What are blood cells? What do they look like? What functions do they perform? How can I recognize the different categories? This is a short description of the blood cells and includes a simple experiment which allows you to become familiar with the cells of this precious liquid. The blood consists of a suspension of special cells in a liquid called plasma. In an adult man, the blood is about 1/12th of the body weight and this corresponds to 5-6 litres. Blood consists of 55 % plasma, and 45 % by cells called formed elements. The blood performs a lot of important functions. By means of the hemoglobin contained in the erythrocytes, it carries oxygen to the tissues and collects the carbon dioxide (CO2). It also conveys nutritive substances (e.g. amino acids, sugars, mineral salts) and gathers the excreted material which will be eliminated through the renal filter. The blood also carries hormones, enzymes and vitamins. It performs the defense of the organism by mean of the phagocitic activity of the leukocytes, the bactericidal power of the serum and the immune response of which the lymphocytes are the protagonists.