Depends on the severity of the anemia. The heart responds to the body's demand for oxygen, which is carried by the hemoglobin in the blood, by substantially increasing work load. The heart workload may even increase three to four fold. Heart rate, “pulse rate,” is usually fast in the presence of anemia. Shortness of breath develops easily because the heart function's reserve is being utilized. Only so much hemoglobin is available for basic needs. Any physical effort needs extra blood to carry oxygen. With lack of extra blood, shortness of breath will occur.