A very efficient circulation system would be needed to transport nutrients and gases to every corner of a large organism's body or a plants's structure. A strong heart and a high blood pressure would be essential. Just imagine the blood pressure required to pump blood up a sauropod's long neck! In addition, very efficient vein and valve systems would be beneficial such that blood would not pool in a large animal's feet and other extremities.
Large plants must have the ability to take up large amounts of nutrients from the ground. They need extensive root systems to increase the surface area of their nutrient intake.