Heart as a Pump, the Cardiac cycle and Cardiac Output

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Describe the sequence of events in the cardiac cycle:

A cardiac cycle consists of systole and diastole of the atria plus systole and diastole of the ventricles. Some points from the cardiac cycle:

• Blood flows from high pressure areas to low pressure areas.

• As a heart chamber contracts, blood pressure within it increases.

• Pressures are greater on the left side of the heart than on the right.

• Systole causes contraction by depolarization.

• Diastole causes expansion by polarization

The cardiac cycle is divided into three parts:

Atrial Systole or Ventricular filling

• Atria are contracting.

• AV valves open.

• ECG marks a P wave.

• Blood gushes from atria to ventricles.

• In the end, ventricles contain about 130 ml of blood, and this is the end-diastolic volume (EDV).

Ventricular Systole or Ventricular systole

• During this time, the ventricles are contracting.

• Both the semilunar and AV valves are closed.

• Ventricular pressure rises sharply, and in the right ventricle, semilunar valves open taking blood to the pulmonary veins and ultimately to the lungs for oxygenation. In the left ventricle, blood gushes to the aorta.

• The blood remaining in each ventricle is the end-systolic volume (ECV).

Relaxation period

• Atria and ventricles both relax.

• Pressure within atria and ventricles begins to fall and blood flows backwards in the aorta and pulmonary valve cusps close causing the dicrotic wave on the ECG.

• This period is called the isovolumetric relaxation.

Describe the position of the semilunar and atrioventricular valves during ventricular filling.

During ventricular filling, the semilunar valves are closed and the atrioventricular valves are open, through which blood flows into the ventricles.

Describe the differences in pressure in the right and left ventricle during ventricular systole. Why are they different? How does the appearance of the ventricles on gross inspection reflect this difference?

During ventricular systole, pressure is greater on the left side than on the right because walls of the left ventricle are thicker and more muscular than walls of the right ventricle. This is because the left ventricle is the main pump of the body and is solely responsible for pumping blood throughout the blood-vessels of the entire body.

How is cardiac output related to stroke volume? How does increased venous return affect stroke volume? How does it affect cardiac output?

Cardiac output (CO) is defined as the amount of blood ejected from the ventricles each minute (into the pulmonary trunk or the aorta). Yes, CO is related to the stroke volume, namely, Cardiac Output = Stroke Volume x Heart Rate. Stroke Volume is the blood ejected by the ventricles during contraction. The more the venous return, the more the stroke volume. Accordingly, the more the venous return, the more the CO.

What is the Frank-Starling Law of the heart? What is its significance?

Frank-Starling law states that the greater the preload (stretch) on the cardiac muscle fibers prior to contraction increases their forces of contraction, i.e., the more the heart fills with blood during diastole, the greater is the force of contraction during systole. This law equalizes the output of the right and left ventricles and keeps the same volume of blood in systemic and pulmonary circulations. As a side note, contractility is the strength of contraction at any given preload, and afterload is the pressure that has to be overcome before the semilunar (or aortic) valves can be open. This pressure is caused by blood in the pulmonary valve and aorta. The more the blood pressure, the more the afterload.

Cardiac output is a product of stroke volume and heart rate. Predict the effect on cardiac output in each of the following situations:

a) Increase in venous return

CO goes up.

b) Decrease in heart beat

CO goes down.

c) Increase in heart rate

CO goes up.

d) An extremely fast heart rate

The time spent in diastole would decrease to the point where ventricular filling time will be shortened, and volume at the end of diastole would be less, thus decreasing the stroke volume. The overall result would be a decrease in CO.

e) An increase in serum calcium

A mild increase in extracellular Ca will increase HR and increase contractility, thus increasing CO.

Additional Reading:

Basic Cardiology

1. Electrical Activity of the Heart
2. Heart Muscle Mechanics
3. Heart Sounds and Murmurs
4. Additional FAQ on Heart Sounds and Murmurs
5. Cardiac Conduction Diagram
6. Blood Pressures in Cardiac Chambers
7. What is Pulsus Paradoxus?
8. FAQ on Heart Murmurs and Mechanisms of Turbulent Flow
9. Notes on Fetal Circulation
10. FAQ on Ischemic Myocardial Infarction
11. FAQ on Electrocardiograms / ECG / EKG
12. FAQ on Cardiac Conduction
13. The Heart as a Pump, the Cardiac cycle and Cardiac Output
14. What are the most common causes of aortic stenosis?
15. What is Pulseless Electrical Activity?
16. Causes and Complications of Arteriovenous Fistulas
17. CHADS2 Score for Atrial Fibrillation Stroke Risk
18. How to Reduce Blood Pressure without Medications?
19. Types of Shock
20. Locations of Heart Murmurs on Chest Wall
21. Types of Heart Blocks

Electrocardiogram (EKG/ECG) Topics

1. EKG Chest Leads
2. EKG Limb Leads
3. Quick 12-Lead ECG/EKG Format

Cardiology Videos

1. Video of Cardiology Examination in a Clinical Setting

Medical Images

Useful Medical Images & Diagrams (link opens in a new window)

Related Topics

1. Thorax Anatomy
2. Vascular Disorders
3. Heart Disorders
4. Histology of the Cardiovascular System
5. Jugular Venous Distention Workup
6. ER Chest Pain Workup
7. Cardiac Examination for Internal Medicine
8. FAQ on Blood Pressure
9. FAQ on principles of fluid and flow dynamics of Blood

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