Body Fluids and Circulation - Notes | Class 11 | Part 4: Cardiac Cycle, ECG, Double Circulation



It is the cyclic contraction and relaxation of heart for pumping blood. 

It involves 3 stages:
  1. Joint diastole: It is the relaxed state of all chambers of heart. When the tricuspid and bicuspid valves open, blood from pulmonary vein and vena cava flows into left & right ventricles respectively through left and right atria. Semilunar valves are closed at this stage.
  2. Atrial (Auricular) systole: SAN generates an action potential. As a result, both the atria contract. It is called atrial systole. This increases the flow of blood into the ventricles by about 30%.
  3. Ventricular systole: The action potential is conducted to ventricular side by AVN & AV bundle from where bundle of His transmits it through the ventricular musculature. As a result, ventricles contract. It is called ventricular systole. During this, the atria undergo diastole. Ventricular systole increases the ventricular pressure causing
* Closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
* Semilunar valves open. So deoxygenated blood enters the pulmonary artery from right ventricle and oxygenated blood enters the aorta from left ventricle.

The ventricles now relax (ventricular diastole) and the ventricular pressure falls causing

* The closure of the semilunar valves which prevents the backflow of blood into the ventricles.
* The tricuspid and bicuspid valves are opened by the pressure in the atria.

The ventricles and atria again undergo joint diastole and the above processes are repeated.
  • A cardiac cycle is completed in 0.8 seconds.
  • One heartbeat = a cardiac cycle.
  • So, normal heartbeat: 70-75 times/min (average: 72/min).
  • Stroke volume: It is the volume of blood pumped out by each ventricle during a cardiac cycle. It is about 70 ml.
  • Cardiac output: It is the volume of blood pumped out by each ventricle per minute, i.e. stroke volume x heart rate (70 x 72). It is about 5000 ml (5 litres).
  • Cardiac output of an athlete is very high.
  • Heart sounds: During each cardiac cycle, 2 sounds are produced. 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.
One heartbeat = a lub + a dub.


It is an instrument used to obtain electrocardiogram.

Electrocardiogram is the graphical representation of the electrical activity of the heart during a cardiac cycle.

To get an ECG, a patient is connected to the machine with 3 electrical leads (one to each wrist and to left ankle) that monitor heart activity. For a detailed evaluation of heart’s function, multiple leads are attached to the chest region.

An ECG consists of the following waves:
  • P-wave: Represents the excitation (depolarization) of atria during atrial systole.
  • QRS-complex: Represents depolarization of ventricles during Ventricular systole.
  • T-wave: Represents the repolarisation of ventricles.

Deviation in the ECG indicates the abnormality or disease. So, ECG has great clinical significance.


It is the circulation in which blood flows through the heart twice for completing its circuit.

It includes Pulmonary circulation and Systemic circulation. 
  • Pulmonary circulation: Circulation b/w lungs and heart.
Deoxygenated blood from right ventricle → to pulmonary artery → to lungs → oxygenated blood → to pulmonary veins → left atrium.
  • Systemic circulation: Circulation b/w heart and various body parts.
Oxygenated blood from left ventricle → to aorta → arteries → arterioles → capillaries → tissues → deoxygenated blood from tissues → venules → veins → vena cava → to right atrium.

Systemic circulation provides nutrients, O2 and other substances to the tissues and takes CO2 and other harmful substances away for elimination. 

Hepatic portal system: 

It is a system which includes the hepatic portal vein that carries blood from intestine to the liver before it is delivered to the systemic circulation.

Coronary circulatory system: 

It is a system of coronary vessels that circulate blood to and from cardiac musculature.


Normal activities of heart are auto-regulated by nodal tissues. So, it is called myogenic heart.

Medulla oblongata regulates cardiac activity through ANS.

Sympathetic nerves of ANS increase the rate of heartbeat, the strength of ventricular contraction and cardiac output.

Parasympathetic nerves of ANS decrease the heartbeat, conduction of action potential and the cardiac output.

Adrenal medullary hormones increase the cardiac output.
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