The ascending aorta branches into the which then branches into two blood vessels. The which provides blood to the right posterior side of the heart and the provides blood to the right anterior side of the heart. These then branch into smaller arteries called and these feed blood into the microscopic vessels called which do the gas and nutrient/waste exchange. Blood leaves through small veins called which then merge to form larger vessels. The drains the right anterior side of the heart and the drains the posterior side of the heart into the which empties the blood into the of the heart to enter pulmonary circulation.