Circulatory System
delivery of oxygen and the removal of carbon dioxide
Relationship Between The Structure and Function of Arteries, Capillaries and Veins
The Arteries are exposed to the pumping of the heart and have higher blood pressures than veins and capillaries. In Veins their low blood pressure is a resort of the bloods long journey from the heart. Capillaries are the links between the arteries and veins and molecules are exchanged between the blood and interstitial fluids.
http://www.phschool.com/science/biology_place/biocoach/cardio2/structure.html#map
Route of Blood Through The Heart
Blood enters the heart through two large veins emptying oxygen-poor blood from the body into the right atrium of the heart. Blood flows from your right atrium into your right ventricle through the open tricuspid valve. As the ventricle contracts into the pulmonary artery and to the lungs where it is oxygenated. The pulmonary vein empties oxygen-rich blood from the lungs into the left atrium of the heart. As the ventricle contracts into the aorta and to the body.
http://www.medicinenet.com/heart_how_the_heart_works/page3.htm