Chapter 15: Body Fluids and Circulation

The atrioventricular bundle (AV), originating from the AV node, passes the cardiac impulse to the walls of the ventricles, whereas the atrioventricular node (AVN) conveys the impulse from the SA node. Both AVN and AV are triggered by the action potential that is originated by the sino-atrial node, thereby conducting the stimulus to the other parts of the heart. This causes the heartbeat, which is conducted to various other parts of the heart.

Cardiac cycle

• The alternate contraction and relaxation of the chambers of the heart cause the blood to circulate in the heart.
• The relaxation or expansion phase is also referred to as diastole, whereas contraction is also referred to as systole.
• Every systole is followed by a diastole.
• The cardiac cycle is the series of events that take place in one complete heartbeat. It lasts for about 0.8 seconds.

Cardiac output

• It is the amount of blood that is pumped by each of the ventricles in a minute.
• It is given by

Cardiac output = Stroke volume x Number of heartbeats per minute

A heartbeat generates two sounds – lub and dub, produced in every cardiac cycle. The sound is produced in a sequence, in sync with every heartbeat. ‘Lub’ is the first sound and is low-pitched. It is caused when the bicuspid valve and the tricuspid valve close. ‘Dub’ is the second sound which arises when the semi-lunar valve closes. This sound is high-pitched. Both sounds are significant in the diagnosis of any disorder of the heart.

The graphical representation of the cardiac cycle that is generated by the electrograph is known as the electrocardiogram or ECG. The diagram below depicts a standard ECG.

In the ECG, each peak indicated by the letters ‘P’ to ‘T’ relates to a particular electrical activity of the heart. The first peak, ‘P’, represents the atrial contraction or the atrial depolarisation. The subsequent QRS complex indicates the depolarisation of the ventricles or the ventricular contraction. Finally, the T-wave indicates ventricular relaxation or ventricular repolarisation.