Pulmonary circulation is the movement of blood from the heart, to the lungs, and back to the heart again. This is just one phase of the overall circulatory system.
The veins bring waste-rich blood containing carbon dioxide back to the heart, entering the right atrium through two large veins called superior vena cava and inferior vena cava. The right atrium gets filled with the waste-rich blood and then contracts, pushing the blood through the right atrio- ventricular opening gaurded by the tricuspid valve to the right ventricle. The right ventricle fills and then contracts, pushing the blood into the pulmonary artery, which leads to the lungs. In the lungs, the exchange of carbon dioxide and oxygen takes place. The fresh, oxygen-rich blood enters the pulmonary veins and then returns to the heart, re-entering through the left atrium. The oxygen-rich blood then passes through the left atrio- ventricular opening to the left ventricle gaurded by the bicuspid/ mitral valve where it will exit the heart through the main artery, called the aorta. The left ventricle's contraction forces the blood into the aorta and the blood begins its journey throughout the body.
The one-way valves are important for preventing any backward flow of blood. The circulatory system is a network of one-way streets. If blood started flowing the wrong way, the blood gases (oxygen and carbon dioxide) might mix, causing a serious threat to our body.