The human heart is a remarkable four-chambered organ that pumps blood throughout your body. It consists of two upper chambers called atria and two lower chambers called ventricles. The right side of the heart handles deoxygenated blood, shown in blue, while the left side pumps oxygenated blood, shown in red. Major blood vessels include the aorta, which carries oxygen-rich blood to the body, the pulmonary artery that takes blood to the lungs, the vena cava that returns blood to the heart, and pulmonary veins that bring oxygenated blood back from the lungs. Four heart valves work like one-way doors to ensure blood flows in the correct direction through the heart.
Now let's follow the complete pathway of blood circulation through the heart. First, deoxygenated blood from the body enters the right atrium through the vena cava. This oxygen-poor blood then flows down into the right ventricle. When the right ventricle contracts, it pumps this deoxygenated blood through the pulmonary artery to the lungs, where it picks up oxygen and releases carbon dioxide. The now oxygen-rich blood returns from the lungs through the pulmonary veins into the left atrium. From there, it flows down into the powerful left ventricle. Finally, the left ventricle contracts forcefully, pumping the oxygenated blood through the aorta to supply the entire body with oxygen and nutrients. This complete cycle repeats continuously, ensuring every cell in your body receives the oxygen it needs.
The cardiac cycle is the sequence of events that occurs during one complete heartbeat. It consists of two main phases: diastole and systole. During diastole, the heart muscle relaxes and the chambers fill with blood. The atrioventricular valves, including the tricuspid and mitral valves, open to allow blood to flow from the atria into the ventricles. The semilunar valves remain closed during this filling phase. This is the lower pressure phase of the cycle. During systole, the heart muscle contracts forcefully. The AV valves snap shut to prevent backflow, while the semilunar valves open to allow blood to be pumped out to the lungs and body. This creates the higher pressure phase. You can see how the chambers change size during contraction and relaxation, and how the valves coordinate their opening and closing. This entire cycle repeats 60 to 100 times per minute, creating the rhythmic heartbeat that sustains life.
The heart has four crucial valves that work like one-way doors to ensure blood flows in the correct direction. There are two types of valves. The atrioventricular or AV valves are located between the atria and ventricles. The tricuspid valve on the right side has three leaflets, while the mitral or bicuspid valve on the left has two leaflets. These AV valves open during diastole to allow blood to flow from the atria into the ventricles, then close during systole to prevent backflow. The semilunar valves are located between the ventricles and the major arteries. The pulmonary valve controls flow to the lungs, while the aortic valve controls flow to the body. These semilunar valves work opposite to the AV valves - they close during diastole and open during systole when the ventricles contract. Watch how the valve leaflets coordinate their opening and closing to maintain proper blood flow direction throughout the cardiac cycle.
The heart has its own electrical system that controls the timing and coordination of each heartbeat. The sinoatrial or SA node, located in the right atrium, acts as the heart's natural pacemaker. It generates electrical impulses 60 to 100 times per minute, setting your heart rate. These electrical signals spread across both atria, causing them to contract and push blood into the ventricles. The impulse then reaches the atrioventricular or AV node, which acts like a relay station. The AV node briefly delays the signal, giving the atria time to completely empty before the ventricles contract. From the AV node, the electrical impulse travels down the Bundle of His, then splits into left and right bundle branches, and finally spreads through the Purkinje fibers throughout the ventricular muscle. This coordinated electrical activity can be measured on an electrocardiogram or ECG. The P wave represents atrial depolarization, the QRS complex shows ventricular depolarization, and the T wave indicates ventricular repolarization. This electrical system ensures that your heart beats in a coordinated, rhythmic pattern throughout your entire life.