BRANCHING OF THE BLOOD-VESSELS. 1 23 



E. Albert estimated the blood-pressure by means of a manometer, placed in connection with 

 the anterior tibial artery of a boy whose leg was to be amputated, to be 100 to 160 mm. Hg. 

 The elevation with each pulse-beat was 17 to 20 mm. ; coughing raised it to 20 or 30 mm. ; 

 tight bandaging of the healthy leg, 15 mm. ; while passive elevation of the body, whereby the 

 hydrostatic action of the column of blood was brought into play, raised it 40 mm. 



The pressure in the aorta of mammals varies from 200 to 250 mm. Hg. As a general rule, 

 the blood-pressure in large animals is higher than in small animals, because in the former the 

 blood-channel is considerably longer, and there is greater resistance to be overcome. In very 

 young and in very old animals the pressure is lower than in individuals in the prime of life. 



The arterial pressure in the foetus is scarcely half that of the newly-born, while the venous 

 pressure is higher, the difference of pressure between arterial and venous blood being scarcely 

 half so great as in adult animals (Cohnstein and Zuntz). 



The arterial blood-pressure is highest in the aorta, and falls towards the 

 smaller vessels, but the fall is very gradual, as shown in fig. 105. A great fall 

 takes place on passing from the area 

 of the arterioles into the capillary 

 area (C), while it is less in the ven- 

 ous area, and negative near the heart, 

 as indicated in the dotted line pass- 

 ing below the abscissa, so that the 

 pressure is lowest in the cardiac 

 nds of the venae cavae (compare fig. h a c v ra 



HI)- , '" Fig. 105. 



(b) Branching of the Blood- Scheme of the blood-pressure, in A, the arteries ; C, 

 Vessels. Within the large arteries capillaries, and V, veins ; O-O, is the abscissa or 

 the blood-pressure diminishes rela- line of no pressure ; L.V., left ventricle, and R.A., 

 tively little as we pass towards the ri |^ uricle ^ B * P " the hdght of the bl d " 

 periphery, because the difference of 



the resistance in the different sections of large tubes is very small. As soon, how- 

 ever, as the arteries begin to divide frequently, and undergo a considerable diminu- 

 tion in their lumen, the blood-pressure in them rapidly diminishes, because the pro- 

 pelling energy of the blood is much weakened, owing to the resistance which it has 

 to overcome ( 99). 



(c) Amount of Blood. The blood-pressure is increased with greater filling of the 

 arteries, and vice versd ; hence it 



Increases. Decreases. 



1. With increased and accelerated action of 1. During diminished and enfeebled action 



the heart; 



2. In plethoric persons ; 



3. After considerable increase of the quantity 



of blood by direct transfusion, or after 

 a copious meal. 



of the heart ; 



2. In anaemic persons ; 



3. After haemorrhage or considerable excre- 



tions from the blood by sweating, the 

 urine, severe diarrhoea. 



The blood-pressure does not vary in the same proportion as the variations in the amount of 

 blood. The vascular system, in virtue of its muscular tissue, has the property, within liberally 

 wide limits, of accommodating itself to larger or smaller quantities of blood (C. Ludivig and 

 Worm Miiller, 102, d). [In fact, a large amount of blood may be transfused without materi- 

 ally raising the blood-pressure.] Small and moderate haemorrhages (in the dog to 2*8 per cent, 

 of the body-weight) have no obvious effect on the blood-pressure. After a slight loss of blood 

 the pressure may even rise ( Worm Miiller). If a large amount of blood be withdrawn, it causes 

 a great fall of the blood-pressure, and when haemorrhage occurs to 4-6 per cent, of the body- 

 weight, the blood-pressure = 0. The transfusion of a moderate amount of blood does not raise 

 the mean arterial blood-pressure. There are important practical deductions from these experi- 

 ments, viz., that the arterial blood-pressure cannot be diminished directly by moderate blood- 

 letting, and that the blood-pressure is not necessarily high in plethoric persons.] 



(d) Capacity of the Vessels. The arterial pressure rises when the capacity of 

 the arterial system is diminished, and conversely. The circularly-disposed smooth 

 muscular fibres of the arteries are the chief agents concerned in this process. When 

 they relax, the arterial blood-pressure falls, and when they contract, it rises. These 



