Cardiac Cycle NEET 2027 PDF Notes | Human Physiology | Deus Learnings

Cardiac Cycle NEET 2027 PDF Notes

Master Human Physiology for NEET UG with high-yield notes, ECG correlations, and interactive practice MCQs.

Introduction

The human heart is myogenic, meaning its rhythm is generated by specialized muscle tissue rather than nerve impulses.

1. The Cardiac Impulse (Electrical Pathway)

Before any muscle contracts, an electrical signal must tell it to do so. This signal travels through a specific pathway of nodal tissue:

Sino-atrial Node (SAN): Located in the right upper corner of the right atrium. It acts as the pacemaker, generating the highest number of action potentials (70-75/min) and initiating the cycle.
Atrio-ventricular Node (AVN): Located in the lower left corner of the right atrium. It receives the impulse from the SAN.
Bundle of His (AV Bundle): Continues from the AVN, passes through the atrio-ventricular septa, and divides into right and left bundles.
Purkinje Fibers: Minute fibers extending from the bundle branches throughout the ventricular musculature.

Remember: The spread of this electrical impulse is what an Electrocardiogram (ECG) measures.

2. The Cardiac Cycle (Mechanical Events)

The cardiac cycle is the sequence of electrical and mechanical events occurring from the beginning of one heartbeat to the beginning of the next. Assuming an average heart rate of 72 beats per minute, one cycle lasts 0.8 seconds.

Phase 1: Joint Diastole (Duration: ~0.4s)

Electrical state: The heart is electrically quiet (between the T wave and the next P wave on an ECG).
Mechanical state: All four chambers are in a relaxed state (diastole).
Blood flow: Blood from the pulmonary veins and vena cava flows into the left and right atria, respectively. Because the pressure in the atria is higher than in the relaxed ventricles, the bicuspid and tricuspid (AV) valves are open. Blood flows passively into the ventricles (about 70% of ventricular filling happens here).
Valves: AV valves = Open | Semilunar valves = Closed.

Phase 2: Atrial Systole (Duration: ~0.1s)

Electrical state: The SAN generates an action potential (represented by the P-wave on an ECG, indicating atrial depolarization).
Mechanical state: Both atria contract simultaneously.
Blood flow: This contraction pumps the remaining 30% of blood into the ventricles.
Valves: AV valves = Open | Semilunar valves = Closed.

Phase 3: Ventricular Systole (Duration: ~0.3s)

Electrical state: The action potential is conducted to the ventricular side by the AVN and AV bundle, then transmitted through the Purkinje fibers to the entire ventricular musculature. (This is the QRS complex on an ECG, indicating ventricular depolarization).
Mechanical state: Ventricles contract. Simultaneously, the atria undergo relaxation (atrial diastole).
Blood flow & Valves:
- As ventricular pressure rises, the AV valves snap shut to prevent backflow into the atria. This closure produces the first heart sound (LUB).
- As pressure continues to build and exceeds the pressure in the pulmonary artery and aorta, the Semilunar valves are forced open. Blood is pumped into the systemic and pulmonary circulation.

Phase 4: Ventricular Diastole (Part of the next Joint Diastole)

Electrical state: The ventricles return to their resting electrical state (represented by the T-wave on an ECG, indicating ventricular repolarization).
Mechanical state: Ventricular muscles relax.
Blood flow & Valves:
- As ventricles relax, ventricular pressure falls. Blood in the aorta and pulmonary artery tries to flow backward, causing the Semilunar valves to snap shut. This produces the second heart sound (DUP).
- When ventricular pressure drops below the atrial pressure, the AV valves are pushed open again by the blood pooling in the atria. The heart is now back in Joint Diastole, and the cycle repeats.

Interactive Multiple Choice Questions (NEET Practice)

Click on an option to check your answer instantly.

1. The pacemaker of the human heart is:

2. The duration of a normal cardiac cycle is approximately:

3. During joint diastole, which of the following valves are open?

4. Atrial systole increases the flow of blood into the ventricles by about:

5. The first heart sound (LUB) is associated with the closure of:

6. The volume of blood pumped out by each ventricle during a cardiac cycle is called:

7. The normal stroke volume in a healthy adult is approximately:

8. Which wave on a standard ECG represents the depolarization of the atria?

9. The QRS complex in a standard ECG represents:

10. The end of the T-wave in an ECG marks the end of:

11. The bundle of His is a network of:

12. The second heart sound (DUP) is heard during:

13. The normal action potential rate generated by the SA node is:

14. Cardiac output is defined as:

15. What is the approximate cardiac output of a normal healthy individual?

16. Which of the following is responsible for initiating and maintaining the rhythmic contractile activity of the heart?

17. The AV node is located in the:

18. During ventricular systole:

19. Why is the human heart termed 'myogenic'?

20. When the ventricular pressure exceeds the pressure in the pulmonary artery and aorta, what happens?

21. The duration of ventricular systole in a normal cardiac cycle is:

22. Read the following statements:
Statement I: The SAN can generate the maximum number of action potentials.
Statement II: The SAN is responsible for initiating and maintaining the rhythmic contractile activity of the heart.

23. During which phase is the heart completely electrically quiet, with no depolarization occurring?

24. A patient's ECG shows an abnormally enlarged QRS complex. This typically indicates:

25. The delay of the cardiac impulse at the AV node is crucial because it:

Answer Key

(1) SA node
(1) 0.8 seconds
(1) Tricuspid and bicuspid valves
(2) 30%
(1) Tricuspid and bicuspid valves
(3) Stroke volume
(2) 70 mL
(1) P-wave
(2) Depolarization of ventricles
(3) Ventricular systole
(4) Muscle fibers found only in the ventricle wall
(2) End of ventricular systole / beginning of ventricular diastole
(3) 70-75 min⁻¹
(2) Stroke volume × Heart rate
(2) 5 liters/min
(1) Sino-atrial node (SAN)
(2) Lower left corner of the right atrium
(4) Ventricular pressure causes the closure of AV valves
(2) The heart beat originates from a muscle (nodal tissue)
(2) Semilunar valves open
(2) 0.3 s
(1) Both statements I and II are correct
(2) Between the T-wave and the next P-wave
(3) Enlargement of ventricles
(2) Allows the atria to completely empty their blood into the ventricles before the ventricles contract

Cardiac Cycle