The heart is a muscular organ that works as a pumping system. It takes in blood with reduced levels of oxygen from the veins (deoxygenated blood), and delivers it to the lungs for oxygenation. When it receives the oxygenated blood back from the lungs, it pumps the blood back into the arteries to be distributed throughout the body.
The heart is divided into four hollow chambers, two on the left and two on the right. The right chambers are the right atrium and the right ventricle. They receive blood from the veins. The left chambers are the left atrium and the left ventricle. They receive blood from the pulmonary circulation, and the left ventricle forces blood into the systemic circulation.
In a cardiac cycle, blood enters the right atrium of the heart from the superior and inferior vena cavae, and flows across the tricuspid valve into the right ventricle. From the right ventricle the blood flows into the pulmonary artery, which is separated from the ventricle by the pulmonary valve. After oxygenation in the lungs, blood returns to the heart via four pulmonary veins that enter the left atrium. From the left atrium, blood flows across the mitral valve and into the left ventricle. From the left ventricle blood is ejected across the aortic valve into the aorta. Together, the mitral and triscupsid valves are known as the atrioventricular valves and the aortic and pulmonary valves as the semilunar valves
The cardiac cycle is the sequence of events that occurs when the heart beats. There are two phases of the cardiac cycle. In the diastole phase, the heart ventricles are relaxed and the heart fills with blood. In the systole phase, the ventricles contract and pump blood to the arteries. One cardiac cycle is completed when the heart fills with blood and the blood is pumped out of the heart.
SAN-SA NODE/AVN-AV NODE
Cardiac Cycle: 1st Diastole Phase
During the diastole phase, the atria and ventricles are relaxed and the atrioventricular valves are open. De-oxygenated blood from the superior and inferior vena cavae flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The SA node contracts triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.
Cardiac Cycle: 1st Systole Phase
During the systole phase, the right ventricle receives impulses from the Purkinje fibers and contracts. The atrioventricular valves close and the semilunar valves open. The de-oxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle.
The pulmonary artery carries the blood to the lungs. There the blood picks up oxygen and is returned to the left atrium of the heart by the pulmonary veins.
Cardiac Cycle: 2nd Diastole Phase
In the next diastole period, the semilunar valves close and the atrioventricular valves open. Blood from the pulmonary veins fills the left atrium. (Blood from the vena cava is also filling the right atrium.) The SA node contracts again triggering the atria to contract. The left atrium empties its contents into the left ventricle. The mitral valve prevents the oxygenated blood from flowing back into the left atrium.
Cardiac Cycle: 2nd Systole Phase
During the following systole phase, the atrioventricular valves close and the semilunar valves open. The left ventricle receives impulses from the Purkinje fibers and contracts. Oxygenated blood is pumped into the aorta. The aortic valve prevents the oxygenated blood from flowing back into the left ventricle.
The aorta branches out to provide oxygenated blood to all parts of the body. The oxygen depleted blood is returned to the heart via the vena cavae.
It is important to note that the events that occur in the first and second diastole phases actually happen at the same time. The same is also true for the events of the first and second systole phases