A blood deficiency. Anaemia is, effectively, reduced levels of the red blood pigment haemoglobin. Since this occurs in red blood cells (erythrocytes), a lack of red blood cells (erythrocytes) is often associated with a lack of blood pigment. The blood pigment transports oxygen in the blood, so a deficiency is associated with insufficient oxygen supply to the body. Anaemia can have many causes, including iron deficiency, increased loss of red blood cells (severe blood loss, tumours, chronic diseases), disorders in the synthesis of red blood cells (lack of erythropoietin in chronic kidney disease, bone-marrow tumours) and congenital disorders.