Merck Manual

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Red Blood Cells of Cats


Susan M. Cotter

, DVM, DACVIM (Small Animal Internal Medicine and Oncology), Tufts University

Last full review/revision May 2018 | Content last modified May 2018

The main function of red blood cells (also called erythrocytes) is to carry oxygen to the tissues. Oxygen is carried within red blood cells by molecules called hemoglobin. Oxygen is used by cells to produce energy the body needs. Carbon dioxide is left behind as a waste product during this process. The red blood cells then carry carbon dioxide away from the tissues and back to the lungs, where it is exhaled.

An animal’s metabolism is geared to protect both the red blood cells and hemoglobin from damage. Interference with the formation or release of hemoglobin, the production or survival of red blood cells, or their metabolism causes disease. 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, causing animals to be weak and fatigued. Cats rarely can have too many red blood cells, which is called polycythemia. In animals with too many red blood cells, the blood can become too thick, impairing the ability of the heart to deliver oxygen throughout the body.

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.

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 how many and how quickly red blood cells are produced. Other factors that affect red blood cell production are the supply of nutrients (such as iron and vitamins) and cell-cell interactions of compounds that aid in their production.

Causes of anemia:

  • Blood loss

  • Destruction of red blood cells (hemolysis)

  • Decreased production of red blood cells

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.

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