Porth's Essentials of Pathophysiology, 4e

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Disorders of Red Blood Cells

C h a p t e r 1 3

corpuscular hemoglobin concentration (MCHC) is the concentration of hemoglobin in each cell. Hemoglobin accounts for the color of red blood cells. Anemias are described as normochromic (normal color or MCHC) or hypochromic (decreased color or MCHC). Mean cell hemoglobin (MCH) refers to the mass of the red cell and is less useful in classifying anemias. A stained blood smear provides information about the size, color, and shape of red cells and the presence of immature or abnormal cells. If blood smear results are abnormal, examination of the bone marrow may be indicated. Bone marrow commonly is aspirated with a special needle from the posterior iliac crest or the ster- num. The aspirate is stained and observed for number and maturity of cells and abnormal types. ■■ The function of red blood cells (RBC), facilitated by the iron-containing hemoglobin molecule, is to transport oxygen from the lungs to the tissues. The biconcave shape of the red cell increases the surface area for diffusion of oxygen across the thin cell membrane. Each red cell has four hemoglobin moieties, each composed of two polypeptide chains consisting of a globin portion and a heme unit that surround an iron atom that reversibly combines with oxygen. ■■ Erythropoiesis, or production of RBC, occurs in the bone marrow and requires iron, vitamin B 12 , and folate. RBC production is governed by tissue oxygen needs.The decrease in oxygen content in blood is sensed by the kidneys, which then produce a hormone called erythropoietin. Erythropoietin, in turn, stimulates the bone approximately 120 days and is broken down in the spleen, liver, or bone marrow. In the process of destruction, the heme portion of the hemoglobin molecule is converted to bilirubin. Bilirubin, which is insoluble in plasma, attaches to plasma proteins for transport in the blood. It is removed from the blood by the liver and conjugated to a water-soluble form so that it can be excreted in the bile. ■■ In the laboratory, automated blood cell counters rapidly provide accurate measurements of red blood cell count and cell indices. A stained blood smear provides information about the size, color, and shape of red cells and the presence of immature or abnormal cells. If blood smear results are abnormal, examination of the bone marrow may be indicated. marrow to increase RBC production. ■■ The red blood cell has a life span of SUMMARY CONCEPTS

Spleen

Hemoglobin

Heme

Globin

Amino acids (reutilized)

Iron

Free, unconjugated bilirubin

Liver

Reused by bone marrow or stored in spleen and liver

Conjugated bilirubin

Bone marrow

Secreted in bile; excreted in feces or urine

FIGURE 13-6. Destruction of red blood cells and fate of hemoglobin.

Because excessive red blood cell destruction can occur in hemolytic transfusion reactions, urine samples are tested for free hemoglobin after a transfusion reaction.

LaboratoryTests Red blood cells can be studied by means of a sample of blood (Table 13-1). In modern clinical laboratories, specialized blood cell counters rapidly provide accurate measurements of red cell content and cell indices. The red blood cell count measures the total number of red blood cells in a microliter ( μ L) of blood. The percentage of reticulocytes (normally approximately 1%) provides an index of the rate of red cell production. The hemo- globin (grams per deciliter [dL] or 100 milliliters [mL] of blood) measures the hemoglobin content of the blood. The major components of blood are the red cell mass and plasma volume. The hematocrit measures the red cell mass in a 100-mL plasma volume. To determine the hematocrit, a sample of blood is placed in a glass tube, which is then centrifuged to separate the cells and the plasma. The hematocrit may be deceptive because it var- ies with the quantity of extracellular fluid, rising with dehydration and falling with overexpansion of extracel- lular fluid volume (Fig. 13-7). Red cell indices are used to differentiate types of ane- mias by size or color of red cells. The mean corpuscular volume (MCV) reflects the volume or size of the red cells. 1 The MCV falls in microcytic (small cell) anemia and rises in macrocytic (large cell) anemia. Some anemias are nor- mocytic (i.e., cells are of normal size or MCV). The mean