There are four chambers inside the heart that fill with blood. Two of these chambers are called atria. The other two are called ventricles. The two atria form the curved top of the heart. The ventricles meet at the bottom of the heart to form a pointed base which points toward the left side of your chest. The left ventricle contracts most forcefully, so you can best feel your heart pumping on the left side of your chest.The left side of the heart houses one atrium and one ventricle. The right side of the heart houses the others. A wall of muscle, called the septum, separates the right and left sides of the heart. The left ventricle is the largest and strongest chamber in your heart. The left ventricle's chamber walls are only about a half-inch thick, but they have enough force to push blood through the aortic valve and into your body.
Four valves regulate blood flow through your heart. 1) The mitral valve connects the left atrium with the left ventricle below it, allowing oxygen-rich blood from the lungs pass through. 2) The tricuspid valve connects the right atrium with the right ventricle. 3) The pulmonary valve controls blood flow from the right ventricle into the pulmonary arteries, which carry blood to your lungs to pick up oxygen. 4) The aortic valve opens the way for oxygen-rich blood to pass from the left ventricle into the aorta, which is the largest artery, delivering it to the rest of the body.
Electrical impulses that travels through nerves connected to the myocardium (heart muscle) cause the heart to contract. This electrical signal begins in the sinoatrial (SA) node, located at the top of the right atrium. The SA node is sometimes called the heart's natural pacemaker. An electrical impulse from this natural pacemaker travels through the muscle fibers of the atria and ventricles, causing them to contract. Although the SA node sends electrical impulses at a certain rate, the heart rate may still change depending on physical demands, stress, or hormonal factors.
From the moment of development through the moment of death, the heart pumps. The heart, therefore, has to be strong. The average heart's muscle, called cardiac muscle, contracts and relaxes about 70 to 80 times per minute without you ever having to think about it. As the cardiac muscle contracts it pushes blood through the chambers and into the vessels. The ventricles of the heart have two states: systole (contraction) and diastole (relaxation). During diastole blood fills the ventricles and during systole the blood is pushed out of the heart into the arteries.
How the human heart works