આ MCQ મોડ્યુલ આના પર આધારિત છે: Cardiac Cycle Ecg Circulation
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Cardiac Cycle Ecg Circulation
🎓 Class 11
Biology
CBSE
Theory
Ch 15 – Body Fluids and Circulation
⏱ ~14 min
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આ મૂલ્યાંકન આના પર આધારિત હશે: Cardiac Cycle Ecg Circulation
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The Cardiac Cycle, ECG, Double Circulation and Disorders
Part 3 built the pump. This part sets it running — following one complete beat from the SAN's first impulse to the closing of the last valve, then reading that beat off an ECG trace, tracing where the blood actually goes, and finally examining what happens when the system fails.
15.3.2 Cardiac Cycle
Stage 1 — joint diastole
To begin with, all the four chambers of the heart are in a relaxed state, i.e., they are in joint diastole. As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and the right ventricle respectively through the left and right atria. The semilunar valves are closed at this stage.
Stage 2 — atrial systole
The SAN now generates an action potential which stimulates both the atria to undergo a simultaneous contraction — the atrial systole. This increases the flow of blood into the ventricles by about 30 per cent.
Read that 30 per cent carefully. It means that about 70 per cent of ventricular filling has already happened passively, during joint diastole, before the atria contract at all. The atria are not the main filling pump; they are a top-up. This is why a person whose atria stop contracting effectively can still survive, whereas failure of the ventricles is immediately catastrophic.
Stage 3 — ventricular systole
The action potential is conducted to the ventricular side by the AVN and AV bundle, from where the bundle of His transmits it through the entire ventricular musculature. This causes the ventricular muscles to contract (ventricular systole), while the atria undergo relaxation (diastole), coinciding with the ventricular systole.
What follows is a strict sequence of pressure changes and valve movements:
- Ventricular systole increases the ventricular pressure, causing the closure of tricuspid and bicuspid valves due to attempted backflow of blood into the atria.
- As the ventricular pressure increases further, the semilunar valves guarding the pulmonary artery (right side) and the aorta (left side) are forced open, allowing the blood in the ventricles to flow through these vessels into the circulatory pathways.
Stage 4 — ventricular diastole, and back to the start
- 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.
- As the ventricular pressure declines further, the tricuspid and bicuspid valves are pushed open by the pressure in the atria exerted by the blood which was being emptied into them by the veins.
- The blood now once again moves freely to the ventricles. The ventricles and atria are now again in a relaxed (joint diastole) state, as earlier.
- Soon the SAN generates a new action potential and the events described above are repeated in that sequence, and the process continues.
The cardiac cycle. This sequential event in the heart, which is cyclically repeated, is called the cardiac cycle, and it consists of systole and diastole of both the atria and ventricles.
The numbers of the cardiac cycle
As mentioned earlier, the heart beats 72 times per minute, i.e., that many cardiac cycles are performed per minute. From this it could be deduced that the duration of a cardiac cycle is 0.8 seconds.
Stroke volume. During a cardiac cycle, each ventricle pumps out approximately 70 mL of blood, which is called the stroke volume.
Cardiac output. The stroke volume multiplied by the heart rate (no. of beats per min.) gives the cardiac output. Therefore, the cardiac output can be defined as the volume of blood pumped out by each ventricle per minute, and averages 5000 mL or 5 litres in a healthy individual.
Cardiac output = Stroke volume × Heart rate
= 70 mL × 72 min⁻¹ = 5040 mL ≈ 5 litres per minute
= 70 mL × 72 min⁻¹ = 5040 mL ≈ 5 litres per minute
The body has the ability to alter the stroke volume as well as the heart rate, and thereby the cardiac output. For example, the cardiac output of an athlete will be much higher than that of an ordinary man.
The heart sounds
During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope.
• The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves.
• The second heart sound (dub) is associated with the closure of the semilunar valves.
These sounds are of clinical diagnostic significance.
• The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves.
• The second heart sound (dub) is associated with the closure of the semilunar valves.
These sounds are of clinical diagnostic significance.
What the sounds tell you. Notice that both sounds are produced by valves closing, not by muscle contracting or blood rushing. “Lub” therefore marks the beginning of ventricular systole, and “dub” marks its end — so the interval between lub and dub is the period during which blood is being ejected. Their diagnostic value follows directly: a valve that fails to close properly produces an abnormal sound at exactly the moment that valve should have shut, which is how a doctor can locate a defect by listening alone.
15.3.3 Electrocardiogram (ECG)
ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle. The machine that records it is an electro-cardiograph.
To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity. For a detailed evaluation of the heart's function, multiple leads are attached to the chest region.
Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.
| Wave | What it represents | Mechanical event |
|---|---|---|
| P-wave | The electrical excitation (or depolarisation) of the atria | Leads to the contraction of both the atria |
| QRS complex | The depolarisation of the ventricles | Initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole |
| T-wave | The return of the ventricles from excited to normal state (repolarisation) | The end of the T-wave marks the end of systole |
Two uses of the ECG. By counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. And since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence it is of great clinical significance.
15.4 Double Circulation
The three layers of a blood vessel
The blood flows strictly by a fixed route through blood vessels — the arteries and veins. Basically, each artery and vein consists of three layers:
| Layer | Composition |
|---|---|
| Tunica intima (inner) | An inner lining of squamous endothelium |
| Tunica media (middle) | Smooth muscle and elastic fibres — comparatively thin in the veins |
| Tunica externa (outer) | Fibrous connective tissue with collagen fibres |
Why the tunica media is thin in veins. Arteries receive blood directly from the ventricles at high pressure, and must both withstand it and recoil elastically between beats. Veins carry blood at low pressure back towards the heart, so a thick muscular and elastic layer would be wasted. That single structural difference — noted in the chapter as the tunica media is comparatively thin in the veins — is the easiest way to tell an artery from a vein in a section.
The two circuits
Pulmonary circulation. The blood pumped by the right ventricle enters the pulmonary artery. The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs, from where the oxygenated blood is carried by the pulmonary veins into the left atrium.
Systemic circulation. The left ventricle pumps blood into the aorta. The oxygenated blood entering the aorta is carried by a network of arteries, arterioles and capillaries to the tissues, from where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium. The systemic circulation provides nutrients, O₂ and other essential substances to the tissues and takes CO₂ and other harmful substances away for elimination.
Two special vascular arrangements.
• Hepatic portal system: a unique vascular connection exists between the digestive tract and liver. The hepatic portal vein carries blood from intestine to the liver before it is delivered to the systemic circulation.
• Coronary system: a special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.
• Hepatic portal system: a unique vascular connection exists between the digestive tract and liver. The hepatic portal vein carries blood from intestine to the liver before it is delivered to the systemic circulation.
• Coronary system: a special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.
15.5 Regulation of Cardiac Activity
The heart is myogenic. Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.
Regulation from outside can only modify that intrinsic rhythm, never create it:
- A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system (ANS).
- Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output.
- Parasympathetic neural signals (another component of ANS) decrease the rate of heart beat, speed of conduction of action potential and thereby the cardiac output.
- Adrenal medullary hormones can also increase the cardiac output.
| Influence | Effect on heart rate | Effect on cardiac output |
|---|---|---|
| Sympathetic nerves | Increase, and also increase the strength of ventricular contraction | Increases |
| Parasympathetic nerves | Decrease, and also decrease the speed of conduction of action potential | Decreases |
| Adrenal medullary hormones | Increase | Increases |
15.6 Disorders of Circulatory System
High Blood Pressure (Hypertension). Hypertension is the term for blood pressure that is higher than normal (120/80). In this measurement 120 mm Hg is the systolic, or pumping, pressure and 80 mm Hg is the diastolic, or resting, pressure. If repeated checks of blood pressure of an individual is 140/90 or higher, it shows hypertension. High blood pressure leads to heart diseases and also affects vital organs like brain and kidney.
Coronary Artery Disease (CAD). Often referred to as atherosclerosis, it affects the vessels that supply blood to the heart muscle. It is caused by deposits of calcium, fat, cholesterol and fibrous tissues, which make the lumen of arteries narrower.
Angina. Also called ‘angina pectoris’. A symptom of acute chest pain appears when not enough oxygen is reaching the heart muscle. Angina can occur in men and women of any age, but it is more common among the middle-aged and elderly. It occurs due to conditions that affect the blood flow.
Heart Failure. Heart failure means the state of heart when it is not pumping blood effectively enough to meet the needs of the body. It is sometimes called congestive heart failure, because congestion of the lungs is one of the main symptoms of this disease.
Three terms that must not be confused. Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply). Heart failure is a chronic inadequacy of pumping; cardiac arrest is a cessation of beating; a heart attack is sudden damage to the muscle. This distinction is a favourite one-mark question.
📐 Activity 15.4 — Measure pulse rate and work out cardiac output
What to do. Working in pairs, place two fingers on the radial artery at your partner's wrist and count the pulse for one full minute while they sit quietly. Record it. Then have them do two minutes of brisk exercise — stepping or jogging on the spot — and immediately count the pulse again for one minute. Continue counting each minute until the rate returns to its resting value, and note how long that takes.
Now calculate, for both the resting and the exercising state, the cardiac output, taking stroke volume as 70 mL. Also work out the duration of one cardiac cycle in each state.
Predict: which changes more during exercise — heart rate or stroke volume? And which division of the autonomic nervous system produced the change?
Typical readings. Resting pulse about 72 per minute, matching the chapter's statement that our heart normally beats 70–75 times in a minute (average 72 beats min⁻¹). After exercise it may reach 120–150, and it returns to resting value over two to five minutes — faster in a fitter person.
The calculations. At rest, cardiac output = stroke volume × heart rate = 70 × 72 = 5040 mL, which is the chapter's figure of 5000 mL or 5 litres. The duration of a cardiac cycle is 60 ÷ 72 = 0.8 seconds. At a pulse of 140, output rises to 70 × 140 = 9800 mL even without any change in stroke volume, and the cycle shortens to 60 ÷ 140 ≈ 0.43 seconds.
Which factor changes more. Both change, but heart rate is the larger and quicker lever — the chapter notes that the body has the ability to alter the stroke volume as well as the heart rate, and thereby the cardiac output. Stroke volume also rises during exercise, because sympathetic signals increase the strength of ventricular contraction as well as the rate. Taken together they can raise output several-fold, which is why the cardiac output of an athlete will be much higher than that of an ordinary man.
Which nerves did it. The sympathetic division, since neural signals through the sympathetic nerves can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output; adrenal medullary hormones reinforce the same effect. The recovery afterwards is largely the parasympathetic division, which decreases the rate of heart beat, the speed of conduction of action potential and thereby the cardiac output.
One conclusion worth writing down. Note what the exercise does not do: it never starts or stops the beat. The rhythm comes from the nodal tissue, which is autoexcitable, and the nerves only moderate it — which is precisely what is meant by calling the heart myogenic.
🎯 Interactive: Where are you in the cardiac cycle?
Valves and blood: AV valves open, semilunar valves closed
All four chambers are relaxed. As the tricuspid and bicuspid valves are open, blood from the pulmonary veins and vena cava flows into the left and right ventricles through the atria. About 70 per cent of ventricular filling happens in this passive stage.
🎯 Competency-Based Questions
Scenario: A cardiologist studies three patients. Patient X has a resting heart rate of 50 per minute and a stroke volume of 100 mL. Patient Y has a heart rate of 100 per minute but a stroke volume of only 40 mL. Patient Z's ECG shows normal P-waves at a regular rate, but only every second P-wave is followed by a QRS complex.
Q1. Calculate the cardiac output of patients X and Y and comment on who is likely to be an athlete. L3 Apply
Using cardiac output = stroke volume × heart rate: X = 100 × 50 = 5000 mL, that is 5 litres; Y = 40 × 100 = 4000 mL, that is 4 litres.
Patient X is likely to be the athlete. X achieves the normal 5 litres per minute with a heart rate well below the usual 70–75, because each beat ejects a far larger stroke volume than the usual 70 mL — a strong, efficient heart that need not beat often. Patient Y is beating fast yet delivering less than normal output, which suggests a weak ventricle compensating by raising its rate. The example shows why cardiac output, not heart rate alone, is the meaningful measure.
Patient X is likely to be the athlete. X achieves the normal 5 litres per minute with a heart rate well below the usual 70–75, because each beat ejects a far larger stroke volume than the usual 70 mL — a strong, efficient heart that need not beat often. Patient Y is beating fast yet delivering less than normal output, which suggests a weak ventricle compensating by raising its rate. The example shows why cardiac output, not heart rate alone, is the meaningful measure.
Q2. What is happening in patient Z, and where in the heart does the fault lie? L4 Analyse
Normal, regular P-waves mean the electrical excitation of the atria is occurring normally, so the SAN is firing properly and the atria are contracting. But the QRS complex represents the depolarisation of the ventricles, and only half of them are appearing — so the impulse is failing to reach the ventricles on alternate beats. The fault must lie in the conducting route between the atria and the ventricles: the AVN, or the AV bundle which passes through the atrio-ventricular septa. This matters because the atrio-ventricular septum is a thick fibrous tissue and the nodal bundle is the only electrical path across it. The result is that the atria beat at the normal rate but the ventricles at half that rate, so cardiac output falls.
Q3. Fill in the blanks: The duration of a cardiac cycle is ______ seconds. Each ventricle pumps out about ______ mL per cycle, called the ______, and the cardiac output averages ______ litres. The first heart sound is due to closure of the ______ valves and the second of the ______ valves. L1 Remember
0.8; 70; stroke volume; 5 (5000 mL); tricuspid and bicuspid; semilunar.
Q4. A student claims the pulmonary artery must carry oxygenated blood because “arteries always carry oxygenated blood”. Correct the error and explain what actually defines an artery. L4 Analyse
The claim is wrong. The deoxygenated blood pumped into the pulmonary artery is passed on to the lungs, from where the oxygenated blood is carried by the pulmonary veins into the left atrium. So the pulmonary artery carries deoxygenated blood and the pulmonary veins carry oxygenated blood — the only such pair in the body.
What actually defines an artery is the direction of flow relative to the heart: an artery carries blood away from the heart and a vein carries it towards the heart, whatever the blood's oxygen content. The structural correlate follows from pressure, not from oxygen: the tunica media of smooth muscle and elastic fibres is comparatively thin in the veins, because arteries must withstand the pressure generated by ventricular systole. The student has mistaken a statistical generalisation — true of the systemic circulation — for a definition.
What actually defines an artery is the direction of flow relative to the heart: an artery carries blood away from the heart and a vein carries it towards the heart, whatever the blood's oxygen content. The structural correlate follows from pressure, not from oxygen: the tunica media of smooth muscle and elastic fibres is comparatively thin in the veins, because arteries must withstand the pressure generated by ventricular systole. The student has mistaken a statistical generalisation — true of the systemic circulation — for a definition.
Q5. “Since the heart is myogenic, the nervous system has no real role in controlling it.” Evaluate. L5 Evaluate
The premise is correct; the conclusion mistakes initiating a beat for controlling a circulation.
What is right. Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic. The nodal musculature has the ability to generate action potentials without any external stimuli, and the SAN generates the maximum, 70–75 per minute. So the heart needs no nerve to beat at all — a denervated or even an excised heart continues beating, which is what makes transplantation possible.
Why the nervous system nevertheless matters. (i) The body's demand is not constant. An intrinsic rate of 72 is right for rest and hopelessly inadequate for running. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system, and sympathetic nerves increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output — raising output several-fold, which is how the cardiac output of an athlete becomes much higher than that of an ordinary man. (ii) Output must also be lowered. Parasympathetic neural signals decrease the rate of heart beat, the speed of conduction of action potential and thereby the cardiac output, conserving the heart at rest. (iii) Hormones add a third layer: adrenal medullary hormones can also increase the cardiac output. (iv) The neural centre is part of a larger loop — the same medulla receives information from the aortic arch and carotid receptors met in Chapter 14, so cardiac and respiratory adjustments are made together.
The accurate formulation: the rhythm is myogenic, but its rate and force are neurally and hormonally moderated. The design is a good one — a self-starting pump that cannot be switched off by accident, with adjustable output layered on top.
What is right. Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic. The nodal musculature has the ability to generate action potentials without any external stimuli, and the SAN generates the maximum, 70–75 per minute. So the heart needs no nerve to beat at all — a denervated or even an excised heart continues beating, which is what makes transplantation possible.
Why the nervous system nevertheless matters. (i) The body's demand is not constant. An intrinsic rate of 72 is right for rest and hopelessly inadequate for running. A special neural centre in the medulla oblongata can moderate the cardiac function through the autonomic nervous system, and sympathetic nerves increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output — raising output several-fold, which is how the cardiac output of an athlete becomes much higher than that of an ordinary man. (ii) Output must also be lowered. Parasympathetic neural signals decrease the rate of heart beat, the speed of conduction of action potential and thereby the cardiac output, conserving the heart at rest. (iii) Hormones add a third layer: adrenal medullary hormones can also increase the cardiac output. (iv) The neural centre is part of a larger loop — the same medulla receives information from the aortic arch and carotid receptors met in Chapter 14, so cardiac and respiratory adjustments are made together.
The accurate formulation: the rhythm is myogenic, but its rate and force are neurally and hormonally moderated. The design is a good one — a self-starting pump that cannot be switched off by accident, with adjustable output layered on top.
🧠 Assertion–Reason Questions
For each pair choose: (A) Both A and R are true and R is the correct explanation of A. (B) Both A and R are true but R is not the correct explanation of A. (C) A is true but R is false. (D) A is false but R is true.
Assertion (A): The first heart sound, lub, marks the beginning of ventricular systole.
Reason (R): It is associated with the closure of the tricuspid and bicuspid valves, which occurs when rising ventricular pressure attempts to drive blood back into the atria.
Answer: A. Both are true and the reason explains the assertion. The AV valves shut at the very start of ventricular contraction, and the semilunar valves shut at its end, producing the dub.
Assertion (A): The pulmonary artery carries deoxygenated blood.
Reason (R): An artery is defined by carrying blood away from the heart, not by the oxygen content of that blood.
Answer: A. Both are true and the reason is the correct explanation. The right ventricle pumps deoxygenated blood into the pulmonary artery, and the pulmonary veins return oxygenated blood to the left atrium.
Assertion (A): Heart failure and cardiac arrest are the same condition.
Reason (R): Heart failure is sometimes called congestive heart failure, because congestion of the lungs is one of its main symptoms.
Answer: D. The assertion is false — heart failure is the state in which the heart is not pumping blood effectively enough to meet the needs of the body, whereas cardiac arrest is when the heart stops beating, and a heart attack is sudden damage to the heart muscle by an inadequate blood supply. The reason is a true statement about heart failure.
Coming next. Part 5 is the exercise part: the chapter summary followed by full worked solutions to all fourteen NCERT exercise questions of Chapter 15, including the Column I–Column II matching, the four “write the differences” pairs, and the ECG diagram question.
Frequently Asked Questions - Cardiac Cycle, ECG, Double Circulation and Disorders
What is the cardiac cycle?
It is the sequential event in the heart which is cyclically repeated, consisting of systole and diastole of both the atria and the ventricles. It begins with joint diastole, then atrial systole, then ventricular systole, then ventricular diastole, after which the SAN fires again.
How long does one cardiac cycle last?
Since the heart beats 72 times per minute, that many cardiac cycles are performed per minute, so the duration of a cardiac cycle is 0.8 seconds.
What are stroke volume and cardiac output?
Stroke volume is the approximately 70 mL of blood pumped out by each ventricle during one cardiac cycle. Cardiac output is stroke volume multiplied by heart rate - the volume of blood pumped out by each ventricle per minute - and averages 5000 mL or 5 litres in a healthy individual.
How much does atrial systole contribute to ventricular filling?
Atrial systole increases the flow of blood into the ventricles by about 30 per cent. The other 70 per cent has already entered passively during joint diastole, when the tricuspid and bicuspid valves are open.
What causes the two heart sounds?
The first heart sound, lub, is associated with the closure of the tricuspid and bicuspid valves at the start of ventricular systole. The second, dub, is associated with the closure of the semilunar valves at its end. Both are of clinical diagnostic significance.
What do the P wave, QRS complex and T wave of an ECG represent?
The P-wave represents the electrical excitation or depolarisation of the atria, leading to contraction of both atria. The QRS complex represents depolarisation of the ventricles, initiating ventricular contraction, which starts shortly after Q and marks the beginning of systole. The T-wave represents repolarisation of the ventricles, and its end marks the end of systole.
What is double circulation?
The presence of two separate circulatory pathways. In pulmonary circulation the right ventricle pumps deoxygenated blood into the pulmonary artery to the lungs, and oxygenated blood returns by the pulmonary veins to the left atrium. In systemic circulation the left ventricle pumps oxygenated blood into the aorta to the tissues, and deoxygenated blood returns by venules, veins and vena cava to the right atrium.
What are the three layers of an artery or vein?
The tunica intima, an inner lining of squamous endothelium; the tunica media, a middle layer of smooth muscle and elastic fibres; and the tunica externa, an external layer of fibrous connective tissue with collagen fibres. The tunica media is comparatively thin in the veins.
What is the hepatic portal system?
A unique vascular connection between the digestive tract and the liver. The hepatic portal vein carries blood from the intestine to the liver before it is delivered to the systemic circulation.
Why is the heart called myogenic, and how is it regulated externally?
Because its normal activities are auto-regulated by its own specialised nodal tissue rather than by nerves. Externally, a neural centre in the medulla oblongata moderates cardiac function through the autonomic nervous system: sympathetic signals increase heart rate, strength of ventricular contraction and cardiac output, while parasympathetic signals decrease them. Adrenal medullary hormones can also increase cardiac output.
What is the difference between heart failure, cardiac arrest and heart attack?
Heart failure is the state in which the heart is not pumping blood effectively enough to meet the needs of the body, and is sometimes called congestive heart failure because lung congestion is a main symptom. Cardiac arrest is when the heart stops beating. A heart attack is when the heart muscle is suddenly damaged by an inadequate blood supply.
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