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• To begin with, all four chambers are in relaxed state called joint diastole. As the
bicuspid and tricuspid valves are open, blood from pulmonary vein and vena cava
flows to left and right ventricle respectively. Semilunar valves are closed at this stage.
• SA node generates action potential that contracts both atria (atrial systole). The
action potential passes to AV node and bundle of HIS transmit it to ventricular
musculature to cause ventricular systole. At the same time atria undergoes
relaxation diastole to close the bicuspid and tricuspid valve.
• Semilunar valves open into circulatory system that relax the ventricle and close the
valves to prevent back flow of blood.
• As the pressure inside ventricle decreases the bicuspid and tricuspid valve open to
repeat the process or cardiac cycle.
• During each cardiac cycle two sounds are produced. The first sound (lub) is due to
closure of bicuspid and tricuspid valve and 2nd heart sound (dub) is due to closure of
semilunar valve.
ECG (Electrocardiograph) is a graphical representation of electrical activity of heart during
cardiac cycle. The electrocardiograph machine is used to obtain electrocardiogram. The
patient is connected to three electrical leads to wrists and left ankle.
• The P-wave represents the electrical excitation of atria (depolarisation) which leads to
contraction of atria.
• The QRS-wave represents the depolarisation of ventricles, which initiates the
ventricular contraction.
• The T-wave represents the return of ventricle from exited to normal state
(repolarization). The end of T-wave marks the end of systole. Counting the number of
QRS complex in given period of time determine the heartbeat rate.
Double Circulation
Flow of same blood twice through the heart once in oxygenated form and other in
deoxygenated form is called double circulation. It includes systematic and pulmonary
circulation.
Systematic circulation includes flow of oxygenated blood from the left ventricle to all parts
of body and deoxygenated blood from various body parts to the right atrium. All systematic
circulation starts form aorta and ends at superior vena cava, inferior vena cava or coronary
sinus to right atrium.
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