THE JOURNEY OF THE BLOOD THROUGH THE BODY

BEFORE the inter-relationships and anatomy of the various organs of the body can be fully understood a working knowledge of the system which carries blood and nourishment to every part of the body must be discussed. The vascular system comprises the whole of this apparatus, and in it are included the heart, the arteries which convey pure, nourishing blood from the heart, the capillaries (or fine veins) where the nourishment is given off to the tissues, the veins which convey blood back to the heart, and a specialised system of vessels called the lymphatics. The system is really a closed circle, or rather two circles, as we shall see later, with the heart at the centre driving blood round and round.

If we start with the impure blood returned to the heart by the veins we find that it passes successively through two chambers situated in the right side of the heart and thence by the pulmonary artery to the lungs where it is purified and saturated with oxygen. After leaving the lungs it passes by the pulmonary veins back to the heart, where after traversing two chambers in the left side of the heart it leaves by the great artery called the aorta and finally reaches the tissues by passing through the smaller arteries. After supplying the tissues with oxygen it becomes impure and is carried back by the veins to the heart and lungs for purification.

It will be seen that in the greater part of the body the arteries contain pure blood and the veins impure blood. The arteries may be compared with the clean water supply of a town, while the veins are the sewers. In the lesser circulation—that is the part in which the lungs are interposed— the reverse is the case, for the pulmonary artery contains the impure blood and the pulmonary vein the pure blood. After this general discussion we can now start on a consideration of each separate part.

THE ROUTES BY WHICH BLOOD TRAVELS TO THE HEART IF we start with the capillaries to which blood is brought by the arteries, the microscope must be used, for they are exceedingly small. Every organ in the body is riddled with these tiny tubes which are the means of nourishing the tissues.

Their walls are excessively thin, for they are composed of flat cells which lie only one cell deep so that a kind of pavement is formed, so delicate that nourishment easily passes through it. Further on the capillaries join together, forming larger vessels which in their turn join until eventually quite a large vein is formed. Veins from all parts of the body converge upon the heart, becoming larger and larger as more tributaries enter them. Many of these veins have special names, but only a few need be mentioned here.

Blood coming to the heart from the head is carried by the jugular veins, internal and external. These join on each side with veins from the arm called the subclavian veins, forming the innominate veins, right and left. Later the right and left innominate veins join together to form the vena cava superior or great vein of the upper part of the body, which directly enters the heart.

From the lower part of the body a vein from each leg— the femoral vein—after passing into the abdomen from the thigh and receiving many tributaries, joins with its fellow of the opposite side to form the inferior vena cava or the great vein of the lower part of the body. This vein lies in front of the vertebral column in the posterior part of the abdomen and runs directly upwards until, after receiving impure blood from the liver, it pierces the diaphragm and immediately enters the heart. We have now, by devious routes, brought all our blood back to the heart, so we must next proceed to describe this organ in detail.

WORK IN THE PUMPING-STATION OF THE HEART THE heart is divided into two parts—right and left—by a partition or septum which runs down the middle. Each part is further subdivided into two cavities called auricles and ventricles. The auricles are thin-walled muscular chambers which receive blood from the veins and pass it on to the ventricles. The ventricles are thick-walled and contain very strong muscles which form the main pumping-station of the heart and drive the blood all over the body.

Starting with the right auricle, this cavity has two large openings in its right wall, namely the superior and inferior vena; cava;. In addition there is a small opening—the coronary sinus—which conveys venous blood from the heart itself back to the right auricle. After passing through the auricle the blood enters the right ventricle through the valve called

the tricuspid valve. This structure is interposed between the auricle and the ventricle in order to prevent the blood regurgitating back into the auricle when the ventricle contracts.

The valve is composed of three thin membranous cusps which meet accurately at the centre and form a completely watertight joint when the valve is closed. Each cusp is like an umbrella with the convex surface upwards so that blood can flow freely over it when the auricle contracts, but concave on its ventricular surface so that, under pressure, the ventricular blood catches under it and closes it. The working of the valve is purely automatic and is under no sort of control. It depends solely on the difference in pressure in the two chambers. The sequence of events is as follows :

The auricle is distended with blood entering it from the veins; it then contracts and the pressure rises sufficiently above that in the ventricle to force the valve open. The blood then passes into the ventricle which, in its turn, contracts, and when the pressure inside it rises sufficiently high the valve automatically closes, preventing the blood from flowing back into the auricle.

It might be thought at first sight that the valve would itself be turned inside out under the pressure. Nature has guarded against this by providing thick strands of fibrous and muscular tissue which stretch from the wall of the ventricle to the under surface of the cusps. The umbrella shape of the cusps and their inelastic nature also tend to prevent this movement which would be so disastrous.

Having been filled with blood by the contraction of the right auricle, the right ventricle contracts and drives the blood through another valve into the pulmonary artery which leads to the lungs. The muscular wall of the right ventricle is much thicker than that of the auricle, for it has to pump blood all round the lungs. Its thickness, therefore, is a response to the greater amount of work it must perform. The pulmonary artery divides into ever smaller and smaller vessels until the capillaries lying in the walls of the air sacs of the lungs are reached. Here the blood is re-oxygenated and collected into the pulmonary veins which lead the blood back to the heart by entering the left auricle and pouring the purified blood into its cavity.

The cavities of the left auricle and ventricle show no very marked differences from those on the right side of the heart.

As might be expected, however, the wall of the left ventricle is many times thicker than that of the right, for whereas the right ventricle pumps blood only through the lungs, the left must be sufficiently strong to force it through the whole of the remainder of the body.

THE VESSELS THAT SUPPLY BLOOD TO THE BODY THE left ventricle pumps blood under pressure into the aorta, which is the great main artery that leads blood from the heart and delivers it to the whole of the system. Just from the point where the aorta leaves the heart arise two arteries of moderate size which are two of the most important arteries in the body, for they supply the heart. They are known as the coronary arteries, right and left, because they run round the top of the heart like a crown. Should one of these arteries become suddenly blocked death will ensue immediately. This is the cause of death in those cases where a man suddenly falls down dead in the middle of the street. Blockage of certain arteries in the brain will cause immediate unconsciousness and death at a later date, but there is nothing which will strike a man down and kill so rapidly as blockage of the arteries to the heart. It is indeed a sudden visitation of the Angel of Death.

After giving off branches which supply the arms (the subclavian arteries) and the head and brain (the carotid arteries) the aorta turns downwards and, passing down the back of the chest in front of and to the left of the vertebral column, it enters the abdomen. Here it gives off large branches which pass to supply the liver and intestines, and eventually the main stem divides into two branches which pass one to each leg, and are known as the femoral arteries.

THE PULSE : THE HEART’S SECOND GEAR

PERHAPS the best-known artery of all is the radial, for it is from this artery that the pulse rate is commonly estimated by doctors. It lies on the thumb side of the wrist and anyone can easily feel the pulsations that are transmitted to it from the heart. The value of the pulse can at once be seen when it is realised that it gives an immediate indication of the rate at which the heart itself is beating. Whenever the heart is working under difficulties, when it is itself diseased or when it suffers with the body in general during fever, it goes,

as it were, into second gear and beats more rapidly. At each beat it need not pump out so much blood, as it is working faster. The result is exactly the same as that attained when a car goes uphill in second gear. The work done eventually is the same, but it is done with less strain on the engine. The normal pulse rate of an adult is about 70 to 80 beats per minute. Anything over 90 is usually an indication that the heart is in difficulties and must be rested by keeping the patient in bed.

The larger arteries that we have mentioned so far divide continually until finer and finer branches are reached. These lead eventually into the capillaries where the blood is at last made use of. This continual division into smaller vessels means that when the blood comes to be used the surface of the blood that is available for the interchange of nourishment with the tissues is enormously increased. Some enthusiast has worked out that the surface which the blood presents in the capillaries of a medium-sized muscle, such as the biceps, corresponds to something like ten whole pages of The Times.

THE VESSELS THAT CARRY FOOD TO THE LIVER w

E have now discussed the circulation in general, but we have yet to mention a small but important system of vessels known as the portal circulation. In the section on Physiology we shall see that food, after it has been digested, is absorbed into the capillaries of the intestines. These capillaries, as they do everywhere else, gather themselves into veins which form eventually a large vein known as the portal vein, which enters the liver. This vein conveys to the liver, which is the storehouse and factory of the body, the blood which contains the food.

The portal vein breaks up into capillaries again inside the liver and blood comes in contact everywhere with the liver cells which extract the food and make it up into new products. The blood is then re-collected in a further set of veins which enter the inferior vena cava, and from there it soon reaches the right side of the heart.

The liver, therefore, is an unusual organ, for it has three sets of blood vessels whereas other organs have only two. Arterial blood brings oxygen which enables it to live, and the veins carry off the impure blood. In addition the portal vein brings food from the digestive tract which serves the special functions of the liver.

THE LYMPHATICS: THE FIRST LINE OF DEFENCE BESIDES the blood vessels there are small vessels which carry a watery fluid known as lymph. This serves quite a different purpose from that of the blood. Lymph spaces lie between the cells all over the body and they communicate with tiny vessels which collect the lymph and pass it on eventually into the veins. The lymph vessels open into a large lymphatic channel known as the thoracic duct which lies in the chest and which passes up into the neck and discharges its contents into the subclavian vein where they enter the general blood circulation. Some of the lymph vessels that enter the thoracic duct come from the intestines and they carry the fat that has been absorbed during digestion, so one function of the lymphatics is to convey food into the blood stream.

A further reason for their existence is to protect the body from the invasions of germs or bacteria. Before they enter the subclavian vein, all the lymphatics have to pass through a series of filters known as lymph glands which strain off any foreign bodies or germs which may be present in the lymph and destroy them. Thus, if a few germs gain entrance to the body through a small cut on the foot, the germs will be carried into the lymphatics and held up at the groin where the main lymph glands are situated. If the glands are sufficiently strong they will destroy the bacteria, but if the germs are numerous and virulent they may cause inflammation of the glands which will then become enlarged and inflamed or may-even form an abscess.

WHAT A SWOLLEN LYMPH GLAND SUGGESTS THE lymph glands do their best, and even if the germs are not destroyed or an abscess forms, the glands have localised the infection and prevented it from spreading all over the body where it would be much more dangerous. A swollen lymph gland will always suggest that there :s some sore in the region which the glands drains. Thus, swollen glands in the neck are often the result of inflamed tonsils, swollen glands in the armpit are caused by sores on the arms or breast, and swellings in the groin arise from trouble in the feet or legs. Swellings in the groin can easily be felt under normal circumstances by anyone who searches for them, for they are much larger than the glands in any other part of the body, and are the only ones that are large enough to be felt in the ordinary way.

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