell (PHSC) in the bone marrow.

CFU-B (Colony-forming unit-blast)

CFU-S (Colony-forming unit-spleen)

PHSC LSC

(Lymphoid stem cell)

CFU-E (Colony-forming unit-erythrocytes)

CFU-GM (Colony-forming unit-granulocytes, monocytes)

CFU-M (Colony-forming unit-megakaryocytes)

- Erythrocytes

Granulocytes

(Neutrophils) (Eosinophils) (Basophils)

Monocytes +

Macrocytes

- Megakaryocytes t

Platelets

T lymphocytes

B lymphocytes

Types Red Blood Cells
Genesis of normal red blood cells (RBCs) and characteristics of RBCs in different types of anemias.

stimulation, large numbers of these cells are formed from the CFU-E stem cells.

Once the proerythroblast has been formed, it divides multiple times, eventually forming many mature red blood cells. The first-generation cells are called basophil erythroblasts because they stain with basic dyes; the cell at this time has accumulated very little hemoglobin. In the succeeding generations, as shown in Figure 32-3, the cells become filled with hemoglobin to a concentration of about 34 per cent, the nucleus condenses to a small size, and its final remnant is absorbed or extruded from the cell. At the same time, the endoplasmic reticulum is also reabsorbed. The cell at this stage is called a reticulocyte because it still contains a small amount of basophilic material, consisting of remnants of the Golgi apparatus, mitochondria, and a few other cytoplasmic organelles. During this reticulocyte stage, the cells pass from the bone marrow into the blood capillaries by diapedesis (squeezing through the pores of the capillary membrane).

The remaining basophilic material in the reticulo-cyte normally disappears within 1 to 2 days, and the cell is then a mature erythrocyte. Because of the short life of the reticulocytes, their concentration among all the red cells of the blood is normally slightly less than 1 per cent.

Regulation of Red Blood Cell Production—Role of Erythropoietin

The total mass of red blood cells in the circulatory system is regulated within narrow limits, so that (1) an adequate number of red cells is always available to provide sufficient transport of oxygen from the lungs to the tissues, yet (2) the cells do not become so numerous that they impede blood flow. What we know about this control mechanism is diagrammed in Figure 32-4 and is as follows.

Tissue Oxygenation Is the Most Essential Regulator of Red

Blood Cell Production. Any condition that causes the quantity of oxygen transported to the tissues to decrease ordinarily increases the rate of red blood cell production. Thus, when a person becomes extremely anemic as a result of hemorrhage or any other condition, the bone marrow immediately begins to produce large quantities of red blood cells. Also, destruction of major portions of the bone marrow by any means, especially by x-ray therapy, causes hyperplasia of the remaining bone marrow, thereby attempting to supply the demand for red blood cells in the body.

At very high altitudes, where the quantity of oxygen in the air is greatly decreased, insufficient oxygen is transported to the tissues, and red cell production is greatly increased. In this case, it is not the concentration of red blood cells in the blood that controls red cell production but the amount of oxygen transported to the tissues in relation to tissue demand for oxygen.

Various diseases of the circulation that cause decreased blood flow through the peripheral vessels, and particularly those that cause failure of oxygen absorption by the blood as it passes through the lungs,

Hematopoietic Stem Cells

Hematopoietic Stem Cells

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Responses

  • Michelle Howe
    Why the Rbc are responsible for most of the acidbase buffering power of blood?
    4 years ago
  • LINDA
    Why is small amounts of haemoglobin passes from the blood into the filtrate?
    4 years ago
  • veli-pekka
    What is the process by which red blood cells squeeze through the capillary membrane?
    4 years ago
  • jakub ross
    What is differentiated in case of red blood cell?
    4 years ago
  • kerstin
    What is normal number of RBCs in blood in micro meters cube?
    3 years ago
  • heribald
    How many RBCs are there in 1 cubic meter of blood in females?
    3 years ago
  • Mari
    Why haemoglobin is not fix in blood.it is always enclose in rbc?
    3 years ago
  • tuula
    Why haemoglobin is not free in blood and why it is enclosed in red blood cell?
    3 years ago
  • j
    Why do the red blood cell not become so numerous so as to impede blood flow?
    3 years ago
  • Patrick
    What happen if rbc leaks through capillary membrane into tissue spaces?
    3 years ago
  • Kedija
    Why is hemoglobin enclosed in erythrocytes?
    3 years ago

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