The sinoatrial (SA) node is the natural pacemaker of the heart. It generates electrical impulses that cause the heart’s chambers to contract, setting the heart rate.

The SA node typically generates 60–100 heartbeats per minute when a person is at rest. It also speeds up the heart rate in response to an increase in demands on the body, such as during exercise or times of stress.

However, if the SA node does not work correctly, the heart may delay or skip beats or beat too rapidly, too slowly, or irregularly.

This article discusses the natural pacemaker of the heart, including the way it works and its location. It also examines problems that may affect it and reasons for needing an artificial pacemaker.

The sinoatrial (SA) node is the heart's natural pacemaker.Share on Pinterest
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The heart’s natural pacemaker is a cluster of cells. Doctors refer to it as the sinoatrial (SA) node or sinus node. The SA node produces electrical impulses that set the heart rate and rhythm.

The SA node is one of the most important tissues in the body as it initiates every heartbeat. Several factors, such as neurotransmitters and endocrine activity, can influence how it works.

Learn more about the heart.

The heart consists of two upper chambers, the atria, and two lower chambers, the ventricles.

Cells in the SA node can spontaneously generate an electrical impulse, which transmits to perinodal cells. Part of the heart’s electrical conduction system, these cells move the impulses across the right atrium.

The impulses cause the right and left atria of the heart to contract, pushing blood into the right and left ventricles.

After a short delay, the impulse then travels along the heart’s electrical system and causes the ventricles to contract. This pushes blood from the right ventricle into the lungs and from the left ventricle to all parts of the body.

While the entire electrical system of the heart is necessary for heart function, the SA node is the initiator.

SA node influences

Both the sympathetic and parasympathetic nervous system influence the SA node. The SA node’s response to these systems allows the heart to respond to varying situations and demands on the body.

The sympathetic nervous system increases the heart rate to boost blood flow to the tissues to deliver more oxygen and nutrients. This happens when someone exercises or experiences stress.

In contrast, the parasympathetic nervous system slows the heart rate when an individual is calm.

Learn more about the different parts of the autonomic nervous system.

The heart’s atria and ventricles have the following functions:

  • Right atrium: This chamber takes in oxygen-poor blood from the veins and sends it to the right ventricle when it contracts.
  • Right ventricle: This chamber pushes oxygen-poor blood into the lungs when it contracts.
  • Left atrium: This chamber takes in oxygen-rich blood from the lungs and sends it to the left ventricle when it contracts.
  • Left ventricle: This chamber pushes oxygen-rich blood into the arteries when it contracts.

The SA node is in the upper wall of the right atrium, where it joins the superior vena cava. This large vein takes oxygen-poor blood from the upper part of the body to the heart.

Problems with the SA node can occur due to either an abnormality in the electrical impulses it generates or dysfunction in the cells that transmit the impulse across the right atrium.

People may inherit problems but more commonly acquire them through other factors.

Sinus pause and arrest

This condition happens when the SA node does not initiate the electrical impulse that leads to a heart contraction. A pause or arrest can last a few seconds to several minutes.

A pause of a few seconds does not always produce symptoms. A longer pause can result in:

In some cases, this condition can even be fatal.

SA nodal exit block

An SA nodal exit block happens when the SA node initiates the electrical impulse, but the impulse does not reach the neighboring tissue in the right atrium.

Therefore, the heart does not receive the signal to contract. This causes a pause in the heartbeat.

Abnormal sinus rhythms

Abnormal sinus rhythms are small variations in heart rhythms. They occur more frequently in younger people or those who use morphine or a drug called digoxin.

If a person uses digoxin, abnormal sinus rhythm may result from digoxin toxicity and require treatment. People in this situation should contact a healthcare professional as soon as possible.

Wandering atrial pacemaker

A wandering atrial pacemaker describes a situation where the pacemaker function of the heart comes from different locations in the right atrium rather than the SA node.

This condition is usually asymptomatic and typically does not require treatment.

A pacemaker is a small battery-operated device that helps regulate heart rhythm.

People with abnormal heart rhythms or a heart that cannot pump enough blood to supply to the body may need a pacemaker.

Abnormal heart rhythms

The most common reason a person will need a pacemaker is to treat a slow or pausing heartbeat that leads to symptoms such as fainting. However, pacemakers are also suitable for fast or irregular heartbeats.

Causes of abnormal heart rhythms include:

A weak heart

When the heart cannot pump enough blood throughout the body, a pacemaker can enable the heart chambers to beat with the correct rhythm.

Causes of a weak heart include:

  • congenital heart abnormalities
  • heart failure
  • a heart attack
  • thick or enlarged heart muscles

The SA node serves as the natural pacemaker of the heart. It generates an electrical impulse that initiates every heartbeat. This structure lies in the upper part of the right atrium at the point when it joins the superior vena cava.

Problems with the SA node can stem from malfunctions in impulse generation or in the cells that transmit the impulse across the atrium.

Reasons for needing an artificial pacemaker include having an abnormal heart rate, such as a fast or slow heartbeat, or a weak heart that cannot pump enough blood around the body.