VASCULAR SYSTEM 107 < lillarv ions (b) Venous Pressure. (1) In the veins near the heart the pressure ssure. is only one twentieth to one-tenth of that of the corresponding arteries. (2) During auricular diastole the pressure in the veins near the heart may become negative ( = - 0039 inch of mercury). (3) There are no periodic variations of pressure in the veins as in the arteries, except in the great venous trunks in the neck and near the heart, where there is a diminution of pressure during auricular diastole and an increase during auricular systole. (4) Great activity of the heart diminishes venous, while it increases arterial pressure. (5) The pressure increases in the veins according to their distance from the heart : thus, in the external facial vein of the sheep it is equal to a column of mercury 12 inch in height ; in the trachial to a column 16 inch high ; in branches of the trachial, 35 inch ; in the crural vein, 45 (H. Jacobson). (6) Plethora increases venous pressure, whilst anaemia diminishes it. (7) Inspiration causes in the great veins near the heart an increase in pressure, whilst expiration diminishes it ; but the respiratory movements "do not affect the venous stream in peripheral veins" (Stirling). (7) Changes in the position of the limbs affect venous pressure hydrostatically : thus, elevation of the extremities favours the flow of blood towards the heart ; but, if the heart hangs down wards, the face becomes turgid, as the outflow by the veins is retarded. (8) Gravity favours the emptying of descending and hinders the emptying of ascending veins, so that the pressure be comes less in the former and greater in the latter. (9) As already stated, muscular movement by compressing the veins, aided by the mechanism of the valves, favours the flow of blood towards the heart, and thus increases the pressure in these vessels. (c) Capillary Pressure. For obvious reasons capillary pressure v ssure. has not been directly measured. Von Kries has measured the amount of pressure necessary to occlude the capillaries in an area abounding in these vessels, such as the skin at the root of the nail on the terminal phalanx or on the ear in man, and on the mucous membrane of the gum in rabbits. He found the pressure in the capillaries of the hand, when the hand is raised, to be equal to 95 inch of mercury, when it hangs down, 2 13, in the ear 79, and in the gum 1*26. Roy and Graham Brown also measured the pressure necessary to close the capillaries in the web of the frog s foot, in the tongue and mesentery of the frog, and in the tail of newts and small fishes. It is evident that any condition favouring the afflux of blood to a capillary area will increase the pressure in the capil laries, such as the dilatation of the small arterioles conveying blood to an area of capillaries, contraction of the venules carrying off the blood from it, or any increase of pressure in the arterioles or in the venules. The arrangement and position of the capil lary network must affect the pressure : the pressure in the capillaries of the glomeruli of the kidney must, for in stance, be greater than in those of the skin, as in the former position there is increased resistance owing to the double set of capillaries (see NUTRI TION, vol. xvii. p. 684). Finally, any Fl - 23 Diagram showing pres- 1 ii i c / J f sure in vascular system. 1, Ven- change in the degree of contraction of tricle . 2j arteries . ; E , D, capil- the wall of the capillary itself will affect the pressure. The general facts regarding pressure in arteries, capillaries, and veins are illustrated by fig. 23. locity Various attempts have been made blood, by Volkmann, Vierordt, Ludwig and Dogiel, Hering, and Chauveau and Lortet to measure the velocity of the circulation, and special instruments have been invented for that purpose. In 1850 Volkmann constructed the hfcmadromonieter ; this was followed by the hoamatachometer of Vierordt in 1858 anil by the h;emadromograph of Chauveau and Lortet in 1867. These instruments are not now much used, having been superseded by the stroinuhr (current-clock) or rheometer of Ludwig and Dogiel, which was invented in 1867. This instrument measures the amount of blood which passes through an artery in a given time. It con sists of two glass bulbs of equal capacity communicating by a tube. One of the bulbs is filled with oil, which is expelled by the blood into the second (empty) bulb. The instrument is then reversed on its socket so that the bulb containing the blood is farthest from the heart, and the former process is repeated. From the time occupied in filling and refilling the velocity of the blood in the artery of supply can be calculated. The following figures give, in inches per second, the velocities of the blood in different vessels. Carotid of dog, 8 07 to 14 06 ; riment carotid of horse, 12 05 ; maxillary of horse, 9 13 ; metatarsal of to horse, 2 2 (Volkmann). Mean velocity in carotid of dog, 10 2S ; locity. in carotid of dog at end of diastole, 8 46 ; at end of systole, 11-69 ; lanes ; 4, veins ; 5, auricle ; A to C, line of pressure in great arter ies ; C to D, in small arteries ; D to E, in capillaries ; E to B in veins. The dotted lines indicate pressure during ventricular sys tole (C) and diastole (a C). Be yond G the blood-pressure is uni form as far as the auricle B, where it is negative during auricular diastole. (Bcaunis.)
- sults
ex- in crural artery of dog at end of diastole, 5 51 ; at end of systole, 9 41 (Vierordt). During systole in carotid of horse, 9 84 ; at time of dicrotic wave, 8 66 ; at end of diastole, 5 9 (Chauveau, Bertolus, Laroyenne). In carotid of rabbits, from 37 to 8 9 ; in carotid of dog, Aveighing 51 2 Ib, poisoned with morphia, from 1374 to 28 86 ; in carotid of another dog, weighing 26 69 lt>, from 9 57 to 20 47 ; in carotid of dog, weighing 7 85 Ib, in which the sympathetic nerve had been cut, from 8 03 to 13 35 ; and in carotid of another dog, weighing 6 97 ft), poisoned with morphia, from 13 35 to 18 03 (Dogiel). The velocity in the capillaries cannot be directly mea sured. E. H. Weber gives it at 032 inch per second in capillaries of mammals and 021 in those of the frog. Vierordt gives the velocity in man as 024 to 035 inch per second. Volkmann states that the flow of blood in mammalian capillaries is five hundred times slower than in the aorta. Donders asserts that the velocity of the current in the smaller arterioles is ten times faster than in the capillaries. When the current reaches the veins it is accelerated in consequence of diminished resistance, but even in the larger venous trunks it is 5 to "75 times less than in the corresponding arteries. The following general conclusions may be drawn. (1) The velocity of the blood is in inverse ratio to the total calibre of the vessels : rapid in the aorta, it diminishes as ve recede from it. (2) Each systole is followed by an increase in the velocity of the blood in the larger vessels. (3) In the smaller arteries, capillaries, and smaller veins the velocity is uniform and constant. (4) The velocity increases in the venous system as we approach the heart. (5) In the large arteries the movements of inspiration retard the velocity, whilst those of expiration increase it. (6) In the large veins the movement of respiration, and also the suction action of the auricle during diastole, cause a rhythmic increase and diminution of the velocity. The explanation of these variations in velocity is obvious. As the arteries pass outwards they give off branches, the united calibre of which is, with rare exceptions, greater than that of the parent vessel. Thus, as Kiiss expresses it, the arterial system may be regarded as a cone, the base of which ends in the capillaries, whilst the summit is at the aorta ; and the venous system is a second cone, the base being also at the capillaries and the apex at the right auricle. Vierordt states that the sectional area of the capillaries is to that of the aorta as 800 to 1 ; but, as the sectional area of the venous orifices at the heart is greater than that of the arterial orifices, the ratio of the sectional area of the capillaries to that of the veins at the heart has been stated as 400 to 1. The increased sectional area retards the velocity, and the velocity of the blood- current in sections of the vessels at various points is inversely as their calibre. The velocity of the blood does not depend on the mean blood-pressure, and, as was pointed out by Ludwig and Dogiel, the velocity in any section of a vessel depends on (1) the vis a tergo (i.e., action of the heart) and (2) the amount of resistance at the periphery. It is important to distinguish between the rapidity of the blood Duration current and the time occupied by a blood corpuscle in making a of circu- complete circuit through the heart and vessels, say, from the left lation. ventricle to the left ventricle again. Attempts have been made to measure the time, starting from the jugular vein. Hering injected into that vein a few drops of a 2 per cent, solution of ferro-cyanidc, of potassium, and then examined the blood of the opposite jugular every five seconds by testing with perchloride of iron, the forma tion of Prussian blue indicating the moment when the ferro-cyanide made its appearance in the blood of the jugular after having made a tour of the circulation. Vierordt modified the method by ex amining the blood received from the jugular each half second. The duration of the circulation as thus determined for various animals is as follows, horse, 31 5 seconds ; dog, 167 ; rabbit, 779 ; hedge hog, 7 61 ; cat, 6 69 ; goose, 10 86 ; cluck, 10 64 ; buzzard, 673 ; and common fowl, 5 17 seconds. Vierordt also made the discovery that in most animals the duration of the circulation is equal to the time in which the heart makes about twenty-seven beats. These tacts are illustrated in the following table : Animal. Weight of body. Pulse Beats per Minute. Number of Pulsations in the Circulation. Guinea-pig Cat 7S3 oz. 46-28 ,, 320 240 26-8 Hedgehog Rabbit 32-13 50-59 ,, 189 220 23-8 28-5 Dog 20-28 Ib 96 26-7 83 -78 55 28-8 46-98 oz. 354 30-5 Buzzard Duek 24-44 ,, 46-7 282 163 31-6 2S"9 99-54 ,, 144 20 1 It may also be shown by another method that a volume of blood equal to that in the whole body passes through the heart in about thirty pulsations. Taking the quantity of blood in the body as one-twelfth of the total weight, a man weighing 140 Ib contains 11^ It), or 5292 grammes, of blood, which represent in capacity 5302 cubic centimetres. Each beat of the heart throws 172 cubic centi
metres into the aorta ; therefore the equivalent of the total quantity
