Cardiac cycle

Your heart works in a loop.

First, the heart relaxes. It grows bigger to hold blood. This is called diastole.

Next, the heart squeezes. This is called systole. It pushes blood out to your body.

One heartbeat takes about 0.8 seconds. Your heart has four rooms. Two rooms are on top. Two rooms are on the bottom.

This loop happens over and over. It keeps your blood moving.

Your heart works in a continuous loop. This loop is called the cardiac cycle. One heartbeat takes about 0.8 seconds to finish.

The heart has four parts called chambers. Two chambers are on top. These are the atria. Two chambers are on the bottom. These are the ventricles.

The cycle has two main steps. The first step is diastole. This is when the heart relaxes. The heart grows bigger to let blood in. The atria also squeeze to push blood into the ventricles.

The second step is systole. This is when the heart muscle contracts, or squeezes. The ventricles pulse to push blood out. One side sends blood to the lungs. The other side sends blood to the whole body.

Small parts of the heart act like a timer. We call this the sinoatrial node. It sends electrical signals to the muscle. These signals tell the heart when to squeeze. This keeps the heart beating in a steady rhythm.

Your heart never stops working. It follows a repeating pattern called the cardiac cycle. This cycle is the performance of the heart from one heartbeat to the next.

A healthy heart beats about 70 to 75 times every minute. This means one full cycle takes only about 0.8 seconds. The heart is a four-chambered organ. It has a right side and a left side. Each side has an upper chamber called an atrium. Each side also has a lower chamber called a ventricle.

The cycle works through two main stages. The first stage is called diastole. During diastole, the heart muscle relaxes and expands. This allows blood to flow into the ventricles. The two atrioventricular valves, or AV valves, open during this time. These include the mitral and tricuspid valves. At the end of this stage, the atria contract. This is called atrial systole. It pushes a final amount of blood into the ventricles.

The second stage is called systole. This is when the heart muscle performs a strong contraction. This stage is called ventricular systole. First, the ventricles contract and the AV valves close. This is the isovolumic contraction stage. Then, the pressure in the ventricles rises very high. This pressure opens the aortic and pulmonary valves. The ventricles then eject blood into the body. One path goes to the lungs through the pulmonary artery. The other path goes to the whole body through the aorta.

Electrical signals tell the heart exactly when to move. These signals come from specialized pacemaker cells. One group of cells is the sinoatrial node. It sits in the upper wall of the right atrium. This node starts the wave of electrical impulses. Another group is the atrioventricular node. It sits between the atrium and the ventricle. This node acts like a gate. It slows the signal down to coordinate the beat. This delay gives blood time to fill the chambers.

Doctors can see these movements on a special chart. This chart is called an electrocardiogram. It shows the electrical activity of the heart. You might see a small P wave on the chart. This wave shows when the atria contract. You will also see a large QRS complex. This spike shows the ventricles contracting.

These electrical waves help us understand how the heart works. The heart uses its own muscle cells, called myocytes, to contract. These cells can start their own contractions without outside nerves. This keeps your blood moving every single second.

The cardiac cycle is the continuous performance of the human heart from the start of one heartbeat to the start of the next.

This cycle is essential for life because it manages the movement of blood through the body. The heart functions as a four-chambered organ divided into a right heart and a left heart. Each side contains an upper chamber called an atrium and a lower chamber called a ventricle. The right atrium and ventricle handle pulmonary circulation, which sends blood to the lungs. The left atrium and ventricle manage systemic circulation, which sends oxygenated blood to the rest of the body.

The cycle is defined by two primary periods: diastole and systole. Diastole is the period when the heart muscle relaxes and expands. This relaxation allows the heart to refill with blood returning from the lungs and other body systems. Systole is the period of robust contraction and pumping. During systole, the heart ejects blood out of the chambers and into the circulatory system. In a healthy heart beating at a typical rate of 70 to 75 beats per minute, one full cycle takes approximately 0.8 seconds. The duration of the cycle is inversely proportional to the heart rate.

Ventricular diastole involves several specific stages of blood inflow. It begins with isovolumic relaxation, where the ventricles relax after a previous contraction. This is followed by stages of inflow, including rapid inflow and diastasis. Toward the end of ventricular diastole, atrial systole occurs. During atrial systole, the two atria contract to pump a final crop of blood into the ventricles under pressure. To allow this filling, the atrioventricular (AV) valves—the mitral and tricuspid valves—must be open.

Ventricular systole begins with a stage called isovolumic contraction. This starts when electrical signals prompt the ventricles to contract. As the ventricles pulse, back-pressure causes the AV valves to close immediately. This closure prevents blood from flowing backward into the atria. As the contraction continues, pressure within the ventricles rises very quickly. Once ventricular pressure exceeds the pressure in the aorta and pulmonary arteries, the aortic and pulmonary valves open. This leads to the ejection stage, where blood is forcefully pushed into the body and lungs.

The entire process is orchestrated by the heart's electrical conduction system. This system consists of specialized pacemaker cells that produce electrical impulses. The sinoatrial (SA) node, located in the upper wall of the right atrium, acts as the primary pacemaker. It initiates a wave of electrical impulses that triggers atrial contraction. The signal then reaches the atrioventricular (AV) node, situated between the atrium and ventricle. The AV node acts as a gate, delaying the signal to ensure the atria finish contracting before the ventricles begin. This delay provides necessary time for the ventricles to fill with blood.

Medical professionals monitor this electrical activity using an electrocardiogram (ECG). An ECG produces a visual trace of the heart's electrical signals over time. The P wave on the tracing indicates the start of atrial systole. The QRS complex, which appears as sharp spikes, represents the onset of ventricular systole.

These electrical events are closely tied to the physical movements of the heart muscle, which is made of specialized cells called myocytes. Interestingly, these myocytes can initiate their own internal contractions without external nerve signals, though metabolic demands can change the heart rate.

Understanding the cardiac cycle also involves studying the complex changes in pressure and volume. A Wiggers diagram is a tool used to show how these different parameters relate to one another.

For example, the diagram tracks how ventricular volume falls during the ejection stage and how pressure fluctuates in the aorta. When the aortic valve closes, it can cause a rapid change in pressure called an incisura. These precise mechanical and electrical coordination efforts ensure that blood is efficiently collected and circulated throughout the entire human body.