by the veins , there's always an exception to every rule and the exception to that rule in the case of the heart and the lung connection is the pulmonary artery and the pulmonary vein, you may not get asked anything about this, but just in case you do it's as well to know, in that instance the flow is reversed, in other words the pulmonary vein takes blood away from the heart and up to the lungs, and the pulmonary artery brings the oxygenated blood back down to the heart from the lungs, if you want to have a look at the diagrams for that and look at it yourself later on, there's no need for you to get concerned or confused about this at all, there's no need.