The Heart and Circulation

The Heart Sounds The valves in the heart close passively whenever there is a tendency for the blood to flow in the reverse direction. Since blood flows from a region of high Pressure to a region of low pressure, it is the relative pressures in the atria, ventricles and arteries that will determine the opening and shutting of the valves.

Since the valves are passive structures and contain no contracting muscle, diseased valves may be replaced by mechanical valves (prosthetic valves) which have been specially designed with smooth surfaces to minimize the risk of blood clotting and red cell haemolysis.

With the onset of ventricular systole, the pressure in the ventricles starts to rise. As soon as it exceeds that in the corresponding atrium, the A-V valves will shut. This simultaneous closure of the mitral and tricuspid valves with the onset of systole can be heard by applying the ear to the chest wall of a subject (or by stethoscope). The sound heard may be likened to the word 'lub' spoken very softly. It is termed the first heart sound

The sound of the valvular closure may be augmented by the impact of the heart against the chest wall and by the noise produced by the contraction of the ventricular muscle fibres. It lasts for 0.15 seconds and the principal frequencies of the sound produced are in the range of 25-45 cycles/second.

The impact of the apex of the heart against the chest wall with each systo1e can be felt, and frequently seen, in the 5th left intercostal space. It is termed the apex beat. The intercostal spaces are named according to the rib that lies immediately above. Thus the 5th interspace lies between the 5th and 6th ribs.

The apex beat lies about 3-3. 5 in. from the midline The apex beat lies about 3-3.5 in. from the midline. A line through the apex beat to the midline passes through the middle of the clavicle (mid-clavicular line). As soon as the ventricular pressures exceed those in the aorta and the pulmonary artery, the aortic and pulmonary valves will open.

The opening of the valves does not produce any detectable sound The opening of the valves does not produce any detectable sound. (A clapping sound is produced when we bring our bands sharply together, but there is no sound when we take them apart again). During the short interval of time, between the closure of the mitral valve and the opening of the aortic valve, the left ventricle is a closed chamber.

At the same time the right ventricle will be closed off by the closure of the tricuspid and pulmonary valves. Blood is incompressible and although the contraction of the ventricular muscle is increasing the pressure in the ventricles there is no actual change in volume during this phase. It is known as the isometric contraction phase.

As soon as the aortic and pulmonary valves open, the ventricles decrease in size as the blood is ejected into the aorta and pulmonary artery. At the end of ventricular systole the pressure in the ventricles drops and the aortic and pulmonary valves close since the pressure in these vessels now exceeds that in the ventricles.

There is a short isometric relaxation phase during which time the ventricles are once again closed chambers, but as soon as the ventricu1ar pressure has fall to below that in the atria the mitral and tricuspid valves will open.

The closure of the aortic and pulmonary valves gives rise to the second heart sound. This is a shorter and sharper sound and has been likened to the word 'dup'. It lasts for 0.1 seconds. The principal frequency is of the order of 50 cycles/second.

Since systole is shorter than diastole the rhythm when listening to the heart is: 'LUB' 'DUP' 'LUB' 'DUP' I - II Pause I - II Pause with a shorter interval between the first and next second heart sound than between the second and next first sound.

Under suitable conditions a 3rd heart sound can be heard Under suitable conditions a 3rd heart sound can be heard. This is caused by the blood rushing into the ventricles during diastole. The third heart sound is probably due to vibrations of the mitral valve cusps since it is no longer heard in patients with prosthetic mitral valves.

The sound of atrial contraction is sometimes audible and when present is termed the A sound or the 4th heart sound. The contribution of each valve to the heart sounds is usually heard best at the sites shown in the Figure, known as the valve areas.

It follows that the 1st heart sound will usually be heard loudest at the mitral and tricuspid areas, whilst the 2nd heart sound will be heard loudest at the pulmonary and aortic areas.

Blood Flow Round the Circulation * With each systole 70 ml. of blood are ejected from each ventricle. * This quantity is termed the stroke volume. * The aorta and large arteries are elastic vessels and they accommodate this blood with a relatively small rise in pressure. During the ensuing diastole, there is no output from the heart, and the pressure in the arteries falls. The blood now in the arteries is pulsatile, and if an artery is cut, the blood shoots out in spurts.

* The blood flow to the tissues is maintained by the elastic recoil of the arterial walls, and by the time the capillaries have been reached, the flow has ceased to be pulsatile; the steady flow of blood through these vessels shows no variation in systole or diastole.

The blood returns to the heart along the veins, and the venous flow is a steady one. If a vein is cut the blood oozes out from the distal end without any pulsation. On approaching the heart, the flow again becomes pulsatile as the ventricles are unable to receive the venous blood during systole

Function of the Atria: *The blood returning to the heart during ventricular diastole passes through the atria and enters the ventricles. *The blood returning during systole is unable to do this as the A-V valves are closed. *The atria act as a storage reservoir for this blood until the end of systole when the A-V valves open. With the opening of these valves the ventricles fill rapidly with the waiting blood.

Further filling occurs as blood returns during diastole *Further filling occurs as blood returns during diastole. One-tenth of a second before the onset of systole, when the ventricles are already 70% full of blood, the atria contract and complete the filling of the ventricles by adding the remaining 30%. *Atrial contraction is not essential for life, but the heart is a much more efficient pump when the atria are contracting. *The flow of blood through the lungs and tissues is continued but the flow is intermittent through the heart.