The most important component of red blood cells is red pigment, hemoglobin, which is connected to the oxygen in the lungs and, by circulation of blood, delivers it into all the body tissues. Anemia occurs when the amount of blood hemoglobin falls below the value required to bring enough oxygen to the tissue.
There are several reasons why the body circulates too little hemoglobin. Disorders can occur in the production of hemoglobin itself or in the production of red blood cells. For hemoglobin production iron supply is required, and the lack of iron in the body means that hemoglobin is not produced enough (this is called iron deficiency anemia).
Due to lack of vitamin B12, red blood cells are poorly produced and defective red blood cells (pernicious anemia) are produced. Lack of folic acid has the same effect (anemia due to lack of folic acid). If red blood cells are produced at normal speed and are rapidly destroyed, then their number in the bloodstream will fall (hemolytic anemia).
Because of hereditary disorders (such as sickle cell anemia or drepanocytosis), pathological hemoglobin is produced.
Characteristic symptoms of anemia are paleness, fatigue, weakness, faintness, loss of breath and heart palpitation.