During what phase does blood flow from the ventricles?

At rest, the heart pumps around 5L of blood around the body every minute, but this can increase massively during exercise. To achieve this high output efficiently, the heart works through a carefully controlled sequence with every heartbeat – this sequence of events is known as the cardiac cycle.

Pressures Within the Heart

The above table shows the range of pressures present throughout the heart during the cardiac cycle. Knowing these values can help us understand the progression between different stages of the cycle. For example, the pulmonary artery has a systolic pressure of 25mmHg, so the right ventricle must match this force to successfully eject blood.

The Cardiac Cycle

The cardiac cycle can be divided into four stages:

• Filling phase – the ventricles fill during diastole and atrial systole.
• Isovolumetric contraction – the ventricles contract, building up pressure ready to pump blood into the aorta/pulmonary trunk.
• Outflow phase – the ventricles continue to contract, pushing blood into the aorta and the pulmonary trunk. This is also known as systole.
• Isovolumetric relaxation – the ventricles relax, ready to re-fill with blood in the next filling phase.

This article will discuss each of the phases in more detail; describing the changes in pressure and the heart valves' actions in the cardiac cycle.

You can read more on the anatomy of the cardiac valves here.

Filling Phase

The ventricles are filled with blood in two stages – diastole (heart relaxation) and atrial systole (contraction of the atria).

In diastole, both the atria and the ventricles are relaxed. Blood flows from the vena cava and pulmonary veins into the right and left atria respectively, before flowing directly into the ventricles. The ventricles fill with blood at a steadily decreasing rate, until the ventricles' pressure is equal to that in the veins.

At the end of diastole, the atria contract, squirting a small amount of extra blood into the ventricles. This increases the ventricles' pressure so that it is now higher than that in the atria, causing the atrioventricular valves (mitral/tricuspid) to close.

Isovolumetric Contraction

As contraction begins both sets of valves are closed, meaning that no blood can escape from the ventricles. Therefore, the start of systole increases the pressure within the ventricles, ready to eject blood into the aorta and pulmonary trunk. The stage of isovolumetric contraction lasts for approximately 50ms, while the pressure builds up.

Outflow Phase

Once the ventricles' pressure exceeds the pressure in the aorta/pulmonary trunk, the outflow valves (aortic/pulmonary) open, and blood is pumped from the heart into the great arteries.

At the end of systole, around 330ms later, the ventricles begin to relax, decreasing the ventricular's pressure compared to the aorta. The decrease in pressure causes the valves to close. As well as this, blood begins to flow backwards through the outflow valves, which also contributes to the valves' closure.

Isovolumetric Relaxation

At the end of the outflow phase, both sets of valves are closed once again. The ventricles begin to relax, reducing the pressure in the ventricles so that the atrioventricular valves open. The ventricles then begin to fill with blood, and the cycle begins once again.

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Clinical Relevance - Cardiac Auscultation

The carefully coordinated sequence of events described in this article means that blood flows smoothly in the healthy heart and so cannot be heard on auscultation. When listening to the heart, what can be heard are two sounds, sometimes described as a “lub” and a “dub”. These sounds are caused by the closure of valves in the cardiac cycle:

• S1(the “lub”): occurs at the end of the filling phase when the atrioventricular valves snap shut
• S2(the “dub”): occurs at the end of the outflow phase when the outflow valves snap shut

Damage to the heart valves can cause them to become narrowed (stenosis) or leaky (regurgitation). Either of these conditions can lead to turbulent blood flow which is heard as a humming sound on auscultation of the heart. These sounds are known as heart murmurs.

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Clinical Relevance - Additional Heart Sounds

Occasionally within clinical practice, additional heart sounds can be heard; these are termed S3and S4and can be heard in the following situations:

• S3 : this sound can sometimes be heard early in diastole (following S2). It is normal in young people or athletes; however, it often indicates congestive heart failure in older patients. It is caused by deceleration of blood moving from the left atrium to the left ventricle.
• S4 : this sound can sometimes be heard during atrial contraction (late diastole, immediately before S1) and is associated with reduced ventricular compliance (“stiff” ventricles) or left ventricular hypertrophy.

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The cardiac cycle is the sequence of events that occurs when the heart beats. As the heart beats, it circulates blood through pulmonary and systemic circuits of the body. There are two phases of the cardiac cycle: The diastole phase and the systole phase. In the diastole phase, heart ventricles relax and the heart fills with blood. In the systole phase, the ventricles contract and pump blood out of the heart to arteries. One cardiac cycle is completed when the heart chambers fill with blood and blood is pumped out of the heart.

The cardiac cycle is vital to proper cardiovascular system function. Comprised of the heart and circulatory system, the cardiovascular system transports nutrients to and removes gaseous waste from the cells of the body. The cardiac cycle provides the "muscle" needed to pump blood throughout the body. Blood vessels act as pathways that transport blood to various destinations.

The driving force behind the cardiac cycle is the electrical system known as cardiac conduction. This powers the cardiovascular system. Specialized tissues called heart nodes send nerve impulses that disperse throughout the heart wall to make the heart muscle contract.

The events of the cardiac cycle described below trace the path of blood from when it enters the heart to when it is pumped out of the heart to the rest of the body. Periods of contraction and pumping are systole and periods of relaxation and filling are diastole. The atria and ventricles of the heart both go through diastole and systole phases and diastole and systole phases occur simultaneously.

Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

During the ventricular diastole period, the atria and heart ventricles are relaxed and the atrioventricular valves are open. Oxygen-depleted blood returning to the heart from the body following the last cardiac cycle passes through the superior and inferior vena cavae and flows to the right atrium.

The open atrioventricular valves (tricuspid and mitral) allow blood to pass through the atria to the ventricles. Impulses from the sinoatrial (SA) node travel to the atrioventricular (AV) node and the AV node sends a signal that triggers both atria to contract. As a result of this contraction, the right atrium empties its contents into the right ventricle. The tricuspid valve, located between the right atrium and right ventricle, prevents blood from flowing back into the right atrium.

Mariana Ruiz Villarreal / Wikimedia Commons / Public Domain

At the beginning of the ventricular systole period, the right ventricle, which is filled with blood passed on from the right atrium, receives impulses from fiber branches (Purkinje fibers) carrying electrical impulses that cause it to contract. As this occurs, the atrioventricular valves close and the semilunar valves (pulmonary and aortic valves) open.

Ventricular contraction causes oxygen-depleted blood from the right ventricle to be pumped to the pulmonary artery. The pulmonary valve prevents blood from flowing back into the right ventricle. The pulmonary artery carries de-oxygenated blood along the pulmonary circuit to the lungs. There, the blood collects oxygen and returns to the left atrium of the heart through the pulmonary veins.

In the atrial diastole period, the semilunar valves close and the atrioventricular valves open. Oxygenated blood from the pulmonary veins fills the left atrium while blood from the venae cavae fills the right atrium. The SA node contracts again triggering both atria to do the same.

Atrial contraction causes the left atrium to empty its contents into the left ventricle and the right atrium to empty its contents into the right ventricle. The mitral valve, located between the left atrium and left ventricle, prevents oxygenated blood from flowing back into the left atrium.

During the atrial systole period, the atrioventricular valves close and the semilunar valves open. The ventricles receive impulses to contract. Oxygenated blood in the left ventricle is pumped to the aorta and the aortic valve prevents the oxygenated blood from flowing back into the left ventricle. Oxygen-depleted blood is also pumped from the right ventricle to the pulmonary artery at this time.

The aorta branches out to provide oxygenated blood to all parts of the body through systemic circulation. After its tour through the body, de-oxygenated blood is returned to the heart via the venae cavae.