The heart has a right side and a left side. Atria are relatively thin-walled chambers, whereas the ventricles are much more muscular.
Blood passes from the atria into the ventricles through two processes.
During the resting phase, when the ventricles are not contracting, the tricuspid and mitral valves open. Some of the blood that has accumulated in the atria passively flows through the tricuspid and mitral valves into the right and left ventricles, respectively. The atria then contract, actively pumping blood out and into the ventricles. Once the ventricles fill with blood, they contract, pumping blood out of the ventricles and to the lungs and body. When this happens, the lungs become congested with fluid, making it difficult to breathe and interfering with the movement of oxygen from the lungs into the bloodstream.