Blood is a fluid binding tissue consisting of plasma and blood cells (erythrocytes, leukocytes, platelets). Through the hematopoetic process blood cells are formed in the bone marrow. In addition to the bone marrow lymph nodes, the spleen, and the thymus play an important role in the maturation of blood cells. Each of the generated cells plays a significant and special role in the body. Erythrocytes are responsible for transferring oxygen from the lungs to the tissues, leukocytes for defence of the body, and platelets participate in the blood clotting process. In a blood transfusion, it is extremely important to know the blood groups whose inheritance is mostly determined by the composition of glycolipids, which are components of the erythrocyte membranes. Although, many types of blood groups systems exist, two systems are the most common, the AB0 system and the Rh system. Inheritance of blood groups is performed through multiple alleles. The blood is susceptible to various disorders, and as the most common type of blood disorder is anaemia or nexborn, disease named erythroblastosis fetalis. The aims of this work were to describe the composition of the blood as a type of connective tissue with the emphasis on each blood component, to describe the role of the blood cells, to present the origin of blood cell types and to explain the importance of knowing the types of blood groups and their principle of inheritance.