The Body at Work: A Treatise on the Principles of Physiology

by Alex Hill

We have said that the heart is so formed that no vibrating fluid vein is produced when it is functioning normally. Murmurs are due to alterations in the valves which are visible after death. This statement needs modification. Not infrequently functional murmurs are heard, which disappear again after a time—in a few weeks, or even days, perhaps. The explanation of murmurs of this class is very difficult. They are heard most frequently in anæmic persons, and appear in these cases to be due to the heart having shrunk, owing to the blood in circulation being deficient in quantity, until the cavities of the ventricles have a smaller diameter than that of the great arteries into which they expel their contents.

Such is the explanation of the physical cause of murmurs given by Chauveau and Marey, the physiologists who have paid most attention to this subject. But it must be remembered that the valves which, when diseased, are the sources of the murmurs are membranous structures. It may be that fluid veins would be produced by them if they were rigid ledges which jutted into the blood-stream; but, being membranous, they are capable of vibration. Certain physicists are of opinion that a murmur is caused, not by the vibration of a fluid vein, as such, but by the vibration of the membranous structure which impedes the passage of the fluid. The physics of the problem is of little consequence to the physician. The murmur is produced at the spot where a diseased valve is situated, and is propagated forwards. It enables him to ascertain with accuracy what is amiss with the heart.

Bloodvessels.—The greater circulation occurs through a closed system of vessels which unite the left ventricle with the right auricle. The aorta gives off lateral branches. Its branches branch. Subdivision continues until the vessels are just wide enough to allow blood-corpuscles to pass in single file, or but little wider. When a bough of a tree divides, the united cross-sections of its twigs, their soft bark being stripped off, may be a little larger than the cross-section of the bough; but the disparity is usually small. The united cross-sections of the smaller arteries is considerably greater than that of the trunks which give origin to them. By the time the capillaries are reached, their total bed—their united cross-section—is about 640 times as great as that of the aorta. This estimate is based upon the diminution in the rate at which blood flows through the vessels. The velocity with which a stream flows through a channel varies as the cross-section of the channel. In a capillary vessel the blood flows at the rate of from 0·5 millimetre to 1 millimetre per second. In the aorta the velocity is about 320 millimetres per second. In the re-formation of the venous system a converse process of reduction occurs, but not with anything like the same rapidity. The united calibre of the two venæ cavæ, in which the reduction is complete, is about twice that of the aorta. From this it follows that the veins hold much more blood than the arteries; and since veins are more easily distended, the amount that they can hold varies within wide limits. They constitute to some extent a reservoir for blood.

The capillary vessels are the tubes of the circulatory system in which blood comes into use. On the average they are about 0·5 millimetre long. Through them the blood flows slowly. Through their walls alone is there any exchange worth mentioning between the blood within the vascular system and the lymph by which it is surrounded. Interest therefore centres in these vessels. Their walls are formed of endothelial tiles. In the centre of each thin transparent tile is a boss, where its lens-shaped nucleus is situate. The outline of the tile is sinuous. Its margin dovetails with the margins of those adjacent to it. Oxygen and carbonic acid, nutrient substances and waste products, pass rapidly through the endothelial cells. Leucocytes have the power of pushing the cells aside, in order that they may make their way out of the blood into the lymph which fills the tissue-spaces. With the exception of the lens and cornea of the eye, cartilage, and the various epidermal structures, all tissues are traversed by capillary vessels. It is not difficult to calculate the number of such vessels in the body exclusive of the liver and the lungs. The diameter of the aorta is 28 millimetres, that of a capillary about 0·008 millimetre. The cross-section of all the capillaries added together is 640 times that of the aorta, as already stated.

Many schemata have been devised to illustrate the vascular system; but all are misleading, inasmuch as they fail to give any idea of the extent to which the subdivision of its vessels is carried. If the water-pipes supplying a town branched until the original conduit was represented by five to six thousand million little pipes, the friction which the pumping-station would have to overcome would be very great. But little force would remain in the water when it reached the smallest pipe. Still greater is the resistance to the flow of blood, which is slightly viscous, and contains solid corpuscles, which increase friction. Two thousand miles of capillary tubing in the body of a man, without reckoning the vessels of his liver and lungs!

Fig. 12.—A Portion of the Wall of a Small Artery cut transversely and highly magnified.

Its inner coat consists of a lining sheet of epithelial scales supported by connective tissue and a strong elastic membrane. This membrane is perforated with holes which place the lymph-spaces on its two sides in continuity. The middle coat is composed of plain muscle-fibres and patches of elastic membrane; the outer coat of elastic fibres, mostly longitudinal, and connective tissue.

Water is supplied to houses in rigid tubes. Arteries are elastic, and their elasticity is self-regulating. The cause of this will be apparent if a section of an artery is examined. It contains much elastic tissue. It also contains plain muscle-fibres. The smaller the artery, the greater is the amount of muscle relatively to the other constituents of its wall. The wall of a vein contains very little muscle, and not much elastic tissue. The muscle of all arterial walls is in a chronic state of tone. To some extent the degree of tone is varied automatically. Pressure within an artery acts as a stimulus to the muscle-fibres of its wall. Any increase leads the fibres to contract more strongly. Any diminution induces them to relax. The arteries resist distension; they do not narrow to any great extent when pressure falls. But more important than this automatic mechanism for maintaining a uniform pressure in the capillaries in general are the changes of pressure in particular localities, brought about by the mediation of vaso-constrictor and vaso-dilator nerves. In almost all organs and parts of the body the automatic tone of arteries is enhanced by impulses which flow continuously down vaso-constrictor nerves. These impulses start from, or, to speak more accurately, pass through, the vaso-motor centre in the medulla oblongata. From every part of the body impulses ascend to this centre, urging it to keep up the blood-pressure by universal constriction. Yet no separate organ would be interested in sending such a message if it were not open to it to ask at the same time that the constriction of its own vessels might be relaxed. Hence it may be said that every individual in the community is crying out for universal economy, with more generous treatment of himself. The response made by the State to the latter part of his demand is in proportion to the vehemence with which it is presented.