Cardiovascular | Cardiac Cycle

Name: Cardiovascular | Cardiac Cycle
Uploaded: 2017-02-23T17:58:02.000Z
Duration: 46 min 59 s

What is the Cardiac Cycle?

Overview of the Cardiac Cycle

The cardiac cycle encompasses all mechanical events involved in blood flow through the heart's chambers, averaging about 0.8 seconds per cycle.

Key components to be discussed include atrial vs. ventricular pressure, arterial vs. ventricular pressure, and the behavior of AV (atrioventricular) and semilunar valves during this cycle.

Phases of Ventricular Diastole

The first event is mid to late ventricular diastole, defined as relaxation of the ventricles where blood returns to the heart from various sources including systemic and pulmonary veins.

Blood accumulates in the atria, leading to increased atrial pressure which opens the AV valves (tricuspid and bicuspid/mitral). This allows passive blood flow into the ventricles without contraction.

Atrial Pressure Dynamics

During this phase, atrial pressure exceeds ventricular pressure, causing AV valves to open and allowing approximately 70-80% of incoming blood to flow passively into the ventricles.

Despite accumulating blood in the ventricles, they do not contract yet; thus, pressures in pulmonary arteries and aorta remain higher than those in ventricles keeping semilunar valves closed.

Semilunar Valve Functionality

The arterial pressures (in aorta and pulmonary trunk) are greater than ventricular pressures preventing semilunar valve opening; these valves remain closed until sufficient pressure builds up in ventricles for ejection of blood.

EKG Correlation with Cardiac Events

Understanding the Phases of Cardiac Cycle

Mid to Late Ventricular Diastole

The period defined as mid to late ventricular diastole is crucial for understanding heart function, specifically during ventricular filling.

Blood accumulates in the ventricles, leading to a state known as end diastolic volume (EDV), which will be discussed further in relation to cardiac output.

Isovolumetric Contraction Phase

As the ventricles begin to depolarize and contract, this phase is termed isovolumetric contraction or isovolumetric systole.

During this phase, despite the ventricles contracting, blood does not leave because the pressure within them remains lower than that in the aorta and pulmonary trunk.

Pressure Dynamics

The pressure in the aorta averages around 80 mmHg while that in the pulmonary trunk is about 10 mmHg; ventricular pressure must exceed these for blood flow.

At this stage, ventricular pressures are approximately 60 mmHg, insufficient to open semilunar valves; thus, they remain closed.

Valve Functionality and Heart Sounds

As blood continues to rise due to ventricular contraction, it pushes against closed semilunar valves without allowing any ejection of blood.

This moment of no blood leaving results in a characteristic sound when atrioventricular (AV) valves snap shut—known as S1 or "Lub," marking an important auditory cue during auscultation.

Transitioning Pressures

Understanding Ventricular Pressure Dynamics

The Basics of Ventricular and Arterial Pressure

The pressure within the ventricles is expected to reach around 25 mmHg, while arterial pressure remains significantly lower at approximately 10 mmHg.

As the ventricles contract, their pressure increases, leading to the opening of semilunar valves which allows blood to flow out into the arteries.

During this phase, both right and left ventricular pressures exceed that of the pulmonary trunk and aorta, causing these valves to open.

Atrioventricular Valves and Blood Flow

Despite blood being ejected from the ventricles, atrioventricular (AV) valves remain closed due to higher ventricular pressure compared to atrial pressure.

The atrial pressure is nearly zero mmHg during this phase, reinforcing why AV valves stay closed as ventricular pressures are greater.

Phases of Cardiac Cycle: Ejection Phase

This segment is characterized by blood being ejected from the ventricles; it can be referred to as mid to late ventricular systole or simply ventricular ejection.

The ongoing contraction of ventricles continues throughout this phase, indicated by consistent QRS complex on an EKG.

Transitioning Between Phases

After blood has been ejected from the ventricles, we transition into another phase where residual blood volume remains known as end-systolic volume (ESV).

Following ejection, some blood enters systemic circulation while other portions go towards pulmonary and coronary circuits.

Pressure Changes in Circulation

Post-ejection, a significant rise in arterial pressures occurs due to accumulated blood in both aorta and pulmonary trunk.

Understanding Cardiac Cycle Dynamics

Blood Flow and Valve Function

The aortic semilunar valve prevents backflow of blood into the ventricles, allowing blood to flow into systemic circulation or towards the head and upper limbs.

As pressure in the aorta and pulmonary trunk rises above ventricular pressure, it causes the semilunar valves to close, resulting in a brief increase in aortic pressure known as the dicrotic notch.

The relationship between arterial pressure and ventricular pressure is crucial; arterial pressure must exceed ventricular pressure for proper valve function during this phase.

During this time, atrial pressure remains low (zero), while ventricular pressure continues to decrease but stays higher than atrial pressure, keeping AV valves closed.

Both AV and semilunar valves are closed momentarily, leading to distinct heart sounds: "lub" from AV closure and "dub" from semilunar closure.

Phases of Ventricular Relaxation

The iso-volumetric relaxation phase begins as ventricles relax; coronary arteries fill with blood supplying oxygen needed for ATP production in muscle cells.