The Cardiac Cycle: Systole, Diastole, Pressure, and Heart Valves

Cardiac Cycle: Systole, Diastole, Pressure, & Valves

Overview of the Cardiac Cycle

The cardiac cycle is the sequence of events that occurs as the heart contracts and relaxes to pump blood. It consists of two main phases: systole (contraction) and diastole (relaxation). These phases coordinate the movement of blood through the heart and the opening and closing of heart valves.

• Systole: Period of ventricular contraction, causing blood to be ejected from the heart. Key event: Pressure in the ventricles rises.

• Diastole: Period of ventricular relaxation, allowing the chambers to fill with blood. Key event: Pressure in the ventricles drops.

Pressure changes in the heart chambers drive the opening and closing of the atrioventricular (AV) and semilunar (SL) valves.

Heart Valves and Pressure Relationships

• Atrioventricular (AV) valves: Separate the atria from the ventricles (tricuspid on the right, mitral/bicuspid on the left).

• Semilunar (SL) valves: Separate the ventricles from the arteries (pulmonary and aortic valves).

• Valves open and close in response to pressure differences between chambers and vessels.

Valve Status During Systole and Diastole

• Ventricular Systole:

  • AV valves: closed

  • Semilunar valves: open

  • Ventricular pressure: rising

• Ventricular Diastole:

  • AV valves: open

  • Semilunar valves: closed

  • Ventricular pressure: falling

Table: Valve Status and Pressure Relationships

Relative Pressure	Valve Affected	Is Valve Open or Closed?	Ventricle in Systole or Diastole?
Aorta > Left Ventricle	LSL/Aortic	closed	diastole
Right atrium > Right ventricle	R. AV valve/tricuspid	open	diastole
Left atrium > Left ventricle	L. AV valve/mitral	open	diastole

Events in the Cardiac Cycle

Main Phases

The cardiac cycle is divided into four main events, based on whether the ventricles are in systole or diastole and whether blood is moving:

1. Ventricular Filling: Blood flows from atria to ventricles. Ventricles: diastole. AV valves: open. SL valves: closed.

2. Isovolumetric Contraction: Ventricles contract, but all valves are closed, so no blood moves. Ventricles: systole. AV valves: closed. SL valves: closed.

3. Ventricular Ejection: Ventricular pressure exceeds arterial pressure, semilunar valves open, and blood is ejected. Ventricles: systole. AV valves: closed. SL valves: open.

4. Isovolumetric Relaxation: Ventricles relax, all valves are closed, and no blood moves. Ventricles: diastole. AV valves: closed. SL valves: closed.

Summary Table: Cardiac Cycle Phases

Phase	Ventricular State	AV Valves	SL Valves	Blood Movement
Ventricular Filling	Diastole	Open	Closed	Blood flows into ventricles
Isovolumetric Contraction	Systole	Closed	Closed	No movement
Ventricular Ejection	Systole	Closed	Open	Blood ejected from ventricles
Isovolumetric Relaxation	Diastole	Closed	Closed	No movement

Heart Sounds in the Cardiac Cycle

Origin of Heart Sounds

• Heart sounds are produced by the closing of heart valves.

• There are two main heart sounds:

  • 1st sound ("Lub"): Closure of AV valves (start of systole).

  • 2nd sound ("Dub"): Closure of semilunar valves (end of systole/start of diastole).

• Abnormal heart sounds (murmurs) may indicate turbulent blood flow or valve dysfunction.

Heart Sound Table by Cardiac Cycle Phase

Phase	Heart Sound
Ventricular Filling	none
Isovolumetric Contraction	S1 ("Lub")
Ventricular Ejection	none
Isovolumetric Relaxation	S2 ("Dub")

Pressure-Volume Relationships and Valve Function

Key Principles

• Valves open when pressure behind them exceeds pressure ahead.

• AV valves open when atrial pressure > ventricular pressure; close when ventricular pressure > atrial pressure.

• Semilunar valves open when ventricular pressure > arterial pressure; close when arterial pressure > ventricular pressure.

Equations

• Pressure-Volume Relationship: $P = \frac{F}{A}$ (Pressure = Force / Area)

• Cardiac Output: $CO = HR \times SV$ (Cardiac Output = Heart Rate × Stroke Volume)

Practice and Application

• When ventricular pressure causes the AV valve to close and the semilunar valve to open, the ventricles are in systole and the pressure in the left ventricle is greater than in the aorta.

• For AV valves to be open, ventricular pressure must be less than atrial pressure.

• Isovolumetric contraction immediately follows AV valve closure and marks the onset of ventricular systole.

• Blood volume in the ventricles is constant during isovolumetric contraction and isovolumetric relaxation (all valves closed).

Summary Diagram and Integration

Cardiac cycle events can be visualized on a pressure-volume or pressure-time graph, showing the timing of valve openings/closings, heart sounds, and pressure changes in the atria, ventricles, and arteries.

Key Points for Exam Preparation

• Know the sequence of valve openings and closings during the cardiac cycle.

• Understand the relationship between pressure changes and valve function.

• Be able to identify phases of the cardiac cycle on a diagram or graph.

• Recognize the origin and timing of heart sounds (S1 and S2).

• Apply knowledge to clinical scenarios involving heart sounds and valve dysfunction.

Additional info: The notes above expand on the provided diagrams and tables, adding definitions, context, and equations relevant to the cardiac cycle for Anatomy & Physiology students.