The main function of red blood cells (also called erythrocytes) is to carry oxygen to the tissues. When the number of red blood cells is too low, this is called anemia. Having too few red blood cells means the blood carries less oxygen (see Blood Disorders of Dogs: Red Blood Cells of Dogs).

Red blood cells are formed in the bone marrow. Within the bone marrow, all blood cells begin from a single cell type called a stem cell. The stem cell divides to produce an immature form of a red blood cell white blood cell, or platelet-producing cell. That immature cell then divides again, matures even more, and ultimately becomes a mature red or white blood cell or platelet. The total number of red cells remains constant over time in healthy animals. Mature red blood cells have a limited life span; their production and destruction must be carefully balanced, or disease develops.

A decrease in the number of total red blood cells (anemia) may be caused by blood loss, destruction of red blood cells (hemolysis), or decreased production. In severe blood loss anemia, red blood cells are lost, but death usually results from the loss of total blood volume. Hemolysis may be caused by toxins, infections, abnormalities present at birth, or antibodies that attack the red blood cells. Certain drugs, such as acetaminophen may also cause hemolytic anemia in cats. Decreased red blood cell production may result from bone marrow diseases or from other causes such as infection with feline leukemia virus, kidney failure, drugs, toxins, or antibodies targeted at developing red blood cells. It is important to remember that anemia is a sign of disease, not a specific diagnosis. The outlook and treatment depend on the underlying cause (see Blood Disorders of Cats: Anemia in Cats).

Last full review/revision July 2011 by Peter H. Holmes, BVMS, PhD, Dr HC, FRCVS, FRSE, OBE; David J. Waltisbuhl, BASc, MSc; Michael Bernstein, DVM, DACVIM; Karen L. Campbell, MS, DVM, DACVIM, DACVD; Nemi C. Jain, MVSc, PhD; Wayne K. Jorgensen, BSc, PhD; Sarah E. Payne, DVM, DACVIM