The heart and the circulating system through arteries, veins, and capillaries must accomplish two goals: (1) delivery of oxygen and nutrients to organs and the peri¬pheral tissues and (2) removal of metabolic waste products that accumulate throughout the body. Oxygen is carried by hemoglobin in red blood cells to the tissues. The normal level of hemoglobin in circulating blood is 11.5–15.5 g/dl (115–155 g/L) in women and 15 to 17 g/dl in men.

A normal structural heart with normal function can tolerate a hemoglobin of 7 g/dl for approximately one year without causing cardiac damage or heart failure. Levels of 6 g/dl or less may cause cardiac dysfunction after approxi¬mately 6 months. In patients with angina, chest pain may be precipitated because lack of oxygen to the myocardium produces myocardial ischemia that causes pain.
Patients with sickle cell anemia commonly have a hemoglobin in the range of 7–8 g/dl. In these individuals cardiac enlargement with dilation occurs early in the course of the disease. Because of the anemia and poor delivery of oxygen to tissues, cardiac output increases to compensate and deliver the supply of nutrients. This entails more work for the heart. By age 40 these individuals manifest symptoms of poor cardiac function.

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