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A Handbook of Biology

01

During this, the atria undergo diastole. Ventricular

systole increases ventricular pressure causing.

Closure of tricuspid and bicuspid valves due to

attempted back flow of blood into the atria.

Semilunar valves open. So deoxygenated blood enters

Pulmonary artery from right ventricle and oxygenated

blood enters the aorta from left ventricle.

02

03

The ventricles now relax (ventricular diastole) and the

ventricular pressure falls causing the closure of

semilunar valves which prevents the backflow of

blood into the ventricles.

The tricuspid and bicuspid valves are opened by the

pressure in the atria.

04

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The ventricles and atria again undergo joint diastole and the above

processes are repeated.

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A cardiac cycle is completed in 0.8 seconds.

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One heartbeat = a cardiac cycle

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So, normal heartbeat: 70-75 times/min (average 72/min.)

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Stroke Volume: It is the volume of blood pumped out by each ventricle

during a cardiac cycle. It is about 70ml.

Cardiac Output: Volume of

blood pumped out by each

ventricle per minute i.e.,

stroke volume × heart rate,

(70×72) is about 5000ml

(5 litres). The body has the

ability to alter the stroke

volume as well as the heart

rate and theirby the cardiac

output. Eg. The cardiac

output of an athlete will be

much higher than that of

an ordinary man.

Heart Sounds: During each

cardiac cycle, 2 sounds are

produced that can be heard

with stethoscope. The first

sound (lub) is due to the

closure of tricuspid and

bicuspid valves. The second

sound (dub) is due to the

closure of the semilunar

valves. (AIPMT 1994)

One heartbeat = lub + dub.