The cardiac cycle has two main components:
Systole: Contraction where the heart's chambers expel blood.
Diastole: The non contractual phase during which the heart's pumping chambers get filled with blood.
The cycle is usually described as consisting of four major stages, and each of the phases is characterized by distinct movements of the heart muscles and valves, which regulate blood circulation.
Atrial systole is the phase when the atria (upper heart chambers) contract. This contraction is triggered by electrical signals from the sinoatrial (SA) node, the heart's natural pacemaker. This impulse then travels through the walls of the atria and causes them to contract, pushing the blood into the ventricles, which are the larger chambers at the lower part of the heart.
They are the tricuspid, on the right side, and the mitral, on the left. These two open during the atrial systole, letting blood flow from the atria to the ventricles.
During atrial systole, the atria contribute the remaining 20-30% of blood to fill the ventricles, after about 70-80% of the blood has already filled passively during the preceding diastole phase.
The blood pumped into the ventricles during atrial contraction is called the atrial kick, which helps ensure optimal blood flow.
By the end of this phase, the ventricles will be filled to maximum capacity with blood, which prepares the organ for the next step as envisaged in the circulatory process.
Following atrial systole, ventricular systole begins. Both the right and left ventricles contract simultaneously, with the right ventricle sending blood to the lungs and the left ventricle sending oxygenated blood to the body. This phase is divided into the isovolumetric contraction and the ejection phase.
This initial phase of the ventricular systole marks the start of ventricles contracting. But at this instant, all the heart's valves are shut, and even though the ventricles have no increase in volume, there is a considerable increase in pressure within the chambers. This pressure buildup is needed to open up the semilunar valves, including the pulmonary and the aortic valves.
When ventricular pressure exceeds that in the pulmonary artery and aorta, the semilunar valves open to pump blood out of the heart. The right side of the heart pumps blood into the pulmonary artery, where it gets to the lungs to acquire oxygen and release carbon dioxide, while the left side of the heart pumps blood into the aorta, which supplies oxygen to the remaining body.
The pulmonary and aortic semilunar valves open due to the rise of pressure, allowing the blood to flow in one direction out of the ventricles and into the pulmonary and systemic circulations, respectively.
This phase forms the pumping mechanism of the heart, whereby oxygen-depleted blood is pumped to the lungs to get re-oxygenated while oxygen-enriched blood is pumped to the tissues and the organs of the body.
When the ventricles are contracted and pumped blood, they follow a period of relaxation called early ventricular diastole or isovolumetric relaxation. During this stage, the ventricles stop contracting, and their pressure decreases. At this point, no blood enters the ventricles because both the AV and semilunar valves are closed.
When the pressure in the ventricles drops and becomes less than that in the pulmonary artery and aorta, the semilunar valves close to ensure that the blood does not flow back into the heart. This closure results in the second sound called 'dub' when the heartbeat is called 'lub-dub.'
Although the ventricles are relaxed during this phase, the AV valves (tricuspid and mitral) do not open. This phase, known as isovolumetric relaxation, is characterized by a constant blood volume in the ventricles as the heart prepares for the next cycle.
As the last process in the cardiac cycle, During late ventricular diastole, the heart, including the atria and ventricles, is relaxed. The ventricular filling process happens as blood flows from the atria into the ventricles. This process happens in two stages:
Approximately 70% of ventricular filling occurs passively when the AV valves open, allowing blood to flow into the ventricles without atrial contraction. Under normal circumstances, pressure in the atria is greater than in the relaxed ventricles, leading to this passive filling.
Ventricular filling may be enhanced by atrial contraction at the end of diastole. This atrial contraction, also known as "atrial kick," ensures that the ventricles are full to allow for the next contraction to happen.
This is the case because, at this stage, the ventricles are so prepared to contract, thus signifying the onset of the next cardiac cycle.
The cardiac cycle is the cyclic process allowing the coordinated and effective blood pumping in the entire organism. Understanding the phases of the cardiac cycle—atrial systole, ventricular systole, and diastole—helps comprehend normal heart function and identify changes in various heart conditions.
Disturbances in any cardiac cycle phase will result in heart dysfunction, hence the importance for healthcare professionals and patients to know these phases. If you are experiencing symptoms such as chest pain, shortness of breath, or irregular heartbeats, it's important to seek medical attention immediately.
Each heartbeat ensures oxygen and nutrients are efficiently delivered to the body's tissues through well-coordinated heart activity. People, in general, will realize the need to keep their hearts in check and manage through the knowledge of the cardiac cycle.
Q1. What is the cardiac cycle?
A: The cardiac cycle refers to the sequence of events that happen in your heart during one complete heartbeat. It involves the heart's contraction (systole) and relaxation (diastole) phases, allowing blood to circulate efficiently through your body.
Q2. What are the main phases of the cardiac cycle?
A: The cardiac cycle has two key components:
These phases are essential for your heart to keep a steady rhythm and ensure blood flows in the right direction.
Q3. Can issues with the cardiac cycle lead to heart disease?
A: Yes, disruptions in the cardiac cycle can lead to conditions like arrhythmias (irregular heartbeats), heart failure, and other cardiovascular diseases. Recognizing issues early can help in managing heart health.
Q4. How can I keep my heart healthy and support its normal cycle?
A: Maintaining a healthy lifestyle with regular exercise, a balanced diet, and stress management is crucial for heart health. If you have any concerns, it’s essential to consult with a healthcare provider.
Q5. Why should I consider Artemis for cardiac care?
A: Artemis Heart offers advanced heart care services that focus on diagnosing, treating, and managing heart conditions.
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