IN the section on Anatomy, we learned that the heart consists of four muscular chambers, and we considered the main points in the anatomy of the circulation as a whole. Here it is our business to discuss the mechanisms by which the circulation is kept going and under what circumstances it varies.
If we cut out the heart from an animal immediately after death, we shall find that it will continue to beat rhythmically for a short period of its own accord. If we now supply it, through the openings of its vessels, with a nutrient fluid, we can keep it going for a longer period, and we can study it as simply as we could study any other purely mechanical machine.
We shall find that the heart of itself has an inherent power
of beating rhythmically and that the direction of the blood flow through the heart is always the same and is determined by the valves. The first thing that happens is that the auricles fill from the veins and then contract, driving the blood through the auriculo-ventricular valves into the ventricles. Both auricles contract simultaneously. One-tenth of a second later the ventricles contract, again simultaneously, and drive the blood into the arteries. Regurgitation of blood, as we have seen, is prevented by the valves.
LAWS THAT THE HEART MUST OBEY
THERE are two ‘laws ‘which govern the working of the heart as a machine and which are of the greatest importance and interest. The first is that each muscular fibre of the heart always contracts to the very best of its ability at any one moment. A fibre can give only one kind of contraction at any moment, namely the maximal. A fibre contracts completely or not at all. This is known as the ‘All or None Law.’ The maximal contraction will vary from time to time according to circumstances, but each contracting fibre is always bound to do its best under the circumstances existing at the moment.
The second law is known as ‘Starling’s Law of the Heart,’ and states that within certain limits the longer a muscle fibre becomes, owing to stretching, the better and more forcibly it will contract. The best that a fibre can do is increased if the fibre is lengthened.
The vast importance of this is that, if a larger than usual amount of blood comes to the heart via the veins, the heart will be unusually distended with blood and its muscle fibres will be lengthened. They will therefore be capable of more vigorous contraction, and the heart will consequently be able to pump out the extra amount of blood without becoming dilated or actually bursting.
It is a defence mechanism, preventing the heart from being disabled and automatically ensuring that the heart responds at once to the work required of it. In exactly the same way if the pressure in the arteries is increased for any reason, as in exercise, the heart must do more work in pumping out blood against a greater resistance. Temporarily blood is dammed back into the heart, but immediately the fibres become lengthened owing to the distension by the blood dammed back, and they contract more forcibly enabling the heart to cope with the increase in work required of it.
THE NERVES THAT CONTROL OUR HEART-BEATS IN the wall of the right auricle is a collection of nerve cells which start the contractions of the heart. From this point the contraction spreads over the heart like a wave. It is by virtue of this collection of cells that the heart is enabled to beat when it is cut out of the body. These nerve cells are known as the sino-auriculo node, and in the body they are controlled by two nerves coming from the central nervous system—one, the vagus, from the brain; the other, the sympathetic, which comes eventually from the spinal cord.
These nerves influence the rate of the heart. This can be proved if the nerves are cut one by one. If the vagus is cut, the heart will beat more rapidly, for its slowing influence is removed. If the cut end which leads to the heart is stimulated by an electric current, the heart will beat more slowly. If the sympathetic is cut, the heart will beat more slowly, and if its cut end is stimulated, the heart will beat faster. The vagus, therefore, slows the heart and the sympathetic quickens it.
In the walls of the great veins entering the heart are certain nerve fibres which are sensitive to stretching when the veins are dilated by a large volume of blood. These nerves collect themselves into a bundle of fibres, which pass to the central nervous system and form connection with the vagus and sympathetic. They subserve a reflex known as Bainbridge’s Reflex, and their function is to drive the heart faster in order to cope with an extra amount of blood coming to it. When the veins are engorged, these nerves are stretched and impulses pass to the nervous system which inhibit the vagus and stimulate the sympathetic so that the heart beats faster.
On the other side of the heart, in the wall of the aorta, are some nerve fibres which join to form the depressor nerve which subserves the depressor reflex. If the pressure in the aorta is increased, these nerve fibres are stretched and impulses pass up them to the nervous system, where the vagus is stimulated and the sympathetic inhibited so that the heart is slowed. This, of course, is a defensive mechanism to prevent the blood pressure from becoming too high. Both these reflexes act purely automatically, of course, and we know nothing about it when they are brought into play, for their activity never comes into consciousness.
HOW THE BLOOD IS SENT BACK TO THE HEART AT first sight it would seem difficult to understand why blood returns to the heart from the capillaries without anything to drive it. The heart itself drives the blood to the capillaries, but there is no heart to drive it back. There are three things that assist the return by the veins, the first being the fact that there are valves interposed in the course of the veins, preventing the blood flowing backwards—away from the heart. This can be demonstrated quite clearly by choosing any vein which stands out sharply in the arm and running a finger firmly down it towards the hand, finally compressing the vein with the finger so that no further blood can enter it. The blood will flow backwards from above until a valve is encountered, where it will be held up. The vein below the valve, and between it and the finger, will remain quite empty so long as the pressure is kept up.
The flow of blood back to the heart is greatly assisted by the contraction of the muscles which under normal circumstances is continually taking place. The muscles squeeze the blood along the veins and help to counteract the influence of gravity. The third factor of great importance is the negative pressure—I.e. pressure less than atmospheric—which exists in the thorax and which is greatly increased when we inspire. This negative pressure which surrounds the great veins entering the heart is constantly tending to suck blood into them, thus returning it to the heart.
WHAT HAPPENS BETWEEN HEART-BEATS
THE pressure which the blood exerts in the arteries is, of course, dependent on the amount of blood which the heart pumps out on each beat. If the arteries were rigid tubes, this pressure would fall and rise with every beat of the heart, or would fall almost to zero between each beat when no blood was leaving the heart or entering the arteries. Nature has guarded against such an untoward result by making the arteries elastic, so that they expand when blood is forced into them and store up the blood.
When the heart finishes its beat and is resting in preparation for the next, the arteries rebound by their elasticity, so that the pressure is kept within reasonable limits and blood is forced into the capillaries in a continuous stream, instead of in an intermittent one. In old age the arteries become dc-
generate and hardened and lose their elasticity, so that the blood pressure becomes higher and fluctuates more when the heart beats.
THE COMPLICATED PROCESS OF RUNNING A RACE w
E have seen briefly how the heart works by itself and how it works when controlled by its nerves. In order to see it in action in relation to the body as a whole, let us see what is the effect upon it of exercise, say the running of a strenuous hundred-yards race. The muscles all over the body, but especially those in the legs, are working much harder than normal, so that they will require a greatly increased blood supply in order to enable them to work efficiently. This extra blood must be supplied by the heart which pumps blood round the circulation more rapidly.
Because the muscles are working and contracting, they will squeeze more blood back to the heart, and this effect will be helped by the increasing rate and depth of breathing, which will suck the blood back into the thorax. Respiration is increased, of course, so that the blood may become more oxygenated and therefore be of more use to the hard-working muscles. The increasing venous return, both because it elongates the muscle fibres of the heart and because it sets the Bainbridge reflex in operation, results in the heart doing more work and throwing out more blood into the circulation at each beat.
Meanwhile the suprarenal glands secrete adrenalin into the blood stream, and this substance increases the force of the heart beat, adding its support to all the other mechanisms. Adrenalin, however, has quite a different action as well, for it has the peculiar property of making the blood vessels of the structures inside the abdomen contract and squeeze out their blood, which then becomes available for a more important purpose. Adrenalin ensures that no blood is wasted in supplying organs that do not need it at the moment. It sends it to organs whose need is greater.
Such are the provisions that Nature makes to enable us to work without undue strain being put upon our hearts or muscles. Such a system works to perfection so long as the strain is not too great. The heart, however, being a vital organ, must be protected from overstrain, and it will always happen that the muscles will become too tired to work before any dangerous strain is placed on the heart. Work will always
cease and allow of a breathing space before the vital structures are endangered.
It follows from this that a person with a perfectly healthy heart cannot possibly strain it by doing too much exercise, whether by road-breaking or by playing a hard game of football. It is only in those cases where the heart is already diseased and where the staying power of the muscles exceeds that of the heart that any damage can be done. The muscles may then outwear the heart and subject an already diseased organ perhaps to breaking strain. This is the reason why people who really suffer from heart disease must be extremely careful in the exercise they take. But the heart is far less easily damaged than is generally supposed.