The systemic circulation is a complex closed circuit of blood vessels which supplies oxygenated blood to every tissue of the body and returns deoxygenated blood from the tissues back to the heart. The oxygenated blood is transported by arteries, and the returning blood flows through the veins. Meanwhile, the capillaries, which are the smallest vessels, constitute the bridge which communicates the arterioles (tiny arteries) with the venules (smallest veins).
In the diagram of the systemic and pulmonary circulation located at the bottom of the post, you can see the heart at the center of the system, since it is the organ that pumps the blood, keeping it circulating (flowing) through the whole system of blood vessels to keep the body tissues alive. Therefore, the blood flows away from the heart and return. It receives oxygen-rich blood from the lungs into its left atrium and its left ventricle pumps it to the rest of the body through the aorta. Arteries and arterioles supply this oxygenated blood to all the organs and body tissues, from which oxygen-poor blood returns to the right atrium of the heart through the superior and inferior vena cava.
Red in color, the arteries are the blood vessels that carry oxygenated blood away from the heart to the rest of the body, with the aorta being the main one, which gives off all the arterial branches as it curves and then descends down the trunk and abdomen. But there is one exception, the pulmonary artery, which carries oxygen-poor blood from the heart to the lungs, where it is oxygenated. So, in medicine, any blood vessel that transports blood away from the heart is called artery.
Veins are the blood vessels that carry oxygen-poor (deoxygenated) blood to the heart, but with the exception of the pulmonary veins, which carry oxygen-rich blood from the lungs to the left atrium. Thus, in medicine, any vessel through which blood flows towards the heart is called vein. The mesenteric and portal veins are other exceptions, because the blood they carry do not go straight towards the heart but straight towards the liver, where the nutrient-rich blood from digestion is processed and metabolized by the liver before going into the general circulation.
Below, a diagram of the systemic circulation
