What Is Cardiac Cycle? Cardiac cycle steps

                       Cardiac Cycle
                Cardiac Cycle | Cardiac cycle refers to the event that occurs from the beginning of one heart beat to beginning of next heart beat.
    It is a continuous process running every time in the heart of each and every living organisms.  
              

• Talking about Healthy human being, the cardiac cycle lasts for 0.8 seconds ( if the heart rate is 72 beats per minute ).
•  The contraction of of Auricles is called auricular systole and the relaxation is called auricular diastol. Similarly, the contraction of ventricles is called ventricular systole and the relaxation is called ventricular diastole.    
•  Joint diastole: Its the period during which entire heart is in diastolic phase. It lasts fpr 0.4 seconds normally.

                           Phases of Cardiac Cycle.
    There are basically 4 different phases of cardiac cycle in human body.
1. Ventricular Systole
2. Ventricular Diastole.  
3. Auricular Systole

4. Auricular Diastole
          So lets learn all these phases in complete details. 
1. Ventricular Systole
          In this phase, The Blood is pumped out of the venticles through arteries. 
          During ventricular Systole ,the auricles receive impure blood from the veins.

     *This process has four main sub-phases.
     a. Isovolumetric contraction Cuspid valves of heart close during this phase and produce "LUBB" sound.
     b. Its the period of ejection of blood 
     c. Also called protodiastole
     d. Volumetric relaxation .   

2. Ventricular diastole.
       Closure of semilunar valves occurs due to which "DUP" sound is heard at the onset of ventricular diastole.
 This phase has two stages:
   ~Rapid inflow :-S3 heart sound is produced during this phase.
   ~Diastasis: In this phase, pressure inside all four chambers is equal and entire heart is in diastole. The AV valves are open but semilunar valves are closed during this phase.

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 3. Auricular systole :
                   In this phase S4 heart sound is produced . Also the Contraction of Auricles take place this phase.

4. Auricular diastole : The phase is identified by the relaxation of walls of auricles.
       This phase has some important topic that needs to known by all.
      ~ stroke volume - It is the volume of blood pumped from heart (left ventricle ) into the systemic aorta in one heart beat.
      ~ cardiac output - Its the volume of blood pumped from heart ( left ventricle ) into the systemic aorta in one minute.                 

         It is also called minute volume.
           Cardiac output =
      Stroke volume * Heart rate=70 ml * 72 times/minute = 5040 ml/ minute = 5 litres/ minute.
       Total amount of blood in human body is about 5 litres (7% of body weight ).,During mild exercise , the cardiac output rises to about 11 litres l. Cardiac output is directly proportional to the size of the organism, metabolic rate but it is inversely proportional to age.
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