Body Fluids and Circulation

Cardiac cycle, cardiac output, ECG; double circulation; regulation of cardiac activity

CARDIAC CYCLE (1 Heat beat 1 systole & diastole)

  • Cardiac cycle comprises of following steps :-
    a) Atrial systole
    (i) SA Node stimulation; triculpid and bicuspid valve open
    (b)Beginning of ventricular systole
  • AV node stimulation ; tricuspid and bicuspid valves closes to produce the first heart sound called "LUBB". (·15 sec) - Low pitch.
    (c) completion of ventricular systole
    (i) semi lunar valves open ;
    (d) Beginning of ventricular diastole
    (i) semi lunar valve closes to produce the second heart sound called "TUBB" (·1sec) - High pitch.
    (e) Completion of ventricular diastole
    (i) Again tricuspid and bicuspid valves open as the pressure in ventricles fall.


  • Cardiac output is the amount of blood pumped by heart in one minute.
    [C.O] = heart beat rate × stroke volume
    = 72 beats/minute × 70 ml/beat
    = 5040 ml / minute = 5.5 L/min.


  • It is measured in the form of graph that is been produced by the excitation of cardiac muscles which was firstly recorded by Waller
  • Father of ECG is EINTHOVEN.
  • P → Atrial depolarization / contraction due to SA node activator.
    QRS complex → ventricular depolarization due to AV node activation.
    T → Ventricular repolarisation end of T wave → end of systole.

Double Circulation :-

  • Both systemic circulation (greater circulation/peripheral circulation) and pulmonary circulation comprise double circulation.
  • In systemic circulation oxygenated blood flows from left ventricle to all parts of body and deoxygenated blood flows from various parts to right atrium.
  • Flow of blood from heart to lungs and vice versa constitute pulmonary circulation.


  • Neural regulation
    Cardiac centre (medulla oblengata)
  • Heart Rate decreases when the impulses receivers from aorta or carotid sinus.
  • Heart rate increases when the impulses received from vena cava
  • Hormonal regulation (influences SA Node)

Watch this video for the topic from 30:19 to 54:27

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