The major function of the heart is to generate force to propel blood to the body. When the heart gets filled with blood, its walls squeeze. This imparts energy to the red blood cells inside it, forcing them to squeeze through the outlet heart valves and on to the body.

The heart is like any pump: it needs to be filled with blood before it can pump. The heart, however, is composed of 4 chambers. Each of them has a double function: they receive blood and eject blood. They are both receiving chambers and pumping chambers at the same time.

To understand the blood flow shifts inside the heart it is important to know that normally the pressure in each heart chamber is different. In a normal adult, the pressure in the Left Ventricle is about 120 mmHg and the pressure in the Right Ventricle is about 30 mmHg.

What is mmHg?
When you go to the doctor to have your blood pressure taken, the doctor records your blood pressure in mmHg. The number he records as the high pressure (systolic) is usually 120 mmHg in an adult. This means that it takes a column of mercury 120 millimeters high to overcome the force in your arteries. In other words, the force from the heart pushing the blood forward in your arteries is 120 mmHg.