Cardiac Cycle

The Cardiac Cycle: The Heartbeat Loop

The Cardiac Cycle is simply one complete heartbeat. It includes the contraction (Systole) and relaxation (Diastole) of both the atria and ventricles.

Duration: The whole cycle takes just 0.8 seconds (assuming a normal heart rate of 72 beats/min).

Key idea: Blood flows because of pressure changes and valves ensure one-way flow.

Step-by-Step Breakdown

A. Joint Diastole (The Relaxed Filling Phase)

• State: All four chambers are relaxed.
• Valves:
  • AV Valves (Tricuspid/Bicuspid): OPEN. (The doors are open so blood can flow in).
  • Semilunar Valves: CLOSED. (Preventing blood from leaking back from arteries).
• Action: Blood flows silently from the veins $\rightarrow$ Atria $\rightarrow$ Ventricles.
• Key Stat: About 70% of the ventricles get filled just by gravity during this phase!

B. Atrial Systole (The "Top-up" Phase) – 0.1 sec

• Trigger: The SAN (Pacemaker) fires an electrical signal.
• Action: Both Atria contract.
• Result: This squeezes the remaining 30% of blood into the ventricles. The ventricles are now full.

C. Ventricular Systole (The Power Pump) – 0.3 sec

• Trigger: The signal travels to the AVN $\rightarrow$ Bundle of His $\rightarrow$ Ventricles.
• Action: The Ventricles contract powerfully.
• The First Sound (LUB): As pressure rises, the AV Valves slam shut to stop blood from going back into the atria. This slamming sound is the "Lub".
• Ejection: The pressure gets so high that it forces the Semilunar Valves OPEN. Blood blasts into the Pulmonary Artery and Aorta.

D. Ventricular Diastole (The Relaxation) – 0.4 sec

• Action: The ventricles relax and pressure drops.
• The Second Sound (DUB): Blood in the arteries tries to rush back in, but catches the Semilunar Valves, snapping them SHUT. This sharp snap is the "Dub".
• Reset: The AV valves open again, and the cycle restarts.

Cardiac Output: The Math of the Heart

How much blood does your heart move?

• Stroke Volume (SV): The amount pumped by one ventricle in one beat.
  • Value: $\approx$ 70 mL.
• Heart Rate (HR): How many times it beats per minute.
  • Value: $\approx$ 72 beats/min.
• Cardiac Output (CO): The total volume pumped per minute.

$$Cardiac \ Output = Stroke \ Volume \times Heart \ Rate$$ $$CO = 70 \ mL \times 72 = 5040 \ mL \approx \mathbf{5 \ Litres/minute}$$

Note: Your body has about 5 liters of blood in total. This means your heart pumps your entire blood volume through your body every single minute!

Cardiac Cycle

• The Sound Logic:
  • Lub (1st sound): Doors closing inside the house (AV Valves).
  • Dub (2nd sound): Doors closing to the outside (Semilunar Valves).
• The 70/30 Rule: 70% of filling is passive (lazy filling); only 30% requires the atria to actually kick (contract).
• 0.8 Seconds Split:
  • 0.1s (Atria squeeze)
  • 0.3s (Ventricles squeeze)
  • 0.4s (Total relaxation/Joint Diastole)
  • Total = 0.8s

💡 Questions and Answers

Q1: Calculate the cardiac output if heart rate is 80 beats/min and stroke volume is 70 mL.

Answer: I will use the formula CO = HR × SV. So, CO = 80 × 70 = 5600 mL/min, which is 5.6 L/min.

Key Points:

• CO = HR × SV
• Convert mL to L: 1000 mL = 1 L

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Q2: Which event produces the “LUB” sound?

Answer: “LUB” happens when the AV valves (tricuspid and bicuspid/mitral) close at the start of ventricular systole.

Key Points:

• LUB = AV valve closure
• Start of ventricular systole

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Q3: What happens to the valves during joint diastole?

Answer: In joint diastole, the heart is relaxed and ventricles are filling. So AV valves are open and semilunar valves are closed.

Key Points:

• Joint diastole: AV open, semilunar closed
• Passive ventricular filling

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Q4: How much time does atrial systole last?

Answer: Atrial systole lasts about 0.1 seconds.

Key Points:

• Atrial systole = 0.1 s

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Q5: Define stroke volume.

Answer: Stroke volume is the amount of blood pumped out by each ventricle in one heartbeat. In a normal adult, it is about 70 mL per beat.

Key Points:

• SV = blood pumped/beat/ventricle
• Approx = 70 mL/beat