Cardiac Cycle

A cardiac cycle is a time period that starts from the contraction of the atria and ends at the relaxation of the ventricular. 

Two terms are very important in the cardiac cycle 

• Systolic (the contraction phase)

• Diastolic (the relaxation phase)

Human Heart and Cardiac Cycle

The heart is a powerful muscular pumping organ that pumps blood from the heart to the body via the circulatory system. The heart is a pumping organ. The triangular, superior-broad portion is tilted slightly towards the right side, its lower narrow portion is tilted towards the left side. The heart wall is made up of 3 layers of endocardium, myocardium, and epicardium. The Heart is an enclosed envelope of two membranes called pericardial membranes (pericardium). There is a protective covering on the outer side of the heart called a pericardial membrane. In between the two membranes, there is a presence of fluid known as pericardial fluid. 

The human heart is divisible into four chambers. Two upper chambers are auricles (atria) while the lower two chambers are called ventricles. 

Right Auricle (Atrium) 

The right auricle has openings of the superior vena cava, inferior vena cava, and coronary sinus. Deoxygenated blood from the veins of the head, neck, and upper limbs enters the right auricles by the superior vena cava and from the rest of the body and lower limbs by the inferior vena cava. The coronary sinus, which auricle blood passes into the right ventricle through a tricuspid valve.

Right Ventricle 

It is guarded by a semilunar valve. 

Left Auricle

This chamber receives four pulmonary veins. The left auricle empties its blood into the left ventricle through a  bicuspid valve. 

Left Ventricle 

This blood is brought back to the heart by coronary veins which join to form the coronary sinus. 

Phases of Cardiac Cycle

The cardiac cycle contains six phases; 

Joint Diastole

When all the chambers are the atria and ventricles are in a relaxed state. Blood flows from the vena cava to the atria and from atria to ventricles. Atria get filled with the blood from the superior vena cava and inferior vena cava. Left atria get filled with the pulmonary veins for pulmonary veins from the left side and from the right side, both the ventricles filled with blood to their respective areas as the tricuspid and bicuspid. 

Atrial Systole 

When the atrial is in the contraction phase. At the time of joint diastole 70 to 80 percent of blood will go from the atria to the ventricles only 20 to 30 percent will remain in the atria to pump out this blood, atria will apply some force. This contraction is done by the contraction phase of the atria which is known as an arterial system as the SA node transfers its impulses; the contraction phase in the atria occurs.

Ventricular Systole 

When the ventricular starts contracting, it is called ventricular systole or the phase of ventricular contraction. In the ventricular contraction, the impulse from the SA node will be transferred to the AV node so further passes it to the Purkinje fibre and his bundle office which start the contraction in the event that occurs as the pressure start or increase in the ventricle with tricuspid and bicuspid valves get closed so that, there is no backflow of the blood. As these walls get closed there is a sound which will be the sound of its ‘lub’.

Complete Ventricular  

When ventricles are in a full contraction phase. As the ventricular compression starts the blood will flow the semolina walls get open and blood from ventricles will go to their respective parts from the right ventricle to the pulmonary aorta and to the left part through the aortic arch. Now semilunar walls are open, the ventricles are incomplete contraction phase and blood flows from pentacles to the pulmonary aorta are in the aortic arch.

Beginning of Ventricular Diastole 

After the systolic phase ventricles come into the relaxation phase. As the ventricular relaxation starts the semilunar walls get closed so that there is no backflow of the blood from the pulmonary aorta and aortic arch. This produces a sound called ‘dub’, the first sound produced at the time of the call. losing of the wall and the second produced at the time of closing of the wall which means when semilunar valves are closed.

Complete Ventricular Diastole 

When chambers or ventricles are fully relaxed, semilunar walls are closed while the tricuspid or bicuspid walls are open. There is a flow of blood from our atria to their respective ventricles and the joint diastolic occurs in which all the chambers are again filling with the blood.

This is our cardiac cycle which is 0.8seconds, the systematic flow of blood throughout the cycle. 

Guarding Your Heart 

Ribs protect the heart from being shocked or injured, thus allowing the heart to beat easily without any kind of external pressure.

The heart is also protected by the pericardium, which is fluid-filled to prevent friction inside the chest.