Hemoglobin is the main intracellular protein of the RBC. Its primary function is to transport oxygen to the cells and to remove carbon dioxide from them for excretion by the lungs. The Hgb molecule consists of two main components: heme and globin. Heme is composed of the red pigment porphyrin and iron, which is capable of combining loosely with oxygen. Globin is a protein that consists of nearly 600 amino acids organized into four polypeptide chains. Each chain of globin is associated with a heme group.

Each RBC contains approximately 250 million molecules of hemoglobin, with some erythrocytes containing more hemoglobin than others. The oxygen-binding, -carrying, and -releasing capacity of Hgb depends on the ability of the globin chains to shift position normally during the oxygenation-deoxygenation process. Structurally abnormal chains that are unable to shift normally have decreased oxygen-carrying ability. This decreased oxygen transport capacity is characteristic of anemia.

Hemoglobin also functions as a buffer in the maintenance of acid-base balance. During transport, carbon dioxide (CO₂) reacts with water (H₂O) to form carbonic acid (H₂CO₃). This reaction is speeded by carbonic anhydrase, an enzyme contained in RBCs. The carbonic acid rapidly dissociates to form hydrogen ions (H⁺) and bicarbonate ions (HCO₃^-). The hydrogen ions combine with the Hgb molecule, thus preventing a buildup of hydrogen ions in the blood. The bicarbonate ions diffuse into the plasma and play a role in the bicarbonate buffer system. As bicarbonate ions enter the bloodstream, chloride ions (Cl⁺) are repelled and move back into the erythrocyte. This "chloride shift" maintains the electrical balance between RBCs and plasma.²⁶

Hemoglobin determinations are of greatest use in the evaluation of anemia, because the oxygen-carrying capacity of the blood is directly related to the Hgb level rather than to the number of erythrocytes. To interpret results accurately, the Hgb level must be determined in combination with the Hct level. Normally, Hgb and Hct levels parallel each other and are commonly used together to express the degree of anemia. The combined values are also useful in evaluating situations involving blood loss and related treatment. The Hct level is normally three times the Hgb level. If erythrocytes are abnormal in shape or size or if Hgb manufacture is defective, the relationship between Hgb and Hct is disproportionate.^27,28

Note: Ratio of hemoglobin to hematocrit = 3:1

Factors that alter the RBC count may also influence Hgb levels

• Routine screening as part of a CBC
• Along with an Hct (i.e., an "H and H"), to evaluate known or suspected anemia and related treatment
• Along with an Hct, to monitor blood loss and response to blood replacement
• Along with an Hct, to monitor hematologic status during pregnancy

Nursing Care Before the Procedure

Client preparation is the same as that for any study involving the collection of a peripheral blood sample (see Appendix I).

A venipuncture is performed and the sample collected in a lavender-topped tube. A capillary sample may be obtained in infants and children as well as in adults for whom venipuncture may not be feasible.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample (see Appendix I).

• Critical values: Notify the physician at once if the Hgb is less than 6.0 g/dL. Prepare the client for possible transfusion of blood products and for further procedures to evaluate cause or source of blood loss.