Tissues and special organs are arranged for working purposes into 'systems' which carry out the functions necessary for the body to remain in health. The skeleton gives shape and support to the body, anchors muscles, protects internal structures, stores calcium and produces blood cells. Bones can be divided into two main groups: axial, consisting of those of the upright parts of the body and appendicular. Muscles make up the largest mass of tissue in the body and account for approximately 40–50% of body weight. The heart pumps blood around the body via the circulatory system, which is a network of blood vessels. Some of these vessels – the aorta, arteries, arterioles and capillaries – carry the blood containing oxygen and nourishment from the heart to the body tissues. Lymph is a fluid drained from tissues and incorporating lymphocytes formed in the lymph glands, situated throughout the body.