Where is heartbeat initiated

Your heart's electrical system controls the timing of your heartbeat by regulating your: Heart rate, which is the number of times your heart beats per minute. Heart rhythm, which is the synchronized pumping action of your four heart chambers. Your heart's electrical system should maintain: A steady heart rate of 60 to beats per minute at rest.

The heart's electrical system also increases this rate to meet your body's needs during physical activity and lowers it during sleep. An orderly contraction of your atria and ventricles this is called a sinus rhythm.

How does the heart's electrical system work? Two different types of cells in your heart enable the electrical signal to control your heartbeat: Conducting cells carry your heart's electrical signal. Muscle cells enable your heart's chambers to contract, an action triggered by your heart's electrical signal. The heartbeat happens as follows: The SA node called the pacemaker of the heart sends out an electrical impulse. The upper heart chambers atria contract.

The AV node sends an impulse into the ventricles. The lower heart chambers ventricles contract or pump. The SA node sends another signal to the atria to contract, which starts the cycle over again.

This cycle of an electrical signal followed by a contraction is one heartbeat. SA node and atria When the SA node sends an electrical impulse, it triggers the following process: The electrical signal travels from your SA node through muscle cells in your right and left atria. The signal triggers the muscle cells that make your atria contract. The atria contract, pumping blood into your left and right ventricles.

AV node and ventricles After the electrical signal has caused your atria to contract and pump blood into your ventricles, the electrical signal arrives at a group of cells at the bottom of the right atrium called the atrioventricular node, or AV node.

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CO can also predict blood pressure based on blood volume. Essentially, this means that higher venous blood return to the heart also called the preload will increase SV, which will in turn increase CO. This is because sarcomeres are stretched further when EDV increases, allowing the heart to eject more blood and keep the same ESV if no other factors change.

The main implication of this law is that increases in blood volume or blood return to the heart will increase cardiac output, which will lead to an increase in MAP. The opposite scenario is true as well. For example, a dehydrated person will have a low blood volume and lower venous return to the heart, which will decrease cardiac output and blood pressure.

Those that stand up quickly after lying down may feel light-headed because their venous return to the heart is momentarily impaired by gravity, temporarily decreasing blood pressure and supply to the brain. The adjustment for blood pressure is a quick process, while blood volume is slowly altered. Blood volume itself is another regulated variable, regulated slowly through complex processes in the renal system that alter blood pressure based on the Starling mechanism.

Privacy Policy. Skip to main content. Cardiovascular System: The Heart. Search for:. Physiology of the Heart. Electrical Events Cardiac contraction is initiated in the excitable cells of the sinoatrial SA node by both spontaneous depolarization and sympathetic activity.

Learning Objectives Describe the electrical events of the heart. Key Takeaways Key Points The sinoatrial SA and atrioventricular AV nodes make up the intrinsic conduction system of the heart by setting the rate at which the heart beats. The SA node generates action potentials spontaneously. The SA node fires at a normal rate of 60— beats per minute bpm , and causes depolarization in atrial muscle tissue and subsequent atrial contraction.

The AV node slows the impulses from the SA node, firing at a normal rate of bpm, and causes depolarization of the ventricular muscle tissue and ventricular contraction. Sympathetic nervous stimulation increases the heart rate, while parasympathetic nervous stimulation decreases the heart rate. Key Terms pacemaker : A structure that sets the rate at which the heart beats. Under normal conditions, the SA node serves this function for the heart.

Learning Objectives Describe electrocardiograms and their correlation with systole. Key Takeaways Key Points An ECG is used to measure the rate and regularity of heartbeats as well as the size and position of the chambers, the presence of damage to the heart, and the effects of drugs or devices used to regulate the heart, such as a pacemaker. The ECG device detects and amplifies the tiny electrical changes on the skin that are caused when the heart muscle depolarizes during each heartbeat, and then translates the electrical pulses of the heart into a graphic representation.

A typical ECG tracing of the cardiac cycle heartbeat consists of a P wave atrial depolarization , a QRS complex ventricular depolarization , and a T wave ventricular repolarization. An additional wave, the U wave Purkinje repolarization , is often visible, but not always. The ST complex is usually elevated during a myocardial infarction. Atrial fibrillation occurs when the P wave is missing and represents irregular, rapid, and inefficient atrial contraction, but is generally not fatal on its own.

Ventricular fibrillation occurs when all normal waves of an ECG are missing, represents rapid and irregular heartbeats, and will quickly cause sudden cardiac death. Key Terms fibrillation : A condition in which parts of the ECG do not appear normally, representing irregular, rapid, disorganized, and inefficient contractions of the atria or ventricles.

ST segment : The line between the QRS complex and the T wave, representing the time when the ventricles are depolarized before repolarization begins. The extra pathway is either separate from or within the AV node. In the first case, this is called an accessory pathway, sometimes referred to as WPW or Wolff-Parkinson-White syndrome.

In either case the extra electrical pathway between the atria and ventricles causes the abnormally fast heartbeat by allowing the electrical impulse to make a continuous loop. The impulse flows down the normal path from the atrium to the ventricle, then returns back to the atrium along the extra pathway.

Each time the impulse completes a circuit, the heart beats. This may result in a very rapid heartbeat. In other cases, instead of an extra pathway in the heart, there is an abnormal "focus," which can act like a second sinus node or natural pacemaker, causing the heart to go much faster than the sinus node would normally go. This focus may be located in either the upper or lower chambers. Certain conditions are associated with a slow heartbeat, or bradycardia.

One of the causes of bradycardia is heart block , which occurs when the electrical signal that starts in the upper portion of the heart cannot get through to the lower chambers. The heart beat and pumping action may not be adequate, causing symptoms such as fatigue, dizziness, near fainting or even fainting.

Another form of bradycardia may be caused when the heart's normal "pacemaker," the sinus node, does not work with regularity. This is known as sinus node dysfunction and may cause a heartbeat that is too slow. Bradycardias may be something that your child is born with, or something that develops with time.

On occasion, bradycardia can be the result of open heart surgery.