CUET UG Biology: Animal Physiology - Circulation, Heart, Blood Vessels, Blood Pressure, ECG

Must-Know (15–20 detailed bullets)

• The human heart is myogenic, meaning it generates its own electrical impulses without neural input; initiated by the sinoatrial (SA) node.
• The SA node is located in the right atrium and acts as the natural pacemaker, generating impulses at 70–75 times per minute under normal conditions.
• The atrioventricular (AV) node is located in the lower part of the right atrium near the interatrial septum and delays the impulse by 0.1 seconds to allow atrial emptying.
• The Bundle of His conducts impulses from the AV node to the ventricles and divides into left and right bundle branches.
• Purkinje fibers distribute the cardiac impulse to ventricular muscle cells, ensuring coordinated contraction.
• The sequence of impulse conduction is: SA node-AV node-Bundle of His-Bundle branches-Purkinje fibers.
• Normal cardiac output is about 5 liters per minute in a resting adult; calculated as stroke volume × heart rate (70 mL × 72 bpm-5 L/min).
• Stroke volume is the volume of blood pumped out by each ventricle per beat; average is 70 mL.
• The lub sound (S1) is produced by the closure of the atrioventricular (tricuspid and bicuspid) valves at the beginning of systole.
• The dub sound (S2) results from the closure of semilunar (aortic and pulmonary) valves at the start of ventricular diastole.
• Systolic blood pressure is the pressure in arteries during ventricular contraction; average normal value is 120 mm Hg.
• Diastolic blood pressure is the pressure during ventricular relaxation; average normal value is 80 mm Hg.
• Hypertension is diagnosed when blood pressure is consistently above 140/90 mm Hg.
• Blood pressure is measured using a sphygmomanometer and reported as systolic/diastolic in mm Hg.
• The ECG (electrocardiogram) records electrical activity of the heart; one cardiac cycle produces a P wave, QRS complex, and T wave.
• The P wave represents atrial depolarization; lasts 0.08–0.11 seconds.
• The QRS complex indicates ventricular depolarization; normal duration is 0.06–0.10 seconds.
• The T wave reflects ventricular repolarization; it is upright in most leads.
• The PR interval (from start of P to start of QRS) measures AV conduction time; normal is 0.12–0.20 seconds.
• The ST segment lies between the end of QRS and start of T wave; elevation or depression may indicate myocardial infarction or ischemia.

Difficulty Level

Intermediate — Requires understanding of sequence, numerical values, and integration of structure-function relationships in the cardiac cycle and ECG.

Common CUET Traps

• Trap: Students confuse the origin of heart sounds; think "lub" is due to semilunar valve closure.
  Avoid: "Lub" (S1) is AV valve closure; "dub" (S2) is semilunar valve closure.

• Trap: Misinterpreting ECG waves — assuming T wave is atrial repolarization.
  Avoid: T wave is ventricular repolarization; atrial repolarization occurs during QRS complex and is masked.

• Trap: Believing the heart requires nervous stimulation to beat.
  Avoid: Heart is myogenic; SA node initiates impulse without external nerves, though ANS can modulate rate.

Practice MCQs

Q1. Which part of the human heart is responsible for initiating the cardiac impulse?
A. AV node
B. Bundle of His
C. SA node
D. Purkinje fibers
Answer: C
Explanation: The SA node, located in the right atrium, is the natural pacemaker that initiates the cardiac impulse.
Why others fail: AV node delays the impulse but does not initiate it; common confusion due to proximity in conduction pathway.

Q2. What does the QRS complex in a standard ECG represent?
A. Atrial depolarization
B. Ventricular depolarization
C. Atrial repolarization
D. Ventricular repolarization
Answer: B
Explanation: The QRS complex corresponds to the depolarization of the ventricles, leading to their contraction.
Why others fail: Atrial depolarization is represented by P wave; QRS is large and prominent, so students may misattribute it to atria.

Q3. In a healthy adult, what is the average stroke volume?
A. 50 mL
B. 70 mL
C. 90 mL
D. 120 mL
Answer: B
Explanation: Average stroke volume in a resting adult is approximately 70 mL per beat.
Why others fail: Option D (120 mL) is close to systolic pressure value (120 mm Hg), leading to confusion between pressure and volume.

Q4. Which of the following correctly describes the sequence of impulse conduction in the heart?
A. SA node-Purkinje fibers-AV node-Bundle of His
B. AV node-SA node-Bundle of His-Purkinje fibers
C. SA node-AV node-Bundle of His-Purkinje fibers
D. Purkinje fibers-Bundle of His-AV node-SA node
Answer: C
Explanation: The correct sequence is SA node (pacemaker)-AV node (delay)-Bundle of His-Purkinje fibers (ventricular spread).
Why others fail: Reversed sequences are tempting due to rote memorization errors; SA node must come first.

Q5. A patient’s ECG shows a prolonged PR interval of 0.24 seconds. What does this most likely indicate?
A. Myocardial infarction
B. Ventricular hypertrophy
C. First-degree AV block
D. Atrial fibrillation
Answer: C
Explanation: A PR interval > 0.20 seconds indicates delayed conduction through the AV node, characteristic of first-degree AV block.
Why others fail: ST elevation is linked to myocardial infarction, so students may jump to that without analyzing interval duration.

Last?Minute Revision (15–20 one?liners)

• SA node is in the right atrium — natural pacemaker.
• AV node delay = 0.1 sec — allows atrial emptying.
• Cardiac output = Stroke volume × Heart rate (avg 5 L/min).
• Stroke volume = 70 mL/beat — standard value.
• “Lub” = AV valves close; “Dub” = semilunar valves close.
• Systolic BP = 120 mm Hg; Diastolic = 80 mm Hg — normal.
• Hypertension = >140/90 mm Hg — diagnostic threshold.
• ECG: P wave = atrial depolarization.
• QRS complex = ventricular depolarization.
• T wave = ventricular repolarization — not atrial.
• PR interval = 0.12–0.20 sec — AV conduction time.
• QRS duration = 0.06–0.10 sec — ventricular activation.
• ST segment — elevated in MI, depressed in ischemia.
• Heart is myogenic — self-excitable, no nerve needed.
• Purkinje fibers — fastest conduction in heart (4 m/s).
• Blood pressure measured in mm Hg — sphygmomanometer.
• SA node rate: 70–75 bpm — sets rhythm.
• Atrial repolarization — hidden in QRS complex.
• Bundle of His — only electrical link between atria and ventricles.
• Mnemonic: “P for Pump (atria), QRS for Quick Run Squeeze (ventricles), T for Take it easy (repolarization).”