During this time the ventricles contract into two blood-filled positions. Bicuspid and tricuspid cusps are open. The pressure between the two ventricles increases. As a result, blood containing CO2 from the right ventricle enters the pulmonary artery and blood containing O2 from the left ventricle enters the aorta or aorta. The duration of this dasha is 0.3 seconds.

During this time the two atria contract. Bicuspid and tricuspid valves open. But the semilunar eyelid remains closed. The pressure between the two atria increases. As a result, blood containing CO2 enters the right ventricle from the right atrium and blood containing O2 enters the left ventricle from the left atrium. The duration of this dasha is 0.1 seconds. The first 0.05 seconds of this is when the contraction is at its maximum. This is called dynamic phase. The second 0.05 seconds fades to fainter. This is called adynamic phase.

During this time the atria are in two dilated states. Bicuspid and tricuspid cusps are closed. The pressure between the two atria decreases. As a result, blood containing CO2 enters the right atrium through the superior vena cava from the upper part of the body and the inferior vena cava from the lower part. At the same time O2-rich blood enters the left atrium through the pulmonary veins. The duration of this dasha is 0.7 seconds.

The sequence of events that occur in the heart to complete each heartbeat is called the cardiac cycle. If heart beats on average 75 times per minute then cardiac cycle time is 60 ÷ 75 = 0.8 seconds.
Phases of the cardiac cycle
The cardiac cycle of the heart is completed in four phases. These are-
1. Diastole of the atrium: During this time the atria are in two dilated states. Bicuspid and tricuspid cusps are closed. The pressure between the two atria decreases. As a result, blood containing CO2 enters the right atrium through the superior vena cava from the upper part of the body and the inferior vena cava from the lower part. At the same time O2-rich blood enters the left atrium through the pulmonary veins. The duration of this dasha is 0.7 seconds.
2. Atrium systole: During this time the two atria contract. Bicuspid and tricuspid valves open. But the semilunar eyelid remains closed. The pressure between the two atria increases. As a result, blood containing CO2 enters the right ventricle from the right atrium and blood containing O2 enters the left ventricle from the left atrium. The duration of this dasha is 0.1 seconds. The first 0.05 seconds of this is when the contraction is at its maximum. This is called dynamic phase. The second 0.05 seconds fades to fainter. This is called adynamic phase.
3. Ventricular systole: During this time the ventricles contract into two blood-filled positions. Bicuspid and tricuspid cusps are open. The pressure between the two ventricles increases. As a result, blood containing CO2 from the right ventricle enters the pulmonary artery and blood containing O2 from the left ventricle enters the aorta or aorta. The duration of this dasha is 0.3 seconds.
4. Ventricular diastole: During this time the two ventricles are in dilated or relaxed state. Bicuspid and tricuspid cusps are open. But the semilunar eyelid remains closed. The pressure between the two ventricles decreases. As a result, blood containing CO2 enters the right ventricle from the right atrium and blood containing O2 enters the left ventricle from the left atrium. The duration of this dasha is 0.5 seconds.
A heart beat or cardiac cycle is completed every 0.8 seconds (0.7 + 0.1 = 0.8 or 0.3 + 0.5 = 0.8) consisting of one systole and one diastole. The sound heard during heartbeat is called lub-dub.

The rhythmic contraction and expansion of the heart together is called the heart beat. An adult’s heart beats 70-80 times per minute (average 75 times). Sympathetic nerves and the hormones adrenaline and thyroxine increase the heart rate. Parasympathetic nerves decrease heart rate. There are two types of heart beats. –
1. Lab: At the beginning of systole, the sound made when the atrioventricular valves (bicuspid and tricuspid) close is called lab.
2. Dub: The sound made when the semilunar valves close at the end of systole is called Dub.

Heart is the recipient and sending center of blood circulation. It pumps 2,000 gallons of blood per day and contracts and expands 100,000 times. The heart of mammals and birds has bi-circular circulation of blood. A two-circuit circulatory system where blood flows through the heart twice before completing one cycle throughout the body. Blood circulation process of heart is-
1. The two atria of the heart are dilated. Atrial volume increases and blood pressure decreases. Blood containing CO2 enters the right atrium through superior venacava-svc from upper body and inferior venacava-ivc from lower body. At the same time, O2-rich blood enters the left atrium through the four pulmonary veins.
2. Both atria contract when filled with blood. Atrial volume decreases and blood pressure increases. At the same time both ventricles dilate. The volume of the ventricles increases and the blood pressure decreases. As a result, blood containing CO2 enters from the right atrium to the right ventricle through the tricuspid valve and blood containing O2 from the left atrium to the left ventricle through the bicuspid valve.
3. When the ventricles are filled with blood, they contract and blood pressure rises. As a result, blood containing CO2 enters the pulmonary artery from the right ventricle through the pulmonary semilunar valve and blood containing O2 enters the aorta from the left ventricle through the aortic semilunar valve.
4. Blood containing CO2 passes through the pulmonary artery to the lungs, purified, enriched with O2, and enters the left atrium through the pulmonary veins.
5. O2-rich blood from the aorta flows to various parts of the body through arteries, arterioles, sub-arteries and capillaries.
In this way the blood flows through the heart periodically and this phenomenon repeats itself cyclically during the heartbeat.

The heart is made up of cardiac muscle. Cardiac muscle has the longest resting period. It never ceases. Normally, when metabolic waste (lactic acid) accumulates in a cell, it is exhausted. But because the cardiac muscle has a long resting period, metabolic wastes are released more easily. In addition, the heart muscle directly uses lactic acid as a nutrient. Therefore, waste cannot accumulate in the heart muscle. So the heart is sometimes exhausted.

The ends of the bundles of His become fine fibers and form a meshwork in the walls of the ventricles. These are called Purkinje fibers. It is made up of specialized cardiac muscle and conducts the heartbeat quickly and efficiently. Its speed is 40-60 times per minute. It causes contraction of the ventricle wall. John Evangelist Perkinzie (1839) discovered it.

Behind the AVN lies the bundle of His. It is composed of multidimensional small fascicles. Of its two branches, one extends to the wall of the left ventricle and the other to the wall of the right ventricle. Swiss cardiologist Wilhelm His (1893) discovered it.

AVN is located in the right septum of the heart. It is composed of cardiac muscle. This is called a protected pacemaker. Because if the SAN fails to generate an electrical signal, it generates it. It receives impulses from the SAN and transmits them to the bundle of His. It takes 0.03 seconds for the impulse from the SAN to reach the AVN. But the AVN has a delay of 0.09 seconds in the arrival of the stimulus. This is called AV Nodal Delay. It then takes another 0.04 seconds for the AVN to travel from the thalamus to the ventricle muscle. That is, the impulse from the AVN takes a total of (0.03+0.09+0.04) 0.16 seconds to reach the ventricle muscle.