Formed elements of blood : hemoglobin

3. Hemoglobin. Each hemoglobin molecule consists of 4 polypeptide subunits, each of which includes an iron-containing heme group. Hemoglobin can bind reversibly to oxygen, forming oxyhemoglobin, and to carbon dioxide, forming carbaminobemoglobin. However, hemoglobin binds irreversibly to carbon monoxide, forming carboxyhemoglobin, which reduces the oxygen-carrying capacity of the blood. Hemoglobin (Hb) exists in a variety of forms, distinguishable on the basis of the amino acid sequence of their subunits. In humans, only 3 forms are considered normal in postnatal life: HbAI constitutes 97%, HbA2 2%, and HbF 1% of the hemoglobin of healthy adults. HbF makes up around 80% of the hemoglobin of newborns, however; this proportion gradually decreases until normal adult levels are reached at about 8 months of age. HbS is an abnormal form of HbA that is found in patients with sickle cell anemia; it differs by a single amino acid substitution in the beta chain (valine in HbS, glutamine in HbA). Unlike HbA, HbS becomes insoluble at low oxygen tensions and crystallizes into inflexible rods that deform the RBCs, giving them the characteristic sickle shape. When the rigid sickled cells pass through narrow capillaries, they cannot bend as normal RBCs do. They may become trapped, obstructing blood flow through the capillary, or rupture, decreasing the number of RBCs available for oxygen transport (anemia).

4. Plasmalemma and stroma. When placed in a hypotonic solution, RBCs swell and release their hemoglobin into the surrounding solution, a process termed hemolysis; they leave behind an empty shell, or red cell ghost, composed of the plasmalemma and the stroma.