Nourish Your Concept Of The Cardiac Cycle:
Cardiac events from the beginning of one heartbeat to the beginning of the next are called the cardiac cycle. This is what we are going to focus on in this article. In the end, we will discuss some frequently asked questions from this topic.

Introduction: Cardiac cycle is the sequential changes taking place during a single heartbeat. A single heartbeat involves contraction and relaxation. The contraction is termed as systole whereas the relaxation is termed as diastole. We know that the heart beats 72 times in 60 seconds in normal physiological conditions. So the time taken by each cardiac cycle or heartbeat is 60/72 which equals 0.8 seconds. So each cardiac cycle lasts for 0.8 seconds. 
Main events of the cardiac cycle: The cardiac cycle consists of two main events.
  1. The atrial cycle
  2.  The ventricular cycle

The ventricular cycle: The ventricular cycle consists of ventricular systole and ventricular diastole. 


  1. Ventricular systole: During the ventricular systole the ventricles pump the blood out of the heart. The ventricular systole lasts for about 0.3 sec. Ventricular systole consists of the following main sub phases:

a. Isometric contraction: The ventricular musculature contracts thus increasing pressure inside the ventricles. Blood yet not leave the ventricles because the semilunar valves are closed. Due to the increased pressure inside ventricles, the atrioventricular or the cupid valves close creating the first heart sound, the LUBB. The cuspid valves are prevented from bulging into the atrial chambers by the chordae tendineae which are bands of connective tissue attached to papillary muscles present in ventricular walls. Thus chordae tendineae prevents regurgitation(backflow) of blood from ventricles to atria.

b. Period of ejection: When the pressure inside ventricles is high enough to open the semilunar valves the blood is pumped out of the heart into the systemic and pulmonary circuit.
The ventricular systole lasts for 0.3 sec which is further subdivided into isometric contraction which takes 0.1 sec and a period of ejection which takes 0.2 sec.

2. Ventricular diastole: The ventricles relax in this state. It lasts for about 0.5 sec. It consists of the following sub phases:

a. Isometric relaxation: The ventricles start to relax but blood yet not enter the ventricles from the atria because AV valves are closed. The pressure inside the ventricles decreases as a result of blood being pumped out of the heart and the relaxation of its musculature. Due to decreased pressure in ventricles, the semilunar valves close created the second heart sound, the DUBB. This closure of semilunar valves prevents backflow of blood from the aorta and pulmonary artery. Isometric relaxation takes 0.1sec.
b. Rapid inflow: The pressure in the ventricles decreases so much due to its relaxation that auricles have greater pressure thus opening AV valves and starting to fill blood in the ventricles. This creates a turbulent sound in ventricles. This is the third heart sound and is known as ventricular diastole gallop. Rapid inflow lasts for 0.2 sec. The pressure decreases significantly in auricles because there blood passes into ventricles. 
c. Diastasis: During diastasis the pressure in all the chambers of the heart is the same and all chambers are in relaxed state. Thus the rate at which the atria receives blood from then veins is equal to the rate at which ventricles receive the blood from the atria. It lasts for 0.2 sec.

The atrial cycle: 

The atrial cycle consists of atrial systole and atrial diastole. Atrial systole lasts for 0.1 sec whereas atrial diastole lasts for 0.7 sec.
1. Atrial systole: The atria contract and pump the blood received from veins into the ventricles. At this time fourth heart sound is heard which is basically a turbulent sound known as atrial systole gallop. Atrial systole lasts for 0.1 sec.
2. Atrial diastole: The atria now start relaxing and due to low pressure in it, it receives blood from the veins. Atrial diastole lasts for 0.7 sec.

Joint diastole: The time during which all the chambers of the heart are in relaxed state is termed as joint diastole. It lasts for 0.4 sec.
Note: When the atria contract for 0.1 sec, the ventricles are in relaxed state thus ventricles are filled with blood and after contraction the atria undergo diastole for 0.7 sec . Then the  ventricles contract taking about 0.3 sec to pump the blood out of the heart. After that the ventricles relax for 0.5 sec. So the time during which atria and ventricles are relaxing is 0.4 sec and is called diastole.

Expected questions for the MDCAT:

1. The heart sound first is heard during
a. Beginning of ventricular systole
b. Beginning of ventricular diastole
c. end of ventricular systole
d. None
The heart sound first is heard during the beginning of ventricular systole in the isometric contraction phase when the AV valves close as a result of hike in ventricular pressure
2. What prevents the bulging of AV valves into the atria
a. Chordae tendineae
b. Papillary muscles
c. Both a and b
3. The time during which all four chambers of the heart are relaxing lasts for
a. 0.4sec
b. 0.2sec
c. 0.5sec
d. none