The main function of red blood cells (also called erythrocytes) is to carry oxygen to the tissues, where it is required for cellular metabolism. Oxygen molecules attach themselves to carrier molecules, called hemoglobin, which are the iron-containing proteins in red blood cells that give the cells their red color. Oxygen is carried from the lungs and delivered to all body tissues by the hemoglobin within red blood cells. Oxygen is used by cells to produce energy that the body needs. Carbon dioxide is left behind as a waste product during this process. The red blood cells then carry that carbon dioxide away from the tissues and back to the lungs, where it is exhaled. 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. The result is that fatigue and weakness develop. When the number of red blood cells is too high, this is called polycythemia. The result is that blood can become too thick, and impair the ability of the heart to deliver oxygen throughout the body. An animal's metabolism is geared to protect both the red blood cells and the hemoglobin from damage. Interference with the formation or release of hemoglobin, or with production or survival of red blood cells, causes disease.

The total number of red cells, and thus the oxygen-carrying capacity, 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.

Production of red blood cells begins with stem cells in the bone marrow and ends with the release of mature red blood cells into the body's circulation. Within the bone marrow, all blood cells begin from a single cell called a stem cell. The stem cell divides to form immature forms of red blood cells, white blood cells, or a platelet-producing cell. Those immature cells then divide again, mature even more, and ultimately become mature red blood cells, white blood cells, or platelets.

The rate of blood cell production is determined by the body's needs. Erythropoietin, a hormone produced by the kidneys, stimulates development of red blood cells in the bone marrow. Erythropoietin increases if the body lacks oxygen (a condition called hypoxia). In most species, the kidney is both the sensor organ that determines how much oxygen the body's tissues are receiving and the major site of erythropoietin production; so chronic kidney failure leads to anemia. Erythropoietin plays a major role in determining whether to increase the number of stem cells entering red blood cell production, to shorten maturation time of the red blood cells, or to cause early release of red blood cells. Other factors that affect red blood cell production are the supply of nutrients (such as iron and vitamins) and cell-cell interactions between compounds that aid in their production. Some disorders are the direct result of abnormal red blood cell metabolism. For example, an inherited enzyme deficiency reduces the life span of red blood cells, causing a condition known as hemolytic anemia (see Blood Disorders of Dogs: Anemia in Dogs).

It is important to remember that a decrease in the total number of red blood cells in the body (anemia) is a sign of disease, not a specific diagnosis. 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, rather than from the lack of oxygen caused by loss of red blood cells. Hemolysis may be caused by toxins, infections, abnormalities present at birth, drugs, or antibodies that attack the red blood cells. In dogs the most common cause of serious hemolysis is an antibody directed against that dog's own red blood cells (immune-mediated hemolytic anemia).

Factors that may prevent red blood cell production include bone marrow failure or malignancy, loss of erythropoietin secondary to kidney failure, certain drugs or toxins, longterm debilitating diseases, or antibodies targeted at developing red blood cells. The outlook and treatment depend on the underlying cause of the anemia.

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