The Cardiac Cycle
Let's take a deeper, more detailed look at how the heart beats, and what controls the rhythm. The various and complex set of events that occur with each heart beat are referred to as the "cardiac cycle." As mentioned in the previous section, the top chambers of the heart are called the atria and the bottom chambers (that do most of the work of pumping) are called the ventricles. The job of the atria is to receive blood from the veins and store it prior to each heart beat. The left atrium receives blood from the lungs via four separate pulmonary veins, and the right atrium receives blood from the main body veins called the "vena cava" (the superior vena cava carries blood from the head, upper chest and arms, while the inferior vena cava carries blood from the lower chest, abdomen and legs). The normal heart beat begins in the top chambers. Both atria contract and push the stored blood across a pair of one-way valves into the ventricles, to help fill them. The valves are known as atrio-ventricular (AV) valves and include the mitral valve located between the left atrium and the left ventricle, and the tricuspid valve which separates the right atrium from the right ventricle. A split second after the atria contract, the ventricles begin to contract. As the pressure in the ventricles increases, the AV valves close (to prevent blood from flowing backwards). The sound made by the AV valves closing is known as the first heart sound (the lub of "lub-dub"). With continued contraction of the ventricles the blood is ejected out the arteries though another pair of valves – the aortic and the pulmonic valves (on the left and right sides of the heart respectively). At the end of the cardiac cycle, all the chambers relax, the aortic and pulmonic valves close (making the dub sound) which keeps blood from flowing backwards into the heart, and the atria begin to fill up with blood again to start the next cycle.