Cardiac Cycle

What is the cardiac cycle?

• The cardiac cycle is the pattern of atrial and ventricle contraction and relaxation that the heart goes through in order to pump blood throughout the body. It has 3 main steps.

Steps in the Cardiac Cycle (0.8 sec)

1. Relaxation (Diastole) Period-the whole heart relaxes (for about 0.4 sec).  Cardiac msucle has a long absolute refractory period. During this time, blood flows passively and about 75% of ventricular filling occurs.
2. Atrial Systole (contraction)-both atria are contracting (for about 0.1 sec). The last 25% of blood in the atria gets squeezed into the ventricles. At this time, the AV valves are still open.
3. Ventricular Systole (contraction)-All four AV valves close temporarily. Pressure in the ventricles goes up until pressure in the ventricles is higher than pressure in the great vessels (pulmonary artery & aorta). The semilunar valves now open, and blood flows out of the heart and into the great vessels.This step takes about 0.3 sec.

Which of the ventricles holds more blood?

• Neither. They hold the same amount, about 130-mL. During ventricular systole, only about 70-mL gets pushed out, the rest is pushed out with the next heartbeat.

Heart Sounds

• Systolic Sound (lubb)-the first heart sound heard, is the sound of the AV valves slamming shut.
• Diastolic Sound (dubb)-the second heart sound heard, is the sound of the semilunar valves opening.

Calculating Cardiac Output

• Cardiac Output (CO)-the blood ejected per minute from the left ventricle.
• Stroke Volume (SV)-the volume of blood pumped per beat from the left ventricle.
• Heart Rate (HR)-beats per minute

So our final equation for Cardiac Output is: CO = SV x HR.

Electrocardiogram

An electrocardiogram (EKG/ECG) is a record of the electrical activity of the heart. It shows us the rate at which the steps in the cardiac cycle occur.

The EKG/ECG has three main parts:

1. P wave-represents atrial depolarization (atrial contraction)
2. QRS Complex-represents ventricular depolarization (ventricle contraction). The atria relax when the ventricle contract. No wave is produced when the atria contract because the wave produced by the ventricular contraction is greater.
3. T wave-represents ventricular repolarization (vetricle relaxation)

This section wasn't the easiest to find decent links for, but here goes:

Details & EKG Animation (You have to click on the word animation to see it!)

• http://www.britannica.com/nobel/micro/189_13.html

EKG Tutorial

• http://education.med.nyu.edu/courses/physiology/courseware/ekg_pt1/ekgmenu.html

Hyperheart: Tutorial/Interactive (If you click on the different tutorials, it will give you more information to help clarify the animation).

• http://www-medlib.med.utah.edu/kw/pharm/hyper_heart1.html