Abstract
A 76-year-old female followed closely for five years with IgM monoclonal gammopathy of uncertain significance developed anemia, worsened plasma creatinine concentration, and markedly elevated serum viscosity. This case illustrates the scope of pathology that can be caused by elevated blood viscosity. Our patient's anemia was a homeostatic response to normalize systemic vascular resistance and resulted from activation of the systemic vascular resistance response. The elevated plasma creatinine resulted from decreased renal perfusion because of elevated blood viscosity. Recent insights in hemorheology (the study of blood flow) are discussed, namely the recent identification of preferential blood flow patterns and erythrocyte autoregulation of deformability. These insights confirm that blood viscosity is part of the "milieu intérieur."