The Heart and Circulation
The heartbeat, originating as a contraction wave at the S. A The heartbeat, originating as a contraction wave at the S.A. node, spreads rapidly through the atrial muscle causing both atria to contract simultaneously.
The blood in the atria is forced through the atrioventricular valves into the ventricles. Rings of cardiac muscle around the entry of the superior and inferior venae cavae, and the pulmonary veins, close off the veins with a sphincter-like action so that blood is not regurgitated back into the veins when the atria contract.
The spread of the contraction wave through the cardiac muscle ceases at the fibrous septum between the atria and the ventricles which contains the four heart valves [plane A-B].
The only pathway through this non-conducting septum is from the atrioventricular node (A.V. node) down the atrioventricular bundle (bundle of His) named after the German physiologist Wilhelm His. This bundle runs into the ventricles in the septum between the right and left ventricles.
The passage of the contraction wave down this bundle of modified cardiac muscle is not visible from the surface of the heart, and there appears to be a slight pause following the atrial contraction, The contraction wave enters the ventricles near the apex and spreads upwards towards the base. The blood in the ventricles is forced upwards towards the base of the heart and out through the aortic and pulmonary valves.
Pericardium: The heart lies in a conical sac known as the pericardium. This consists of an inner serous pericardium and an outer fibrous pericardium. The inner serous pericardium is composed of two smooth layers, the visceral and parietal layers, one attached to the heart and the other to the fibrous sac. These two smooth layers allow the heart to beat in the mediastinum of the thorax with the minimum of friction.
The pericardium: 1 - Sets a limit to the maximum size of the chambers of the heart. 2 - Prevents excessive stretching of the cardiac muscle fibres due to overfilling with blood. The pericardium is attached to the diaphragm, and when the heart beats, it behaves as if the apex were relatively fixed.
Thus when the ventricles contract, instead of the apex moving upwards towards the base, the base, and particularly the atrioventricular ring, descends towards the apex. This has the effect of increasing the size of the atria at the same time as blood is ejected from the ventricles.
Cardiac Cycle-The Heart as a Pump: The sequence of events may, therefore, be summarized as follows: The heart beat originates in the S.A. node, and shortly afterwards the atria contract. This is followed by a short pause whilst the contraction wave is moving down the bundle of His. Then the ventricles contract, the atrioventricular ring moves downwards and blood is ejected into the arteries. The ventricular muscle relaxes and the atrioventricular ring returns to its initial position. There is then a long pause, when all chambers are relaxed before the next beat occurs.
60/0.8 ~70 cycles/minute. This is the heart rate. The ventricular contraction phase is termed systole (pronounced sis'to.lee). It lasts for 0·3 seconds. The ventricular relaxation phase is termed diastole (pronounced dye.ess'to.lee) and this lasts for 0.5 seconds. The complete sequence of events, the cardiac cycle, lasts for 0.8 seconds, so that there are: 60/0.8 ~70 cycles/minute. This is the heart rate. The heart beats continuously for the whole of a person's life and its only period of rest is after each contraction, during diastole which becomes shorter.
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.