Cardiac Cycle

Introduction

The cardiac cycle is the sequential, repetitive events occurring in the heart. It is composed of systole and diastole of both the atria and ventricles. The heart rate denotes the occurrence of the cardiac cycle. 

The series of steps that allow the heart to pump in and pump out blood is known as a cardiac cycle. There are 7 phases of the cardiac cycle covered by its two major phases. These two phases are the muscle relaxation phase or diastole and the muscle contraction phase or systole.

The heart beats seventy-two times in a minute. It means that it states that seventy-two cardiac cycles take place in a minute. If you calculate this rate, the entire cycle becomes roughly 0.8 seconds. Let us learn about the cardiac cycle in detail.

Understanding Cardiac Output

Cardiac output (CO) refers to the volume of blood pumped out by each ventricle per minute by strict definition. A healthy heart’s cardiac output is about 5 to 6 litres of blood each minute in a resting position. It is known as normal cardiac output and is measured in mL/minute. 

Low output can lead to health complications. Similarly, high output heart failure is also a common cause of worry.

Calculating Cardiac Output

Two variables determine cardiac output.

• Heart rate or HR: Cardiac output can be determined using the heart rate, which is essentially the number of heartbeats per minute (beats/min)

• Stroke volume or SV: Stroke volume refers to the amount of blood pumped by each ventricle on each beat and is measured in mL/beat

Thus,

Normal Cardiac Output

The average human heart beats 75 times in a minute, with each beat pumping roughly 70 mL of blood. Thus, we can conclude that average cardiac output for a person is about 5.25 L.

Cardiac Output is About 4 Determinants

Given that stroke volume is a factor that helps determine cardiac output, the following variables are direct factors that help us understand the matter –

1. Heart rate: As already discussed, heart rate is directly proportional to the cardiac output value.

2. Preload: As the name suggests, it is the volume of blood that enters each ventricle during diastole. An increase in input volume directly increases the stroke volume, thus affecting the cardiac output proportionally. 

3. Afterload: The name, in this case, is a little misleading. It does not refer to the blood output. Instead, it relates to the resistance the blood must overcome to circulate throughout the body. 

Thus, the more the afterload, the less is the stroke volume. Cardiac output is therefore inversely proportional to afterload.

4. Contractility: The force with which the myocardium contracts for a given preload is known as contractility. It is directly proportional to cardiac output.

Cardiac Cycle Process Duration

The beating of the heart takes place seventy-two times in a minute. It takes 0.8 seconds for completing a particular cardiac cycle. Below is a breakdown of the exact timings of various parts: 

• The duration of the Atrial Systole is 0.1 seconds

• The duration of the Cardiac Quiescence is 0.4 seconds

• The duration of the Ventricular Diastole is 0.5 seconds

• The duration of the Atrial Diastole is 0.7 seconds

• The duration of the Ventricular Systole is 0.3 seconds

Physiology of the Cardiac Cycle

There are four chambers in the human heart, comprising left and right halves. The left and right atria comprise two upper chambers, and the left and right ventricles form the lower chamber. Newly oxygenated blood is pumped to the body by the left ventricle through the aorta. 

Sequential Phases in the Cardiac Cycle Process

There are two fundamental cardiac cycle phases. These two primary phases cover the more minor 7 phases of the cardiac cycle. These 7 phases are as follows:

The Diastolic Phase

The cardiac cycle is described from late diastole when the atria and ventricles are relaxed, and the atrioventricular valves are open. During this phase, the filling of the ventricles with the blood takes place. 

There are four stages in this phase which are as follows:

• Rapid Ventricular Filling

• Atrial Contraction

• Slow Ventricular Filling

• Isovolumetric Relaxation

The Systolic Phase

Here, the contraction of ventricles happens due to which the ejection of blood takes place into the aorta and the pulmonary artery. This particular has the following stages:

• Rapid Ejection

• Reduced Ejection

• Isovolumetric Contraction

Effect of the Autonomic Nervous System on the Heart and the Cardiac Cycle

• Due to this recoiling of the aorta, the blood rushes forward into the systemic circulation. This phenomenon occurs throughout the arterial system to maintain the diastolic pressure

• If the blood pressure becomes too high, it can damage our blood vessels. However, if it becomes too low, the blood may be inadequate for flowing through the tissues

Stroke Volume

Stroke Volume is the volume of blood ejected from the LV per heartbeat. 

The blood volume in the LV before contraction is the left ventricular end-diastolic volume (LVEDV). The volume of blood remaining in the LV after contraction is the left ventricular end-systolic volume (LVESV). 

Stroke volume will normally be in the range of 70–80 ml

Stroke volume = EDV – ESV 

Conclusion

The heart’s primary purpose is to pump blood throughout the body; it does so in a repeating sequence called the cardiac cycle. 

The cardiac cycle is the coordination of the filling and emptying of the heart of blood by electrical signals. It causes the heart muscles to contract and relax. It works on the heartbeat from its production to the commencement of the next beat. 

One can explain the cardiac output using the statement the volume. The SAN will generate an action potential that stimulates both the atria to undergo a contraction at the same time, called atrial systole.