Cardiac cycle

Category : 11th Class

During the completion of one heart beat is called as cardiac cycle. Following events are repeated in a cyclic manner during each heart beat.

(1) Auricular systole : The atria contract due to wave of contraction stimulated by S.A. node contraction of auricles drives most of their blood into respective ventricles as the A.V. valves are open. There is no backflow of blood into the large veins as the contraction begins at the upper end and passes towards ventricles and moreover, the valves present at the opening of these veins close. Also, blood is already present in large veins which offers resistance to the blood that may return from the atria. At the end of a atrial systole, there starts the relaxation of auricles (auricular diastole) and contraction of ventricles (ventricular systole) simultaneously. Atrial systole takes 0.1 second while atrial diastole is of about 0.7 seconds.

(2) Ventricular systole : The ventricles begin to contract due to a wave of contraction stimulated by A.V. node. Due to ventricular systole, the pressure of blood in ventricles immediately rises above that in the auricles. With this pressure, the bicuspid and tricuspid valves close rapidly to prevent the backflow of blood. This closure of A.V. valves at the start of ventricular systole produces first heart sound called “Lubb” or Systolic sound. The semilunar valves are also close at this time. When the pressure of blood in the ventricles exceeds that in the great arteries, the semilunar valves open and blood enters into the great arteries. This marks the end of ventricular systole which takes about 0.3 seconds. Now the ventricles start relaxing (ventricular diastole which lasts for about 0.5 sec.)

(3) Joint diastole : The ventricles and auricles are in the diastolic phase simultaneously. As the ventricular diastole progresses, the pressure in the ventricles falls below that in the great arteries. So, to prevent backflow of blood from great arteries into ventricles, the semilunar valves close rapidly. This rapid closure of semilunar valves at the beginning of ventricular diastole produces second heart sound “Dup” or diastolic sound.

The quality of heart sounds indicates the state of the heart valves. Defective or damaged heart valves lead to the backflow of blood either from ventricles to auricles or from aortae to ventricles. Such defects are detectable as abnormal hissing sound called “Murmur”. Defective valves may be replaced or repaired surgically. Syphilis and Rheumatic fever cause Murmur. The instrument used to magnify and record the heart sound is called Phonocardiogram.

During joint diastole, blood from great veins and coronary sinus flows into the atria and some blood also passes from auricles into the respective relaxing ventricles due to less pressure in ventricles. This phase takes only 0.4 seconds and is also called as blood receiving period of heart. Thus a cardiac cycle is completed in 0.8 seconds.

Cardiac output : Volume of blood pumped from heart (left ventricle) into the systemic aorta in one minute is called cardiac output. It is also called minute volume. It is calculated as the product of stroke volume (amount of blood pumped by left ventricle each time it contracts) and rate of heart beat.

i.e. Cardiac output = Stroke volume \[\times \] Rate of heart beat

\[=70\,\,ml\times 75\] times/minute = 5250 ml/minute or 5.25 liters/min.

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. During intense exercise, cardiac out put rises to 19.5 liters/min., and heart beat may rise to130 beat/minute.  Cardiac output is directly proportional to the size of the organism, metabolic rate etc. but is inversely proportional to age.

(1) Fractions of cardiac output : Amount of pure blood going to an organ per minute is called as fraction of the organ.

(i) Cardiac fraction\[\text{ }200\,\,ml/min.\]

(ii) Hepatic fraction (maximum) \[\text{ }1500\,\,ml/min.\] (28% of blood as liver is the busiest organ of body and has maximum power of regeneration).

(iii) Renal fraction \[\text{ }1300\,\,ml/min\] (25% of blood)’

(iv) Myofraction\[\text{ }600-900\,\,ml/min.\]  

(v) Cephalic organs (brain) \[\text{ }700-800\,\,ml/min.\]

(vi) Remaining organs - Remaining blood.

(2) Cardiac index : Cardiac output per square metre of body surface area per minute. As area of normal young adult is 1.7 metre square, so, cardiac index is 3 litres/min/square metre.

(3) Cardiac reserve : Maximum amount of blood that can be pumped by left ventricle under the conditions of maximum needs. In this condition, heart beat can go upto 250 and stroke volume can go upto 100 ml per systole. Cardiac reserve is 25-30 litres which is about 5-6 times of cardiac output.

(4) End diastolic volume (EDV) : Amount of blood present in left ventricle at the end of diastole. It is the maximum volume of the cavity of left ventricle and is equal to \[120-130\,\,ml.\]

(5) End systolic volume (ESV) : Amount of blood present in left ventricle at the end of systole. It is the least volume of the cavity of left ventricle and is equal to 50-60 ml.

(6) Stroke volume : (70 ml) is equal to the difference between the end diastolic volume and end systolic volume.


(7) Venous return : Amount of impure blood returning to righ atrium per minute is called venous return and is equal to 5.25 litres.


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