Atrial systole is the contraction of cardiac muscle cells of both left and right atria after electrical stimulation and conduction across the chambers. It starts at the beginning of a heartbeat and ends when another heartbeat begins. The systole causes the pumping of blood in the aorta and pulmonary trunk. The atrial blood pressure during systole, also known as systolic blood pressure, peaks at 90 to 120 mm of mercury in humans. 

Measuring Atrial Systole

• It is measured by an electrocardiogram; ECG or EKG.
• The technical name is electrocardiography. 
• Here the electrogram of the heart is measured and recorded using electrodes.
• The electrodes are placed on the skin. 
• It records the electrical signals of the heart that cause the beats. 
• A healthy ECG shows a PQRST shape. 

Atrial Systole parts

The sinoatrial node (SA node) is an area of specialised cells. It is located near the superior vena cava on the right atrium. The SA node is also called the pacemaker of the heart as it can contract faster than any other tissue in the heart. It can spread its impulses across the left and right atria by special pathways. 

Then comes the Atrioventricular valves or AV valves. These valves connect the atriums to the ventricles. There are a total of 2 valves – mitral valve and tricuspid valve – located between the atria (upper heart chambers) and the ventricles (lower heart chambers) in an individual. 

The mitral valve aids with the blood flow from the left atrium into the left ventricle. On the other hand, the tricuspid valve allows the flow of blood into the right ventricle. It closes to prevent the backflow of blood into the right atrium.  

Atrial Systole causes

• It occurs at the almost end of the ventricular diastole. This completes the filling of the ventricles. 
• Atrial systole also happens because of atrial depolarization. The P wave deflection shows this in the ECG. The word ‘systole’ means a contraction in this process.  
• As the atrial muscles contract, the pressure in the atria and blood vessels increases. 
• The blood is then pumped through the AV valves (mitral and tricuspid) into the ventricles. 
• At the start of the systole, the ventricles fill up with blood up to 70 to 80%. 
• The other 20 to 30% is filled due to atrial contraction, which is also known as atrial kick.
• The systole lasts 100ms, it ends before the ventricular systole, and the atrial muscles return to diastole, also known as relaxing. 
• Atrial systole ends, and ventricular systole begins just before the atrial muscle contraction.  
• It lasts about 270ms, and during this, the ventricles contain approximately 130ml of blood in a resting or standing position. 
• The volume is also known as EDV End Diastolic Volume or preload. 

Diastole

• Diastole is the first thing that occurs in a cardiac cycle. 
• Diastole occurs before the SA node sends an electrical impulse to the atria. 
• During diastole, the blood enters through the superior and inferior vena cava into the right atrium and by pulmonary veins in the left atrium. 
• The AV valves are closed in diastole, and blood gathers in the atria.
• When the pressure is higher in the atrium than in the ventricle, the AV valves open up and allow the blood flow in the ventricles, through osmosis, so the blood flows from high pressure to low-pressure. 

Differences between the systole and diastole 

Conclusion 

To conclude, the atrial systole is the contraction of atrial muscles in the heart. The contraction increases pressure in the atria, causing the AV valves to open and allow the transfer of blood from the atria to the ventricles. So the blood flows from high-pressure volume to low-pressure volume. The systole is caused when the SA node sends an action potential throughout the atria, resulting in electrical depolarization and contraction of the atria. 

As the atria contracts, it causes the blood in the atria to flow from the upper chambers to the lower chambers of the heart. The systole is measured by an ECG report which comes in PQRST shape.