Figure 18.7 S.A. and A.V. node with conducting or Purkinje fibers

i) The heart beat: The heart is a remarkable organ. The heart beats continuously throughout an animal’s life. The heart contracts rhythmically, pumping a certain volume of blood from each ventricle to different parts of the body. This volume is called cardiac output.

The contraction of the heart is called systole and the relaxation, diastole. The contraction and relaxation of the heart forms a heart beat. The human heart beat can be heard as "lub-dub" sound (heart sound) with a stethoscope. When the body is at rest, the heart beats approximately 70 times per minute, and each cardiac cycle lasts for about 0.8 seconds.

Functioning: (1) The cardiac cycle (heart cycle) begins with atrial contraction (atrial systole), which is initiated by electrical stimulus coming from the sino-atrial (S.A) node, or "pacemaker". The stimulus is transmitted through the atrio-ventricular (A.V.) node to bundle of His, which spreads to form Purkinje fibers on the ventricle. (Fig. 18.7) During atrial contraction, the blood from the atria is forced into the respective ventricles. The bicuspid and tricuspid valves prevent back flow of the blood.

(2) The ventricles now undergo powerful contraction (ventricular systole), during which bicuspid and tricuspid valves remain closed. When the ventricular blood pressure increases (systolic pressure) to more than that in the aorta and pulmonary arteries , the blood enters these arteries through the semilunar valves.

(3) When the blood enters the arteries, the ventricles relax (ventricular diastole) and the pressure (diastolic pressure) in them falls. Simultaneously the atria have relaxed (atrial diastole) and the atria fill with blood again. The cardiac cycle continues in this way.

Heart sounds: During ventricular systole, simultaneous closure of atrio-ventricular valves makes the first sound --"lub". The second sound -- "dub"- which is higher-pitched, shorter and sharper, is caused by the simultaneous closure of aortic and pulmonary valves. Extra heart sounds, called murmurs or a "hiss" sound, occurs when blood leaks from the artery back into the ventricle, usually because one of the valves is defective. (In that case the rhythm would be: Lub-hiss-dub or lub-dub-hiss.)