# The Cardiac Cycle ## What is the Cardiac Cycle?

The cardiac cycle is the sequence of events that occur during one complete heartbeat, including contraction and relaxation of heart muscle

- The cardiac cycle involves the coordinated contraction and relaxation of the heart chambers - **Systole** = contraction of heart muscle - **Diastole** = relaxation of heart muscle - One cardiac cycle follows another in a continuous, uninterrupted process - Blood flow through the heart never stops between cycles - There is no pause or gap where the heart is completely still ## Volume and Pressure Changes - Changes in volume within the heart chambers drive the cardiac cycle - When heart muscle **contracts**, chamber volume **decreases** - When heart muscle **relaxes**, chamber volume **increases** - Volume changes create corresponding pressure changes in the chambers

Decreasing volume causes increasing pressure; increasing volume causes decreasing pressure

- These pressure changes control the opening and closing of valves - Valves open when pressure behind them exceeds pressure in front - Valves close when pressure in front exceeds pressure behind ## Events of the Cardiac Cycle ### Atrial Systole - The **atria contract**, starting the cardiac cycle - Atrial volume **decreases** - Atrial pressure **increases** above ventricular pressure - The pressure difference forces the **atrioventricular (AV) valves to open** - Blood is **pushed from the atria into the ventricles** - Ventricular pressure rises slightly as blood enters - Ventricular volume increases as chambers fill - During atrial systole, the ventricles are relaxed - Ventricular diastole occurs at the same time as atrial systole ### Ventricular Systole - The **ventricles contract** powerfully - Ventricular volume **decreases** rapidly - Ventricular pressure **increases** dramatically - Pressure in ventricles rises above atrial pressure - This **forces the AV valves to close** - Prevents backflow of blood into the atria - Ventricular pressure continues rising above aorta/pulmonary artery pressure - This **forces the semilunar (SL) valves to open** - Blood is ejected from ventricles into arteries - Meanwhile, the atria are relaxing (atrial diastole) - Blood continues flowing into the atria from veins - The relaxed atria begin filling with blood again ### Diastole - Both **ventricles and atria are relaxed** during this phase - Ventricular pressure **drops below** arterial pressure - This causes the **SL valves to close** - Prevents backflow from arteries into ventricles - Blood continues returning to the heart - Vena cava delivers blood to right atrium - Pulmonary vein delivers blood to left atrium - Atrial pressure rises as the atria fill with blood - When atrial pressure exceeds ventricular pressure, the **AV valves open** - Blood flows **passively** from atria into ventricles - No atrial contraction is needed for this initial filling - Think of it like water flowing downhill due to gravity - The ventricles continue filling until the next atrial systole begins the cycle again ## Valve Movements During the Cardiac Cycle | Stage in Cardiac Cycle | Atrioventricular Valves | Semilunar Valves | |------------------------|-------------------------|------------------| | Atrial systole | Open | Closed | | Ventricular systole | Closed | Open | | Diastole | Open | Closed | - Valves respond automatically to pressure changes - No active control is needed - They function like one-way doors responding to pressure on either side ## Interpreting Cardiac Cycle Graphs - Graphs show pressure changes in different heart chambers and blood vessels over time - The graph below shows pressure changes in the left atrium, left ventricle, and aorta during one complete cardiac cycle:

- Key features to identify on a typical cardiac cycle graph: ### Point A: End of Diastole - Atrium has filled with blood during the previous diastole - Pressure higher in atrium than ventricle - AV valve is open ### Point A-B: Atrial Systole - Left atrium contracts - Atrial pressure increases, forcing blood into ventricle - Ventricular pressure rises slightly as it fills - Pressure remains higher in atrium than ventricle - AV valve stays open ### Point B: Beginning of Ventricular Systole - Left ventricle begins to contract - Ventricular pressure rises rapidly - Atrial pressure drops as atrium relaxes - Ventricular pressure exceeds atrial pressure - **AV valve closes** ### Point C: Ventricular Systole - Ventricle continues contracting - Ventricular pressure exceeds aortic pressure - **Aortic valve opens** - Blood is forced into the aorta ### Point D: Beginning of Diastole - Ventricle has emptied most of its blood - Ventricular muscle relaxes - Ventricular pressure falls below aortic pressure - **Aortic valve closes** ### Point D-E: Early Diastole - Ventricle remains relaxed - Ventricular pressure continues decreasing - Blood flows into relaxed atrium from pulmonary vein - Atrial pressure increases ### Point E: Diastole - Relaxed atrium fills with blood - Atrial pressure exceeds ventricular pressure - **AV valve opens** ### After Point E: Late Diastole - Ventricle expands as muscles relax - Internal volume of ventricle increases - Ventricular pressure decreases further - Blood flows slowly through open AV valve into ventricle - Both atrial and ventricular pressures increase slowly as they fill

The right side of the heart appears on the left when viewing diagrams, as if looking at someone facing you

## Calculating Heart Rate from Graphs - Heart rate can be determined from the duration of one cardiac cycle - The time for one complete cycle represents one heartbeat **Example calculation:** If one cardiac cycle takes 0.7 seconds: 1. Calculate beats per second: - $1 \div 0.7 = 1.43$ beats per second 2. Convert to beats per minute: - $1.43 \times 60 = 85.8$ beats per minute

Formula: Heart rate (bpm) = $\frac{60}{\text{time for one cycle (seconds)}}$

## Understanding Pressure-Volume Relationships - The relationship between pressure and volume is inverse - Like squeezing a balloon: reducing volume increases internal pressure - Like releasing a balloon: increasing volume decreases internal pressure - In the heart: - Muscle contraction = reduced chamber volume = increased pressure - Muscle relaxation = increased chamber volume = decreased pressure - These pressure gradients drive blood flow in one direction through the heart - Blood always flows from high pressure to low pressure - Valves ensure flow only occurs in the correct direction ## Key Points About the Cardiac Cycle - The cycle is continuous and uninterrupted in a healthy heart - Pressure differences, not active pumping, move blood through valves - The septum keeps left and right sides completely separated - Oxygenated and deoxygenated blood never mix - Timing is crucial: - Atria contract before ventricles to ensure complete filling - Ventricles must relax before atria contract again - The left ventricle generates much higher pressures than the right - Left side pumps blood around entire body - Right side only pumps blood to nearby lungs - This explains why the left ventricle wall is much thicker