Pathway of Blood through the HeartCredit: http://www.starsandseas.com/SAS%20Physiology/Cardiovascular/Cardiovascular.htm
Before we had the technology to fully understand the dynamics of the heart, people used to believe that blood moves from the right to the left side by seeping through small pores found in the septum. We now have knowledge that truly heart passages exist but it is not an opening from one side to the other but rather a vertical opening exists. Through efforts in trying to understand the pumping mechanism of our body, we know that the heart is actually a two side-by-side pump, with each pump serving a unique blood circuit.
The Blood CircuitThe Blood Circuit - Pathway of Blood through the HeartCredit: http://heart-conditions.knoji.com/heart-function-how-the-heart-works/
After the blood has been enriched with oxygen by the lungs, it is then pumped by the ventricles to the aorta and then distributes throughout the body. The vessels which carry deoxygenated blood towards and from the lungs are the vessels that form the pulmonary circuit. They serve an important role as pathways for gas exchange to occur. The other blood circuit is the systemic circuit which carries the functional blood, both oxygenated and deoxygenated, to and from all the body tissues.
The Pulmonary Circuit PumpThe Pulmonary Circuit Pump - Pathway of Blood through the HeartCredit: http://www.daviddarling.info/encyclopedia/C/circulatory_system.html
The right side of the heart is what’s known as the pulmonary circuit pump. Blood that is distributed by the aorta is relatively oxygen-rich. Once it arrives to its destination, the oxygen and nutrients are used up by the cells and the returning blood is now deprived of oxygen and rich in carbon dioxide. The blood then enters the heart through the right atrium and is pushed down, with minimal contraction, to the right ventricle. The right ventricle then contracts and pushes the blood towards the lungs via the pulmonary trunk. Gas exchange occurs in the lungs and the returning blood is now rich in oxygen. The pulmonary veins direct the oxygenated blood to the left side of the heart. 
The pulmonary circulation is unique in the sense that the veins are the ones that carry oxygen rich blood and the arteries carry the oxygen-deprived blood. This exact opposite condition exists in the pulmonary circulation. 
The Systemic Circuit PumpThe Systemic Circuit Pump - Pathway of Blood through the HeartCredit: http://training.seer.cancer.gov/anatomy/cardiovascular/blood/pathways.html
The systemic circuit pump refers to the left side of the heart. The oxygenated blood from the lungs returns to the left side of the heart. It enters through the left atrium and then through the left ventricle. The left ventricle then pumps the blood to the aorta which is distributed to the different parts of the body via smaller systemic arteries. The same thing happens again and again; the oxygen and nutrients being delivered by the blood is then used up by the cells and the blood now is deoxygenated, it returns to the right atrium by way of the superior and inferior vena cava and the process starts all over again.
 Workload - Pathway of Blood through the HeartCredit: http://erwinadr.blogspot.com/2011/04/hypertension.html
At any moment, equal volumes of blood are pumped to the systemic and pulmonary circuits. However, the left and right ventricles have unequal workload. Since the blood in the pulmonary circuit travels only a short distance, it faces lesser friction and requires only a low pressure circulation. The systemic circuit, which is associated to the left ventricle, needs to travel longer to reach every part of the body. As it travels, the blood is faced with about five times more friction or resistance hence requiring a greater force to overcome the resistance. 
This difference is evident in the differences of the anatomy of the two ventricles. The left ventricles have walls that are almost three times thick compared to the other ventricle. The cavity of the left ventricle is almost circular, as compared to the right ventricle. The left ventricle is far more powerful and has the ability to generate much more pressure.