Cardiac Cycle and Cardiac Output, Practice Problems and FAQs

You must have seen doctors placing the stethoscope near the chest and listening to the heart beat. Have you ever tried to listen to your heart? Not in a figurative sense, but in the literal sense? Try to put your hand on the left side of your chest and you will feel a thumping in your chest. If you can get hold of a stethoscope, try and listen to your heart like a doctor does, you can hear two distinct sounds - lub and dub repeating one after the other, in a rhythmic pattern. Do you know why the heart makes these sounds? Don’t worry we will enlighten you through this article.

You must have noticed that whenever we are running or doing exercise our heart starts to beat fast, but when we are sitting ,watching TV or just reading a book, our heart beats at a normal rate. Why do you think this happens and how can you quantify the beating of your heart?

Let’s find out in this article.

Table of contents

Overview of cardiac cycle
Heart sounds
Heart beat
Stroke volume
Cardiac output
Practice Problems
FAQs

Overview of cardiac cycle

The heart pumps around 7000 litres of blood to different parts of the body daily. To pump out this big amount of blood ,the chambers of the heart contract and relax rhythmically. The human heart consists of four chambers. Two upper chambers are left and right atria and the two lower chambers are left and right ventricles. Contraction of the muscles of the chambers is called ‘systole’ and relaxation of chamber muscles is called ‘diastole’.

These muscular movements are essential for the forceful pumping of blood throughout the body and keep repeating in a cyclic manner during each heartbeat, making the blood flow through the heart chambers in a particular direction only. The events in each heartbeat together constitute a cardiac cycle

Events in a cardiac cycle

A cardiac cycle describes all the mechanical events that occur during a complete heartbeat when the blood flows through the different chambers of the heart. On an average it takes about 0.8 secs to complete one cardiac cycle.

During a cardiac cycle, the events that occur in a sequence are-

Atrial systole and ventricular diastole (0.1 seconds)
Atrial diastole and ventricular systole (0.3 seconds)
Joint diastole (0.4 seconds)

Atrial systole and ventricular diastole

At the end of a cardiac cycle all the four heart chambers are in joint diastole, i.e, they are in the relaxed state. At this time, the bicuspid and tricuspid valves and the semilunar valves remain closed. Blood from the superior and inferior vena cavas and the pulmonary vein enter the right and left atria, respectively. At the end of joint diastole, the fall in pressure in the ventricles results in opening of the bicuspid and tricuspid valves due to which around 70% of the blood flows into the ventricles. This phase is known as the general pause and lasts for around 0.4 seconds, before the next cycle starts.

A cardiac cycle starts with atrial systole, that is, contractions of the atrial muscles. The ventricles are in the state of diastole (relaxed) at this point. Remaining 30% of the blood is pushed to the ventricles during atrial systole. The semilunar valves remain closed at this point. Atrial systole lasts for around 0.1 to 0.15 seconds.

Fig: Atrial systole

Ventricular systole and atrial diastole

At the end of atrial systole, atrial muscles start relaxing (atrial diastole) whereas ventricular muscles start contracting resulting in ventricular systole. During ventricular systole the pressure in ventricles starts increasing, causing the closure of tricuspid and bicuspid valves to prevent the backflow of blood towards the atria. The blood from the right ventricle is pushed out to the pulmonary artery and from the left ventricle it is pushed out to the aorta. At this time the semilunar valves remain open while the bicuspid and tricuspid valves remain closed. Ventricular systole lasts for around 0.3 seconds.

Fig: Ventricular systole

Joint diastole

Just after the ventricular systole, the ventricles start relaxing due to which the ventricular pressure falls resulting in the closure of semilunar valves which prevents the backflow of blood into the ventricles. The atria are still in their diastole. Thus, at this point, both the atria and ventricles are in diastole and hence this phase is called ‘joint diastole’. It lasts for around 0.4 seconds. The bicuspid and tricuspid valves remain closed during early joint diastole but open up during the later phase. This stage is followed by the atrial systole which marks the beginning of the next cardiac cycle.

Fig: Joint diastole

Heart sounds

Whenever there is the closure of the heart valves, heart sounds can be heard. During each cardiac cycle, two sounds are produced which can be heard by a stethoscope.

Lub sound

The closure of atrioventricular valves (bicuspid and tricuspid valves) during ventricular systole, causes the first sound, lub.

Dub sound

The closure of semilunar valves during joint diastole, causes the second sound, dub.

Heart beat

The rhythmic contraction and relaxation of the muscles of the heart chambers is called ‘heartbeat’. Each heartbeat represents the completion of one cardiac cycle. Atrial systole, ventricular systole and joint diastole take about 0.1 sec.,0.3 secs.,and 0.4 secs, respectively, in adult humans.

Thus, the duration of each heartbeat in adult humans is about 0.8 secs and the adult human heart beats around 72 times per minute. This is known as the heart rate. This means that 72 cardiac cycles are performed per minute. The heart rate of newborn infants is about 130 times per minute because smaller the animal, higher is the heart rate.

Stroke volume

During a cardiac cycle, the volume of blood pumped out by the ventricles is called the stroke volume. It is approximately 70 ml for the human heart.

Cardiac output

Heart beats 72 times per minute and pumps out about 70 ml of blood per heart beat. Hence about 5 litres of blood is pumped through the heart per min.,which is termed as cardiac output.

Cardiac output = Heart rate x Stroke volume

= 72 x 70 = 5,040 ml or about 5 litres.

Practice problems

1.Which condition is not true regarding the ventricular systole?

a) The ventricles relax

b) The ventricles contract

c) The semilunar valves open

d) The atrioventricular valves close

Solution: The events occurring during ventricular systole are -

Contraction of ventricles
Simultaneous relaxation of atria
Closing of the atrioventricular (bicuspid and tricuspid) valves
Opening the semilunar valves

Thus, the correct option is a.

2. Which pair is correctly matched?

a) Dub - Sudden opening of semilunar valves at the beginning of ventricular diastole

b) Lub - closure of AV valves at the beginning of ventricular systole

c) Cardiac cycle in humans - 0.7 seconds

d) Initiation of heartbeat - Purkinje fibres

Solution: The closure of atrioventricular valves (bicuspid and tricuspid valves) during ventricular systole, causes the first sound, lub. Whereas the closure of semilunar valves during joint diastole, causes the second sound, dub.

Each cardiac cycle in adult humans lasts for 0.8 seconds.

The initiation of heartbeat occurs at the sino atrial node (SAN).

Thus, the correct option is b.

3. After exercising, the heart rate of an individual was around 130 beats per minute. What will be the cardiac output?

Around 11 L
Around 9 L
Around 5 L
Around 25 L

Solution: The volume of blood pumped out by the ventricles during each cardiac cycle = 70ml = 0.07L

The heart rate of the person = 130 beats per minute

Cardiac output = Heart rate x Stroke volume

= 130 x 0.07 = 9.1L which is around 9 L.

Thus, the correct option is b.

4. What is joint diastole?

Answer: Just after the ventricular systole, the ventricles start relaxing due to which the ventricular pressure falls resulting in the closure of semilunar valves which prevents the backflow of blood into the ventricles. The atria are still in their diastole. Thus, at this point, both the atria and ventricles are in diastole and hence this phase is called ‘joint diastole’. It lasts for around 0.4 seconds.

FAQs

1. What are the factors affecting cardiac output?

Answer: Cardiac output is directly proportional to size of the organism, amount of blood entering the heart and metabolic rate but is inversely proportional to age and peripheral resistance (resistance posed by arteries to the flow of blood).

2. What are systolic and diastolic pressure?

Answer: The pressure exerted by blood on the arterial walls during ventricular systole is the systolic blood pressure. The pressure exerted by blood on the arterial walls during ventricular diastole is the diastolic blood pressure.

3. How is backflow of blood to the vena cavas prevented during atrial systole?

Solution: During atrial systole, blood does not rush back from the right and left atria to the vena cavas or the pulmonary vein because the roots of these veins are compressed to block the backflow of blood.

4. What will be the effect of exercise on cardiac output and heart rate?

Answer: During exercise, our heart beats faster to supply ample amounts of oxygen to body parts. Our heart can also increase its stroke volume by pumping more blood forcefully or pouring more blood in the ventricle before its supply to the body parts. This would result in an increase in cardiac output. For mild exercises it can be around 11 litres whereas for vigorous exercises it can increase up to 25 to 40 litres.

Youtube link- https://www.youtube.com/watch?v=sKFfgnlKSJw