A textbook of human physiology / translated from [the] 6th German edition by W. Stirling.

  • Landois, Leonard
Date:
1888
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Licence: Public Domain Mark

Credit: A textbook of human physiology / translated from [the] 6th German edition by W. Stirling. Source: Wellcome Collection.

Provider: This material has been provided by Royal College of Physicians, London. The original may be consulted at Royal College of Physicians, London.

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    BKANCHING OF THE BLOOD-VESSELS. B.Rr o o E. Albert estimated tlie blood-pressure by means ol' a manometer, placed in connection with the anterior tibial artery ot a boy whose leg was to be amputated, to be 100 to 160 mm. Hg. l lie elevation with each pulse-beat was 17 to 20 mm. ; eougliing raised it to 20 or 30 mm. ; tight bandaging ot the healthy leg, 15 mm. ; while passive elevation of tlie body, whereby tlie Jiydrostatic action of the column of blood was brouglit into play, raised it 40 mm. Ihe pressure m tlie aorta of mammals varies from 200 to 250 mm. Hg. As a general rule, the blood-pressure m large animals is higher than in small animals, because in the former the blood-chaunel 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. ihe arterial pressure m the foetus is scarcely half that of the newly-born, while the venous pressure is higher, the ditlerence of pressure between arterial and venous blood being scarcely half so great as in adult animals {Cuhnstciu 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. lOo. A great fall takes place on passing from the area of tlie 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 ends of the venae cavae, (compare fig. h a c v ---Ih ^ ^ ^ L.V. R.A. {/>) Branching of the Blood- scheme of the blood-pressure, in A, the arteries ; C, vessels.—Within the large arteries capillaries, and V, veins; 0-0, 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 '''^^1 auricle; H.P., the height of the blood- periphery, because the difference of l*'*^^^''^- 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). (<•) Amount of Blood.—The blood-pressure is increased with f/reaier filling of the arteries, and vice versa; hence it Increases. I Decreases. 1. With increased and accelerated action of 1. During diminished and enfeebled action the heart; 2. Ill plethoric persons; 3. After considerable increase of the quantity of blood by direct transfusion, or after a copious meal. of the heart; 2. In anremic persons; 3. After hremorrhage or considerable excre- tions from the blood by sweating, tlie 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. Ludioig and IVorni Mailer, § 102, d). [In fact, a large amount of blood may be transfused without materi- ally raising the blood-pressure.] Small aud moderate hajmorrliages (in the dog to 2'8 per cent, of the bod3'-weight) have no obvious effect on the blood-pressure. After a slight loss of blood the pressure may even rise ( JVorni Miillcr). If a large amount of blood be withdrawn, it causes a great fall of the blood-pressure, and when ha-morrhage occurs to 4-6 per cent, of the body- weight, the blood-pressure = 0. The transfusion of a. niodrrate Amornni oi blood does not raise the mean arterial blood-pressure. There are important practical deductions from these experi- ments, viz., that the arterial blood-prcssuro 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
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