Pressure and Volume Changes in the Human Heart (With Diagram) | Biology

In this article we will discuss about the pressure and volume changes in the human heart and blood vessels with the help of suitable diagram.

1. Pressure Changes in the Human Heart and Blood Vessels:

In Animals—Optical Method of Wiggers:

A vertical glass tube filled with an anticoagulant solution is used as a cannula. One end of it is introduced into the chamber whose pressure is to be recorded. The other end of the cannula is covered with a tense elastic membrane, upon which a small, mirror is set.

A beam of light is so arranged that it is reflected by the mirror and falls on moving photographic plate. The pressure changes in the chamber are transmitted through the solution in the cannula and moves the mirror. The oscillations of the reflected beam are recorded on the moving photographic plate.

In Man—(a) Cournand’s Method:

Pressure changes in the right atrium and ventricle have been directly measured in man by introducing a thin rubber tube into the antecubital vein and gradually pushing it up through the corresponding bigger veins into the right atrium and then into the right ventricle. The pressure changes are transmitted through this tube and are recorded by suitable apparatus.

(b) Jugular Pulse Tracing:

From the jugular pulse tracing indirect information about the pressure changes in the right atrium can also be obtained.

2. Volume Changes in the Human Heart and Blood Vessels:

In animals it can be studied by Cardiometer (Fig. 7.52). This is a rounded funnel in which the heart is placed in such a way that the ventricles remain inside and atria outside the funnel- the fitting being made airtight around the atrioventricular groove.

When ventricles contract, pressure in the funnel falls; when it relaxes, pressure rises. These pressure changes are transmitted into a Tambour (Fig. 7.53) or pistons recorded through rubber tubing’s and are recorded on a moving drum.

With such studies the following four facts about the pressure and volume changes are known:

1. Intraventricular Pressure Changes:

The ventricular curve in Fig. 7.54 is to be carefully followed.

The following pressure changes are found on it:

During Ventricular Systole:

i. In the isometric contraction period both valves remain closed, blood cannot run out and ventricles forcibly contract upon the locked-up blood. Hence, intraventricular pressure sharply rises.

ii. In the next phase, maximum ejection period comes. Since blood is running out, pressure should fall in the ventricles. But the force of contraction is stronger than the rate of outflow, hence intraventricular pressure continues to rise for a brief while, but at a much slower rate than before. Gradually, the force of contraction and the rate of outflow equalise. Hence, a horizontal plateau is produced at the summit.

iii. In the next phase—the reduced ejection phase—the force of contraction is much reduced and is propor­tionally less than the rate of outflow. Hence, intraventricular pressure gradually falls.

During Ventricular Diastole:

i. In the protodiastolic period the pressure continues to fall as in the previous stage.

ii. In the isometric relaxation period the ventricles are actively relaxing as closed cavities and the intraventric­ular pressure sharply drops. This continues until the A.V. valves open.

iii. As soon as the A.V. valves open atrial blood rushes into the ventricles. But the rate of relaxation being more than filling, ventricular pressure continues to fall slowly to some extent.

iv. Then comes the period of diastasis or slow inflow phase. Ventricles are no more relaxing, blood accumulates in it and pressure slowly rises.

v. In the last phase—corresponding to atrial systole-blood is pumped in to the ventricles and ventricular pressure shows a small but sudden rise. Then ventricular systole comes again and the changes repeat.

2. Intra-Atrial Pressure Changes:

In the atrial curve of Fig. 7.54 there are three positive waves 1, 2 and 3, and three negative waves:

i. During atrial systole, intra-atrial pressure rises causing the first positive wave. In the later part of systole, the pressure falls. Because, most of the atrial fibres have started relaxing.

ii. During atrial diastole, the atria relax and pressure should continue to fall. But instead of that, there occurs a sharp rise causing the second positive wave. This corresponds to the isometric contraction period of the ventricles. As soon as ventricles contract, the A.V. valves become shut and bulge into the atrial cavity in a dome-shaped manner. Hence, intra-atrial pressure suddenly rises.

iii. Then the pressure very sharply falls and corresponds to the maximum ejection period of the ventricles.

This sudden fall is due to the following three reasons:

a. Atrial relaxation continues.

b. As the ventricular muscles shorten, the A.V. ring is pulled down, so that atrial cavity enlarges.

c. Due to reduction of ventricular volume, mediastinal pressure falls. Owing to this negative pressure— the thin-walled atria dilate and atrial pressure falls.

iv. In the later part of ventricular systole, intra-atrial pressure slowly rises causing the third positive wave. This is due to accumulation of blood in the atria. A.V. valves remaining closed. This rise slowly continues until the A.V. valves open (i.e., up to the end of isometric relaxation period).

v. During isometric relaxation period, the A.V. ring rises up and is an additional cause for pressure rising.

vi. As soon as the A.V. valves open, atrial blood rushes into the ventricles, so that atrial pressure falls. The fall continues till about the middle of ventricular diastole. Then as ventricles fill up (diastasis), atrial pressure slowly rises. After this, atrial systole comes again.

3. Intra-Aortic Pressure Changes (Aortic Curve-Fig. 7.54):

i. During isometric contraction of the ventricles, a slight rise of the intra-aortic pressure takes place due to the bulging-out of the semilunar valves.

ii. With the opening of the S.L. valve, blood enters the aorta and aortic pressure smoothly rises and falls-running parallel to the intraventricular pressure.

The fall of intra-aortic pressure in the reduced ejection phase is due to three causes:

a. Ventricle is contracting less forcibly than before, so that a comparatively less amount of blood is entering the aorta now.

b. More blood is running out into the periphery than is entering the aorta from the ventricles.

c. Reflex vasodilatation through Sino-arotic nerves, thus reducing the peripheral resistance and facilitating better ventricular emptying.

iii. With the onset of diastole, ventricular pressure sharply falls causing a backward flow of the aortic blood towards ventricles. Owing to this, aortic pressure drops causing the incisura. It corresponds to the dicrotic notch of the radial pulse. The blood column is reflected back by the sudden closure of the semilunar valves, thus causing a sharp rise in the aortic pressure. This is the cause of the dicrotic wave on the radial pulse. After this, aortic pressure shows a few elastic oscillations (after vibrations) caused by the recoil of the aortic wall.

iv. The aortic pressure then slowly falls due to the continuous passage of blood to periphery (propelled by elastic recoil of the vessels). The fall continues till ventricles contract again.

4. Jugular Pressure Tracing (Venous Pulse – Fig. 7.54):

The pressure changes in the right jugular vein will give a good idea as to those in the right atrium, because the right jugular vein is the direct continuation of the superior vana cava.

It may be studied in a number of ways:

i. A small metal cup is placed on the vein. The cup communicates through a rubber tube with Marey’s tambour (Fig. 7.53). As the vein pulsates pressure changes take place in the cup and are transmitted to Marey’s tambour. The recorder of the tambour moves and the oscillations are recorded on a moving drum.

ii. Mackenzie’s polygraph. This is an instrument by which tracing of jugular pulse and radial pulse can be obtained simultaneously. This is a very useful instrument by which the pressure changes as well as their time-relations can be known.

Since jugular pressure follows the atrial pressure, it is obvious that jugular pulse will have three positive waves and three negative waves following those in the atria. But due to its distance, the waves will come a little later than the corresponding atrial waves. The three positive waves are called, a, c, v and three negatives ones as x, x_1, y.

The waves are as follows:

i. The first positive wave a due to atrial systole. As atrial muscles contract, pressure in it rises, blood in the jugular vein cannot enter the atria, hence, jugular pressure rises. [It is not due to the regurgitation of atrial blood into the jugular vein. Because a sleeve of atrial muscles surrounds the openings of great veins. When atria contract, these sleeves of muscles also contract thus closing the openings of great veins like a purse string.]

ii. This is followed by the first negative wave x. This is due to the fall of atrial pressure during the adynamic phase of atrial systole.

iii. After this, the second positive wave c comes during isometric contraction phase. In this phase there is bulging of the A.V. valves into the atrium causing rise of intra-atrial pressure. The a – c interval indicates the conduction time of the bundle of His and corresponds to the P-R interval of the Electrocardiogram. (The c wave begins 0.1 sec. before the primary wave of the radial pulse.)

iv. x₁ is the second negative wave after c. It is due to the corresponding fall of pressure in the atrium.

v. v is the third positive wave, caused by the gradual filling of the atria and the return of the A.V. ring to its original position. The summit of the v wave indicates the end of ventricular systole.

vi. The third negative wave y sets in after the opening of the A.V. valves and is caused by the corresponding fall in the atrial pressure.

Many information’s regarding the conditions of heart in health and disease may be obtained from the jugular tracing.

Ventricular Volume Changes:

The volume changes of the ventricles are to some extent the reverse of its pressure changes.

The following phases are seen:

i. During atrial systole, ventricular volume increases due to rapid filling. This rise is maintained during the isometric contraction phase of the ventricles, because no blood is going out.

ii. As soon as ejection starts, ventricular volume smoothly and continuously falls up to the end of systole.

iii. In the isometric relaxation period, volume remains same, because no blood is entering.

iv. In the next phase ventricular filling starts and its volume rises rapidly corresponding to the first rapid filling period.

v. After this, during diastasis or slow inflow phase, ventricular volume very slowly increases (Fig. 7.54).

Composite Representation of the Sequential Changes in the Pressure and Volume Events in the Heart:

Composite representation of the sequential changes in the pressure and volume events in the heart and blood vessels during the cardiac cycle correlating With phonocardiogram and electrocardiogram (Fig. 7.58):

1. I-II is the ventricular isometric contraction phase. This phase starts with the closure of atrioventricular (A.V.) valve and with the occurrence of the first sound. In this phase the ventricle is a closed chamber and with the contraction of the ventricular muscle, the intra-ventricular pressure rises first slowly but at later stage, rapidly. Notch c₁ in the aortic pressure curve and notch c in the atrial pressure curve—both are due to the transmission of the intraventricular pressure through the bulging of semilunar (aortic) valve and atrioventricular valve respectively.

2. II-III is the ejection phase of the ventricle. At II the ventricular pressure overcomes the aortic pressure and the semilunar valve opens. The blood begins to flow through the aorta at a higher pressure head. The ventricular pressure remains at a higher level throughout this phase.

3. The ventricular volume begins to decrease with the onset of ventricular systole. The atrial pressure though remains below the ventricular pressure head but begins to rise due to the venous filling.

4. III-TV is the protodiastolic phase. The ventricular systole ceases and the ventricle begins to relax. At IV the ventricular pressure falls below the aortic pressure and the semilunar valve is closed with the occurrence of the second sound. The ventricle is again a closed chamber and relaxes isometrically. Due to the closure of the semilunar valve a depression in the aortic pressure curve is generally observed. There are multiple oscillations in the aortic pressure curves which are known as postincisural vibrations.

5. At IV-V the ventricular pressure curve drops abruptly and at V it falls below the atrial pressure and then the blood from the atrium begins to rush (first rapid filling phase) in the ventricle rapidly. Here the third sound is heard. Throughout the diastolic phase of the ventricle, the atrial pressure remains in higher pressure head (V-I).

6. The ventricular volume curve begins to rise and maintains this stage until the systole begins. The third sound occurs with the rapid rush of blood in the ventricle.

7. At VII-I the atrial systole coincide with the ventricular diastole and the blood rushes in the ventricle from the atrium very rapidly and the fourth sound occurs.

8. a in the atrial pressure curve represents the cessation of the ventricular diastasis (slow filling) and the onset of the atrial contraction.

9. At I the atrial systole ceases and the ventricular systole starts and the cycle is repeated.

10. The ventricular volume begins to decrease with the onset of the ventricular systole.

Asynchronous Ventricular Pressure Curves of Two Ventricles:

Theoretically, the dynamic events of the two ventricles or the two atria are the same, but it has been understood that the right atrium contracts earlier than the left one. On the other hand, the left ventricle contract earlier than the right one, but the right ventricle ejects blood earlier and ceases later than the left ventricle.

Electrocardiographic Correlation of Cardiac Cycle:

Prior to atrial systole, P wave, the depolarisation wave of atrium occurs. So the interval between the electrical events and the mechanical events of the right and left atria are 0.08 sec. and 0.06 sec. respectively. Similarly the ventricular depolarisation wave QRS starts prior to the onset of ventricular systole.

The interval between the electrical events and mechanical events of the left ventricle and right ventricle is approximately 0.05 sec. and 0.04 sec. respectively. T wave which is the repolarisation wave of the ventricle mostly occurs during the systole of the ventricle and thus ends just before the incisura of the aortic pressure curve.