Cardiopulmonary Physiology

Millersville University

Dr. Larry Reinking

Chapter 5 - Pumping Action of the Heart

General Structures and Terminology

As you know, the heart has four contractile chambers and a series of valves that permit a one-way flow of blood. Systole is a term used for the contracting period of a chamber while diastole refers to the resting, expansion interval. The cardiac cycle refers to all of the events that occur during one complete systole and one complete diastole. Cardiac output is the volume of blood pumped in a given time interval.

Atria

The left and right atria are thin walled structures and this wall thickness reflects the low pressures generated by the chambers (5-6 mm for the right side, 7-8 mm Hg for the left). Most of ventricular filling depends on 'passive' venous return since the contraction of both atria add only an additional 20-30% to the ventricular volume. For this reason, the atria are sometimes thought of as 'primer pumps'. At rest, the heart will work satisfactorily without atrial pumping.

Ventricles

The ventricles are the power pumps of the heart and have thick muscular walls. These walls are arranged in multiple sheets of myocardium that spiral from the base of the heart to the apex. The thickness of the left ventricular wall is about three times that of the right. Cardiac output for each chamber is the same, however the driving pressures differ significantly. Typical pressures for the left are 120/80 mm Hg (systolic/diastolic) but only 25/1 mm Hg for the right.

Shape of the Ventricles and Pressure

Wall thickness alone does not account for all of the pressure difference between the left and right ventricles. At least one other factor of important consideration is the shape of these chambers. The right ventricle wraps halfway around the left ventricle which results in different chamber shapes:

Figure 5.1
Shape of the Right
and Left Ventricle
RV = right ventricle
LV = left ventricle
® = direction of flow

Note that the right ventricle, in longitudinal section, forms a shallow ‘U’ shape while the left ventricle is narrower and has a ‘V’ shape. The different widths and shapes of these chambers influences the ability to generate pressures.

Law of LaPlace

The Law of LaPlace states that the pressure (P) in a vessel is directly related to the wall tension (T) and inversely related to the vessel radius (r):

Law of LaPlace Equation 5.1

Thus, small vessels are able to withstand (or generate) more pressure than large vessels having the same wall thickness. This is why oxygen (a high pressure gas) is compressed in long, small radius tanks while propane (a low pressure gas) is bottled in squat, wide containers. Likewise, the left ventricle, having a smaller radius will have a distinct geometrical advantage over the right ventricle. A smaller radius, combined with a more muscular wall allows this side of the heart to produce pressures 5-6 times those of the right.

Interesting Nature Note - A giraffe must pump blood to the head against large hydrostatic columns and left ventricular pressure can exceed 300 mm Hg. In accordance with LaPlace’s Law, the left ventricle has an almost tubular shape.

We will encounter the Law of LaPlace several times this semester in other applications.

Mechanical Events of the Cardiac Cycle

The heart really is an amazing organ. At a rate of 65 beats/min, a human heart will complete 3,900 beats/hr... 93,600 beats/day... 34,164,000 beats/year... 2,391,480,000 beats/70 year life span! This is especially impressive when you consider that just a few missed beats ‘spells curtains’. At a cardiac output of 6 L/min, a heart will pump 3.2 million liters/year or 221 million liters in a 70 year life span. For comparison, a regulation Olympic water polo pool holds 1.2 million liters.

At a typical heart rate of 65 beats/min, each cardiac cycle takes 0.92 sec. During this interval the chambers partition their time between systole and diastole in the following manner:

atrial systole 0.15 sec ventricular systole 0.36 sec

atrial diastole 0.77 sec ventricular diastole 0.56 sec

In other words, the atria are at rest for 84% (.77/.92) of the time and the ventricles for 61% (.56/.92) of the time. This is important, as we will see in the next chapter, because ventricular contraction strength is governed by degree of filling during diastole. As the heart rate increases, the duration of the cardiac cycle decreases and the diastolic filling times are reduced.

Heart rate is limited, in part, by the refractory period (the plateau, Figure 4.5 in chapter 4) of the cardiac action potential. The absolute refractory period of the ventricle and atria are about 0.3 and 0.15 seconds, respectively. As a result, the contraction rate of the atria can be much faster than the ventricles during pathological situations.

Wiggers Diagram and the Cardiac Cycle

A series of events take place during a cardiac cycle that include pressure changes in the chambers, valve movement, electrical events, heart sounds, flow of blood and pressure changes in the major vessels. Figure 5.2, below, incorporates these events, over time, into a single figure (also see your lecture packet). Named in honor of Carl J. Wiggers, this diagram is one of the cornerstones of cardiology. Become totally familiar with the Wiggers Diagram!

• On the top of this diagram are three lines that deal with pressures: aortic pressure, left ventricular pressure and left atrial pressure.

• The next line across is left ventricular volume, that is, the volume of blood contained within the left ventricular chamber at any given point in the cardiac cycle. The difference between the maximum and minimum value is the volume of blood ejected by the left ventricle.

• Next is the electrocardiogram, a recording made at the skin surface, of the electrical events of the heart. It is a surface recording that ‘sums’ all of the action potentials of individual muscle fibers. The P wave corresponds to the electrical depolarization that takes place just before atrial contraction, the QRS complex is the depolarization that precedes ventricular contraction and the final wave, the T wave, is caused by ventricular repolarization. We will examine the electrocardiogram in detail in lab.

• On the bottom is the phonocardiogram, a recording of heart sounds using a surface microphone on the chest. These sounds are the result of turbulent blood flow caused by valve closure or rapid chamber filling.

Figure 5.2 The Cardiac Cycle (Wiggers Diagram)

This diagram depicts events for the left ventricle, left atrium, aorta, aortic valve, mitral valve, electrocardiogram and phonocardiogram. , , and  refer to the first, second and third heart sounds.

Phases of the Cardiac Cycle

The events of the cardiac cycle, on the Wiggers Diagram, are divided into a number of phases. There is no real end or beginning to a cycle, but we will use atrial systole as a starting point. Remember that this diagram depicts only the left side of the heart.

1. Atrial Systole - This phase begins as the atrium contracts causing a small rise in atrial pressure (atrial pressure wave a) as well as ventricular pressure. Prior to this interval, blood had been flowing through the atrium into the ventricle under the influence of venous pressure. During this phase the mitral valve is open and the aortic valve is closed. Note that as the atrium contracts a small additional volume is added to the left ventricle (the primer pump concept). Also note that aortic pressure gradually drops ('diastolic run-off') as the elastic walls of the artery recoil from the previous systolic ballooning (p. 5, chapter 1). The P wave of the electrocardiogram overlaps the beginning of this phase. Recall that this wave represents the summed depolarization of the atrial myocardium and that there is a slight time lag between cellular depolarization, cross-bridging and contraction. The QRS complex, in preparation for ventricular contraction, is seen at the end of this phase. The phonocardiogram is silent during atrial systole.

At the end of atrial systole the ventricle is at its maximum volume which is called the end-diastolic volume (diastolic, in this case, refers to ventricular diastole). End-diastolic volume is an important concept we will use in later lectures.

2. Isovolumic Contraction -This term means that although the ventricle is contracting, the volume does not change, as can be seen on the ventricular volume curve. As contraction of the ventricle begins (following the start of the QRS complex), ventricular pressure rapidly rises and the mitral valve slams shut. The valve closure creates turbulence and vibrations that are detected as the first heart sound on the phonocardiogram. This phase is isovolumic because both valves are closed. Aortic pressure continues to drop and reaches its minimum pressure (≈80 mm Hg). The blip in atrial pressure (atrial pressure wave c) is due to the massive contraction of the ventricle which distorts the shape of the attached atrium. At the end of this phase, the ventricular pressure equals and then exceeds aortic pressure, causing the aortic valve to open.

3. Ejection - As the aortic valve opens, blood is rapidly ejected into the aorta (note rapid drop in ventricle volume) and pressures in the ventricle and aorta rise and are essentially matched. At this point the aorta will stretch and the elastic componets will store energy. Pressure gradually rises in the atrium because it is filling, via venous pressure, against a closed mitral valve. About midway in the cycle, the muscle stops contracting and begins to repolarize. As a result, the aortic and ventricular pressures drop and the T wave appears on the ECG. Although the pump has stopped, blood flow continues at a reduced rate due to inertia. The ejection phase can be subdivided into the period of maximum ejection (steep slope on volume curve) and the period of reduced ejection (lesser slope on volume curve). At the end of the ejection phase ventricular pressure falls below aortic pressure and the aortic valve slams shut.

The amount of blood ejected with each heart beat is referred to as the stroke volume and is typically about 70 ml. The fraction of the end-diastolic volume ejected is called the ejection fraction and has a value of about 60% (more correctly, 0.6, however the former is the usual convention).

4. Isovolumic Relaxation - The ventricle is now at rest and because both valves are closed, the volume is constant and at its lowest value (see ventricular volume curve). The noisy closure of the aortic valve at the beginning of this phase is detected as the second heart sound. Atrial pressure continues to rise as blood dams up against the closed mitral valve. Following closure of the aortic valve, aortic pressure is no longer under ventricular influence, however, a precipitous pressure drop is prevented by the energy stored in the elastic walls of this artery. The notch (called the dichrotic notch) in the aortic pressure curve is due to a momentary back-flow of blood caused by the aortic valve snapping closed. Ventricular pressure rapidly drops during this phase and when atrial pressure is matched, the mitral valve opens. Repolarization of the ventricular myocardium is complete (see T wave) by the end of isovolumic relaxation.

5. Rapid Filling - With the mitral valve open, blood rapidly flows into the ventricle (steep slope on the ventricular volume curve) and exits from the dammed up atrium causing atrial pressure wave c. Aortic pressure gradually drops during this resting phase. Sometimes, a third heart sound is heard at the end of this phase, possibly caused by turbulent flow into a nearly filled chamber. No activity is seen on the electrocardiogram.

6. Diastasis - This period falls in the middle of ventricular diastole. Only a small amount of blood enters the ventricle and ventricular pressure increases slightly during diastasis. Atrial pressure also rises slightly because of continued venous return. Since the blood flows from atrium to ventricle, left atrial pressure will be a little higher than that in the left ventricle. Aortic pressure continues to fall and the phonocardiogram is silent. Near the end of diastasis the P wave appears on the electrocardiogram as the atrial myocardium prepares for atrial systole.

The following are characteristic time intervals for the cardiac cycle phases in an adult human. These values vary with heart rate.

Table 5.1. Durations of Cardiac Phases

Heart Rate = 75 beats/min

Phase / Duration (sec)
Atrial systole / 0.11
Isovolumic Contraction / 0.05
Ejection / 0.26
Isovolumic Relaxation / 0.08
Rapid Inflow / 0.11
Diastasis / 0.19

Heart Valves

Blood flows through the heart from the right atrium ® right ventricle ® pulmonary artery and from the left atrium ® left ventricle ® aorta. This one-way flow is the result of the cardiac valves. There are two types of valves; the atrioventricular valves (A-V valves) and the semilunar valves. Both types of valves are fibrous tissue, covered with endothelium, that passively open and close in response to pressure differences between the heart chambers. Around each valve is a supporting ring of fibrous connective tissue.

Atrioventricular Valves

The tricuspid valve lies between the right atrium and ventricle and is comprised of three flaps. Positioned between the left atrium and ventricle is the mitral (or bicuspid) valve which has two flaps. The thin flaps (also called leaflets or cusps) of the A-V valves are extremely flexible and will close when the pressure in the ventricles only slightly exceeds that in the atria. On the edges of the valve leaflets are the strong chordae tendineae which connect to the papillary muscles at the apex of the ventricular walls (Figure 5.3, below). During ventricular systole, the anchoring action of the chordae tendineae and the forceful contractions of the papillary muscles prevent the valve leaflets from everting into the atria. In both the A-V valves, the leaflets have considerable overlap when closed and create a tight seal.