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Circulationofthe Blood

Circulation of the Blood. The course of the circulation in mammals (and birds). Arterial blood coming from the lungs reaches the left auricle by the pulmonary veins; from the left auricle it is conveyed into the left ventricle, and thence is forced by the ventricular contraction into the aorta. In its course from the left ventricle to the right auricle, the blood completes the systemic part of the circulation. The aorta and its branches, by repeated division and subdivision, merge into the capillary network of the several parts and organs of the system; and the blood of the capillaries is gradually collected again by the veins and conveyed back to the heart. From the right auricle the next step is to the right ventricle, and then the blood is forced into the pulmonary artery. On the completion of the pulmonary part of the circulation, the circle is completed. The systemic and pulmonary parts of the circulation are sometimes called the greater and the lesser circuits respectively.

The right and left auricles contract simultaneously, and so do the right and left ventricles; so that at the same instant that the contents of the left ventricle are discharged into the systemic vessels, the contents of the right ventricle are propelled into the pulmonary vessels. The amount of blood discharged at each ventricular contraction is in an adult about four ounces, and it has been calculated that the complete round of the circulation can be made by substances injected into the blood-stream in about two minutes. In the greater and lesser circuits, as already described, only one system of capillaries is traversed by the blood, on its way from left ventricle to right auricle, or from right ventricle to left auricle. Under certain circumstances, however, there is a double capillary system interposed in the path of the blood from the left ventricle to the right auricle. Thus, the veins of the stomach and intestines receive the contents of the capillaries and join together to form the vena porta, and then this great vein, instead of directly proceeding to the heart, breaks up again into a second capillary system in the liver. This peculiar portal circulation, as it is called, will be described in connection with the liver (q.v.). Again, in the kidney (q.v.), there is a double capillary system.

The facts concerning the circulation of the blood as just detailed have only been made out within quite recent times. It was supposed by the ancients that blood was carried from the heart to distant parts of the body by the veins; while the arteries, being found empty after death, were credited with conveying some kind of air or vital spirit to the tissues. Galen first showed that the arteries did actually contain blood, in addition - as he maintained - to the vital air. Communication between the right and left sides of the heart he supposed to be brought about by minute invisible openings in the septum between the ventricles. These doctrines were generally accepted for many centuries. The first step in advance was made by Servetus. This anatomist, writing in 1553, suggested the possibility of a means of communication existing in the lungs between the blood of the pulmonary artery and that of the pulmonary veins. It was, however, William Harvey who first conceived the idea of the circulation of the blood, and to him is due the honour of demonstrating the same by a series of masterly experiments and deductions. Harvey conclusively showed that the blood in the veins and in the arteries is the same blood; that the connection between the right and left sides of the heart is not by direct openings in the septum, but that all the blood of the right ventricle is sent through the pulmonary arteries, by way of the lungs, to return to the left auricle by the pulmonary veins; while, similarly, the whole of the blood of the left ventricle is returned by the great veins to the right auricle. In Harvey's great work - Exercitatio de Motu Cordis et Sanguinis - published in 1628, he describes his observations on living animals, and the experiments which he conducted to show the effect of blocking the blood-stream in particular situations by ligatures or by compression. He shows that the heart is a muscular organ, points out that the arrangement of the valves is such that blood can only flow in one direction, and demonstrates the same principle in the case of the valves of the veins. Lastly, he adduces the effect of poisons introduced at a single point upon the system generally, as an additional proof of the truth of his contentions. The advances made by science in the last twenty-five years have abundantly confirmed the truth of Harvey's ideas; in particular, the employment of the microscope has supplied the means of tracing by actual observation the course of the blood through the capillary network. Thus, to-day, the merest tyro can witness in the web of a frog's foot the marvellous race of blood corpuscles through the small arteries, their distribution into, and slower progress through, the capillaries, and their gathering together again in the venules. It is difficult for the modern student to understand how so obvious a condition of things should have remained undiscovered for so many hundreds of years, and the greatness of Harvey's discovery is well nigh overlooked in the confusion of its abundant confirmation.

Physiology of the Circulation. It is now necessary to discuss the circulation from a mechanical standpoint, and to inquire into the several conditions of pressure and of velocity of flow which obtain at different points.

The three factors which are of first importance are: the force and frequency of the heart's beat, the elasticity of the arteries, and the capillary resistance. The heart's action is intermittent; each complete cardiac cycle occupies about 8/10 of a second. With each ventricular systole a fresh amount of blood is propelled into the arteries, the capillaries offer a certain resistance to its onward flow, and hence the result of the repeated injection of fresh quantities of fluid is to bring about a condition of distension of the highly elastic coats of the large vessels. But the arterial walls being ever on the stretch, are ever tending to contract upon their contents; hence, during the period when the left ventricle is passively dilating, the arterial tension must make itself felt, and tend to drive some of the blood out of the large arteries. It cannot drive it back into the heart, the semilunar valves excluding the possibility of a backward flow; therefore, it must drive it onward. In other words, the flow of blood through the capillary network is not affected by a series of forward movements corresponding with each emptying of the left ventricle, but takes place during the dilation as well as during the contraction of the ventricle. The flow in the capillaries is, in fact, continuous and not intermittent.

The pressure of the blood in the large vessels has been investigated by means of the mercurial manometer. An artery or vein is exposed and the flow of blood through it controlled by pressure forceps, a canula, i.e. a suitably-shaped tube, is then introduced (in the case of an artery, so that the pressure forceps lies between the heart and the canula opening) and connected by a leaden pipe with a U-shaped tube containing mercury. The apparatus is filled with a saline solution, so as to obviate escape of blood from the artery into the system of tubes with which it is now continuous. Moreover, the mercury in the far limb of the tube is raised by pressure to a height such as it is judged will be necessary in order that the column of mercury supported will just about correspond with the pressure of the blood. It is clear, then, on removing the pressure forceps, and placing the blood inside the artery in direct continuity with the fluid in the canula and the leaden tube, that the variations in pressure inside the vessel will be represented by oscillations in the column of mercury. If a piston attached to a marking point be allowed to float on the free surface of the mercury, the marking point will oscillate and may be caused to record its oscillations on a revolving drum. In this manner tracings representing the alterations of level of the manometer column have been obtained, and such a tracing is called a blood-pressure curve.

It is estimated that the average pressure in the carotid artery of man is about equivalent to the weight of a column of mercury six inches high. The blood pressure rapidly diminishes on proceeding towards the periphery, and still further diminishes on passing from the capillaries to the veins. Indeed, in the large veins near the heart the pressure is very low.

The velocity of the blood, on the other hand, while it is greatest in the large arteries and decreases towards the capillaries, begins to increase again on passing from the capillaries to the veins. This, of course, arises from the fact that, the velocity depends upon the calibre of the channels through which the blood is flowing. Cooped up within small dimensions in the aorta, the blood flows very rapidly (about one foot per second in man); as it approaches the capillaries, it is distributed into multitudinous channels, which, though minute individually, are collectively of much greater sectional area than the main artery from which they originate. Hence, in the capillary network, the movement of the blood may not exceed a rate of one inch per minute. When, however, the capillaries merge into veins, the gross sectional area diminishes again and the velocity increases.

It must not be imagined that the blood pressure and the rapidity of flow in a particular vessel are fixed quantities. On the contrary, in accordance with the requirements of the parts and organs of the body, modifications are effected in the local or general circulation. These modifications are brought about through the mediation of the nervous system.

In certain organs local peculiarities in the circulation are recognisable, The liver and kidneys have already been alluded to, and will be fully dealt with under the articles devoted to their consideration. In erectile structures special conditions obtain, the venous sinuses permitting of great distension by increased afflux of blood. The capillaries of the lungs again present peculiarities. Lastly, the circulation in the brain differs in several respects from that in other parts of the body. The large arteries supplying the brain, passing as they do through bony canals, are incapable of becoming much distended; moreover, by their tortuous course and free anastomosis the impetus of the blood in them is modified and uniformity of blood supply ensured. Again, the large venous sinuses of the cranial cavity present unique characters.