Higher animals possess an elaborated circulatory system that consists of a muscular and chambered heart, a network of blood vessels, and an extracellular fluid called the blood.  The heart pumps the blood to the body organs. The mammalian heart comprises four chambers, i.e., two atria and two ventricles. The blood flows through the cardiac (heart) chambers in a specific direction due to the rhythmic contraction and relaxation of the heart chambers.

This sequential flow of blood between the respective atria and ventricles and their respective blood vessels is repeated with every heartbeat and is called the cardiac cycle. Let’s read the article to study the sequence of events in a cardiac cycle and know about the interrelation of blood pressure and heartbeat rate.

Definition of Cardiac Cycle

The sequence of events that occur from the beginning of one heartbeat to the beginning of the next heartbeat (completion of one heartbeat) is called the cardiac cycle. The cycle involves systole or contraction and diastole or relaxation of atria and ventricles.

Phases of Cardiac Cycle

The different phases of the cardiac cycle with respect to the configuration of valves and the pressure built in the heart chambers can be tabulated as follows:

Stages	Atrioventricular valves (AV valves)	Semilunar valves (SL valves)	Atrial & Ventricular systole or diastole with respect to pressure
Maximum Ejection Phase	Closed	Open	Pressure in ventricles increases and 70% of the stroke volume of the blood is pumped out from the ventricle into the aorta.
Reduced Ejection Phase	Closed	Open	The remaining 30% of the stroke volume is pumped out into the aorta during this phase and leads to a gradual decrease in intraventricular pressure.
Protodiastolic Phase	Open	Closed	It is the time interval between the end of ventricular systole and the closure of the aortic SL valves. A fall in the intraventricular pressure below the aortic pressure allows the backflow of blood into ventricles that are prevented by the sudden closure of aortic valves.
Isovolumetric Relaxation Phase	Closed	Closed	The blood returning to the heart keeps getting accumulated in the atria and leads to a slow rise in the atrial pressure. The pressure continues to increase until the AV valves open, which occur at the end of isovolumetric ventricular relaxation.
Initial Rapid Filling Phase	Open	Closed	It is also called the ventricular filling phase. About 70% of ventricular filling occurs during this phase due to the pressure gradient between the atrium and ventricle, and there is no active contraction of the atrial muscle involved.
Rapid Filling Phase	Open	Closed	The pressure gradient gradually falls after the ventricular filling phase, and the blood flow stops. The remaining about 30% of the blood is pumped due to the active contraction of the atrial muscle.

Physiology of Cardiac Cycle

The complete cardiac cycle is comprised of the following steps that take place in a sequential manner:

1. Atrial (Auricular) Systole

I. Under normal conditions, blood continuously enters the right atrium through the inferior and superior vena cava.
II. The blood enters the left atrium from the lungs through pulmonary veins.
III. From the atria, nearly 70% of the blood passively flows into their respective ventricles.
IV. During atrial systole, the wall of atria contract and exert the pressure on remaining 30% blood and pushing it to the respective ventricles. The wave of contraction is stimulated by the action potential generated by the Sino-auricular (SA) node.
V. The atrioventricular valves facilitate the flow of blood from the atria to the ventricles.
VI. On the other hand, the closure of semilunar valves restricts the flow of blood from the ventricles to the respective blood vessels during atrial systole.

2.Atrial (Auricular) Diastole

I. The completion of atrial systole is further followed by atrial diastole.
II. The blood enters the left and right atria from the large veins coming from the lungs and other body organs.
III. The relaxation of the atria and contraction of ventricles occur simultaneously during this phase.

3. Ventricular Systole

I. The ventricles contract in this phase, which results in the rise of ventricular pressure that leads to the closure of atrioventricular valves to prevent the backflow of the blood into their respective atria.
II. The conduction of action potential to the ventricular side occurs by AV node and AV bundles from where the bundle of His transmits the action potential to the entire ventricular musculature. The contraction of ventricles thereby increases the ventricular pressure. The contraction of the ventricle is so strong that the ventricular pressure exceeds the aortic pressure. As a result, the semilunar valves open.
III. Further, the blood is pumped to the lungs through the pulmonary artery and other body organs through the main aorta.

4. Ventricular Diastole

I. The ventricular systole is followed by ventricular diastole. In this phase, the ventricular pressure decreases.
II. The semilunar valves are now closed to prevent the backflow from the pulmonary artery and aorta.
III. At the same time, the auriculo-ventricular valve opens to allow the flow of blood from the atria to their respective ventricles.

5. Joint Diastole

It is the phase when all the heart chambers are in a relaxed state. The SAN (sinoauricular node) again generates a new action potential after the completion of ventricular diastole, and the events described above are sequentially repeated to proceed with another cardiac cycle.

Fig: Phases of Cardiac Cycle

Cardiac Cycle & Heart Sounds

The two main heart sounds are produced during each cardiac cycle. These prominent sounds are repeated rhythmically and can be heard with the help of a stethoscope. These sounds result from the closure of the heart valves at two different stages of the cardiac cycle.

1. The first sound LUBB is produced by the closure of atrioventricular valves. This sound marks the end of the atrial systole and the beginning of the ventricular systole. It is the louder one that persists for a longer duration of about 0.16- 0.90 seconds.
2. The second sound DUP (DUB) is produced by the closure aortic semilunar valves. This sound marks the end of the ventricular systole. It is of a short duration of about 0.10 seconds.

The time interval between the 1^st and 2^nd heart sounds is known as clinical systole.

Cardiac Cycle Time

The time period consumed by the different events of one cardiac cycle is called the cardiac cycle time. The human heart beats at the rate of 72 per minute in normal conditions. Thus the time period consumed for one heartbeat is 72/60 = 0.8 seconds. The time taken in each event of the cardiac cycle can be described as follows:

Fig: Cardiac Cycle Time

Cardiac Cycle Events	Time in Seconds
Auricular Systole	0.1 Seconds
Auricular Diastole	0.7 Seconds
Ventricular Systole	0.3 Seconds
Ventricular Diastole	0.5 Seconds

Auricular event: 0.1 + 0.7 = 0.8 Seconds
Ventricular event: 0.3 + 0.5 = 0.8 Seconds

Cardiac Output & Heart-Rate

During each cardiac cycle, (i.e. in one beat), each ventricle pumps about 70 ml of blood to the body organs through the respective blood vessels. This is known as stroke volume. The volume of the blood that is pumped out by each ventricle in a minute is called cardiac out.

Since our heart beats about 72 times in a minute and pumps 70 ml of blood to the body organs each time, therefore the cardiac out per minute will be 72 x 70 = 5040 ml or approximately.
Thus, Cardiac output = Stroke volume x Numbers of beats per minute

Cardiac Reserve

The cardiac reserve is defined as the ratio between the maximum cardiac output of a person that a person can achieve and the cardiac output at rest. The following factors affect the cardiac output and cardiac reserve of a person:

1. Ventricular end-diastolic volume
2. Venous return
3. Strength of myocardial return
4. Blood volume
5. Autonomic nerve stimulation
6. Activity and exercise
7. Emotional states
8. Gender & Age
9. Body temperature
10. Baroreceptor reflex.

Blood Pressure

The pressure exerted by the flow of blood on the elastic wall of the arteries is called blood pressure.

1. During a cardiac cycle, arterial blood pressure increases during ventricular contraction and decreases during ventricular filling and atrial systole.
2. The blood pressure value is different during the diastole and systole phases. It is greater during systole than diastole. In normal conditions (at rest), the systolic pressure is 120mmHg, and diastolic pressure is 80 mmHg. The age and health of a person affect the values of systolic and diastolic blood pressure.

Blood Pressure related Issues

Abnormal variation in the value of systolic and diastolic blood pressure causes hypotension or hypertension.

1. Hypertension: A rise in blood pressure above 140-90 mmHg is called hypertension.
I. A rise in blood pressure may lead to the deposition of cholesterol on the walls of blood vessels, causing arteriosclerosis, i.e. stiffness of arteries.

2. Hypotension: The blood pressure below 100-60 mmHg in an adult signifies hypotension.

I. It may result from chronic vasodilation of arterioles, anaemia, blood loss, or failure of pumping action of the heart.  

II. In general, hypotension causes no alarming problem. However, in some instances, it may cause transient episodes of dizziness or faintness.

Summary

The heart is the main organ of the circulatory system. It is made up of cardiac muscles characterised by their continuous functioning of contraction and relaxation to pump the blood through the blood vessels to the body organs to supply the nutrients, gases, and other essential substances to the body cells. Moreover, the heart also receives impure blood from the body organs through the blood vessels. This cyclic flow of blood follows a sequence of events called the cardiac cycle. The events involved in a single cardiac cycle are atrial systole, atrial diastole, ventricular systole, ventricular diastole, joint diastole. The cardiac cycle is repeated after every 0.8 seconds and thereby pumps about 5 litres of blood in a single heartbeat.

Frequently Asked Questions (FAQs) on Cardiac Cycle, Heart Rate & Blood Pressure

Q.1. What is the cardiac cycle?
Ans: Cardiac cycle is defined as the sequence of events in a complete heartbeat that involve the alternative contraction and relaxation of atria and ventricles in order to pump the blood throughout the body organs.

Q.2. What is systole and diastole?
Ans: Systole is the state when the muscles of the cardiac chambers contract. The atrial systole forces the blood to the ventricles, and the ventricular systole forces the blood to the main blood vessels. In contrast, diastole is when the muscles of the heart chambers relax, and the blood enters the chambers.

Q.3. What is the normal blood pressure range?
Ans: The value of normal arterial blood pressure in the contraction and relaxation phase is:
1. Systolic pressure: 120 mmHg
2. Diastolic pressure: 80 mmHg

Q.4. What is the normal heart rate of an adult?
Ans: The normal heart rate of an adult is about 72 beats per minute.

Q.5. What is meant by cardiac output?
Ans: The term cardiac output refers to the amount of blood that is pumped out by the heart in a minute.

Learn About Double Circulation Here

We hope this article on Cardiac Cycle, Blood Pressure, and Heart Rate helps you in your preparation. Do drop in your queries in the comments section if you get stuck and we will get back to you at the earliest.