Question

why the transportation system in human body is called two way process

Answer 1

Human body contains two types of blood- oxygen rich blood (oxygenated) and carbon-dioxide rich blood (deoxygenated). The oxygenated blood comes to the heart from the lungs (via pulmonary vein) and via heart (aorta), oxygen rich blood is transported to the whole body. Similarly deoxygenated blood is transported from heart to lungs (via pulmonary artery) for oxygenation. So the transportation system in human body is a two way process.