The Cardiac Cycle | Measuring and Managing the Output of the Heart
The Cardiac Cycle | Measuring and Managing the Output of the Heart
Objective 4
Define the steps in the cardiac cycle. Correlate the cardiac cycle with the electrocardiogram and with the pumping action of the heart. Explain how the output of the heart is measured. Interpret a graph of the pressure in different parts of the systemic circuit.
The Cardiac Cycle
The last objective concerned the electrical activity of the heart. We saw how the rhythmic electrical activity of the pacemakers, and of the cardiac muscle cells themselves, created an electrical wave spreading over the surface of the heart which is detected as the electrocardiogram (ECG or EKG) on the surface of the chest.
In this objective, we’ll see how this wave of electrical activity is translated into heart muscle contraction, and how that rhythmic contraction leads to the movement of blood within the heart and through the pulmonary and systemic circuits.
We’ll start our analysis of the diagram above at 12 o’clock on the clock face. Here, there is a wave of electrical activity spreading between the SA node and the AV node in the right atrium and over the surface of the left atrium. This wave of electrical activity causes both atria to contract simultaneously (the so-called “atrial kick“). Formally, this is called atrial systole. Note the darker gold color of the inner ring of the diagram. This is represented by the P wave on the ECG.
This is the only time the atria contract; the remainder of the time, they are relaxed (the lighter gold color of the inner ring). The relaxed state of the atria is called atrial diastole. Blood enters the atria during this relaxation phase, as shown by the small black arrows.
Now we switch to about 1 o’clock to 5 o’clock on the diagram. Here, the outer ring is a darker green color. This is ventricular systole. The electrical activity is now spreading through the AV bundle (bundle of HIs), into the bundle branches, and across the Purkinje cell network on the surface of the myocardium. At first, the ventricles contain the same volume of blood. This is called isovolumic or isovolumetric contraction. The contraction of the heart muscle against a relatively incompressible liquid medium increases its pressure, just as you can feel the pressure increase on a blocked syringe as you push on the plunger. The increase in pressure opens the outflow (semilunar) valves. Blood is pushed out of the heart through the pulmonary and aortic valves into the pulmonary trunk and the aorta, respectively. This is ventricular ejection. Electrically, ventricular systole corresponds to the QRS complex of the ECG.
The ventricles repolarize (i.e. the electrical potential of the myocardial cells goes back to –60 mV or so). As they do, the ventricles relax (ventricular diastole). At first, the volume of the remaining blood in the ventricles remains the same. This is called isovolumic or isovolumetric relaxation. Like pulling up on a plunger on a closed syringe, this will reduce the pressur