THE circulatory or vascular system is divided for descriptive purposes into:— 1. The blood circulatory system which consists of: The heart, which acts as a pump. The blood vessels through which the blood circulates. The heart and the blood vessels form a completely closed and continuous system through which the blood flows. 2. The lymphatic system which consists of:— Lymphatic glands. Lymphatic vessels and capillaries through which a colourless fluid known as lymph circulates. The two systems communicate with one another and are intimately associated. There are several types of blood vessels:— Arteries. Arterioles. Veins, {d) Venules, (e) Capillaries. The Arteries. These are the blood vessels which transport blood away from the heart carrying oxygen and nutritive material to the tissues and cells. They vary considerably in size, but have much the same structure. They consist of three coats:— An outer coat of fibrous tissue known as the tunica advent itia. A middle coat of elastic and muscular tissue, the tunica media. An inner lining known as endothelium composed of flattened squamous epithelial cells. This lining is known as the tunica Ultima. The tunica media is composed of a mixture of involuntary muscle fibres and elastic tissue. The amount of muscular tissue and elastic tissue varies in the arteries depending upon their size. In the large arteries there is much more elastic and less muscle tissue than in the smaller arteries and arterioles. In the smaller arteries and arterioles there is more muscle than elastic tissue. The Veins. The veins are the blood vessels which transport deoxygenated blood from the tissues to the heart. The walls of the veins like those of the arteries have three coats:— Tunica adventitia. Tunica media. Tunica intima. The main difference between the walls of the veins and arteries is the comparative thinness and weakness of the tunica media of the veins. There is relatively less muscular and elastic tissue present. Due to the weakness of the walls of the veins, when they are cut they will collapse, but if an artery be cut the walls remain separated. Most veins possess valves which prevent the back flow of blood. The valves are formed by a fold of tunica intima strengthened by connective tissue. They are semi-lunar in shape with the concavity towards the heart. The Capillaries. The small arteries known as the arterioles break up into a number of extremely minute vessels known as capillaries. The wall of a capillary is composed of a single layer of endothelial cells which is exquisitely thin and permits the passage of water, substances of small molecular size and oxygen, but not the passage of red blood corpuscles or plasma proteins. Their diameter is approximately that of a red blood corpuscle.