The heart is a muscle. It has four chambers: two upper ones, called the atria, and two lower ones, called the ventricles. Blood flows in and out of the heart through these chambers.
The Heart As A Pump
The heart pumps blood to the lungs to absorb the oxygen we inhale. It then pumps this oxygen-rich blood to the rest of the body through the arteries. After the blood delivers the oxygen to the body's tissues, it returns to the heart through the veins. The cycle is then repeated.
Here's how the pumping system works:
Oxygen-poor blood returns from the body to the right upper chamber of the heart, called the right atrium. This chamber contracts to pump the blood to the right ventricle.
The right ventricle pumps the blood into the lungs, where it takes up oxygen and gets rid of carbon dioxide.
Next, the oxygen-enriched blood travels to the left atrium, which pumps it into the left ventricle.
The left ventricle then pumps the blood to tissues throughout the body via the arteries.
What Makes The Heart Beat?
Heartbeats - the contractions of the heart muscle - are controlled by electrical signals. These signals are low-voltage signals that are normally generated in an area of the heart called the sinus node, which is the heart's natural pacemaker.
Here's how the heart's electrical system, also called the conduction system, works:
The sinus node, also called the sinoatrial (SA) node, lies in the wall of the right atrium (the upper right chamber of the heart). Electrical impulses generated here are spread to both atria and cause them to contract (pump).
Special pathways carry impulses from the sinus node to the atrioventricular (AV) node. The role of the AV node is to momentarily delay the impulse to allow the atria to complete their contraction before the ventricles contract. This helps the ventricles fill with blood, especially at fast heart rates.
From the bottom of the AV node emerges the atrioventricular bundle. This cable-like structure - the only normal route for conduction of impulses from the atria to the ventricles - splits into two pathways, called the right and left bundle branches.
The bundle branches divide into fibers that spread electrical impulses throughout the ventricles. The high speed at which impulses travel through these fibers ensures that both ventricles contract almost simultaneously, increasing the effectiveness of contraction and pumping of blood.
How Is The Heartbeat Regulated?
The ability of the heart to contract is a property of the heart muscle itself; it is not dependent on the body's nervous system. So even if nerve connections to the heart are cut, the heart continues to contract, as demonstrated by transplanted hearts.
However, the heart is well supplied by nerve fibers, and these can affect the basic activity of the heart. Other factors that determine the heart's rate (the number of beats per minute) and rhythm (the sequence of heartbeats) include:
Levels of electrolytes (salts) in the blood
Normally, the sinus node sets the pace of the heart. At rest, the sinus node usually fires between 60 and 100 impulses per minute, and the associated rhythm is called sinus rhythm.
But under certain conditions (for example, exercise, stress, decreased blood volume, low blood pressure), the sinus node speeds up the rate at which it generates impulses. This happens in order to speed up the heart rate to provide the increased blood supply that the body needs.
The message to the sinus node to speed up is sent by the nervous system and by chemical messengers called catecholamines, which consist of adrenaline (epinephrine) and noradrenaline (norepinephrine). Another chemical messenger, acetylcholine, has the opposite effect, causing the heart rate to slow.
What Other Factors Influence The Heartbeat?
A variety of other factors also influence the heartbeat:
Lifestyle factors (for example, stress or the use of nicotine, caffeine, or alcohol)
Irregularities in the normal rate and rhythm of the heart are called arrhythmias. Examples of arrhythmias are:
A fast heartbeat, called tachycardia
A slow heartbeat, called bradycardia
An irregular heartbeat
Symptoms of an arrhythmia may include:
A skipping or fluttering sensation in the chest (palpitations)
Shortness of breath
Nice To Know:
Not everyone experiences symptoms, however. In some people, arrhythmias may go unnoticed.
Arrhythmias may occur if the sinus node develops an abnormal rate. In other cases, arrhythmias may mean that another region of the heart (such as the AV node or ventricle) has assumed the role of cardiac pacemaker.
Under normal conditions, these other regions do not control the heartbeat, as they fire impulses at slower rates than the sinus node. However, any of these regions may assume the role of pacemaker if the sinus node fails or if its impulses are blocked or interrupted. In addition, various diseases (including heart disease), drugs, and nervous system stimulation may contribute to this condition.