Blood is composed of blood cells and plasma, including red blood cells, white blood cells and platelets, which have different physiological structures and perform different physiological functions. 1. red blood cells: they are in the shape of a single double-sided concave disk, and mature red blood cells do not have a nucleus, but the cytoplasm contains a large amount of hemoglobin, which can combine with oxygen and thus play the role of oxygen transport. If the number of red blood cells decreases, the concentration of hemoglobin decreases, and the body’s ability to carry oxygen decreases, the symptoms of anemia will occur. 2. platelets: the body is responsible for stopping the blood cells, the number of platelets decreases after the symptoms of bleeding will occur. 3. plasma: mainly composed of water, albumin, globulin, inorganic ions, clotting factors and other substances. The main components of plasma are globulins and clotting factors, and after these components are changed, the function will be significantly affected. When the activity of clotting factors decreases, the body will experience symptoms of bleeding, such as hemophilia A, hemophilia B, vitamin K deficiency, etc., which are caused by a decrease in the activity of clotting factors. Blood is mainly composed of blood cells and plasma, each performing a different physiological function.