332 TEXT-BOOK OF PHYSIOLOGY. 



BLOOD-PRESSURE. 



From theoretic considerations alone it may be inferred that the blood, 

 as it flows through the vascular apparatus, exerts a pressure against the 

 walls of the vessels, and that this pressure is greatest at the beginning of the 

 aorta, and least at the ends of the venae cavae. The fact that the blood 

 flows from the aorta to the venae cavae indicates that there is a higher pres- 

 sure in the former than in the latter. The same holds true for the pulmonary 

 artery and veins. So long as these conditions are maintained, the blood 

 must flow from the point of high to the point of low pressure. 



To this pressure the term blood-pressure is given, and may be defined 

 as the pressure exerted radially or laterally by the moving blood-stream 

 against the sides of the vessels. That there is such a pressure within the 

 arteries, capillaries, and veins, different in amount in each of these three 

 divisions of the vascular apparatus, is evident from the results which follow 

 division of an artery or a vein of corresponding size. When an artery is 

 divided, the blood spurts from the opening for a considerable distance and 

 with a certain velocity. The reason for this lies in the fact that the vessel 

 has been distended by the pressure from within and its walls thrown into a 

 condition of elastic tension, so that at the moment there is an outlet, the 

 vessel suddenly recoils and forces the blood out with a velocity and to a 

 height proportional to the distention. When a vein is divided, the blood as 

 a rule merely wells out of the opening with but slight momentum, and for 

 the reason that the vessel has been but slightly, if at all distended by the 

 pressure. These results indicate that the blood in the arteries stands under 

 a pressure considerably higher than that of the atmosphere, while that in 

 the veins stands under a pressure perhaps but slightly above that of the atmos- 

 phere. Especially true is this of the larger veins. 



The same facts may be demonstrated in another and more striking way. 

 A dog or cat is anesthetized and securely fastened in an appropriate holder. 

 The carotid artery on the right side and the jugular vein on the left side are 

 freely exposed and clamped. Into the artery there is inserted on the distal 

 side of the clamp and in the direction of the heart a cannula to which is 

 connected a tall glass tube, 200 cm. high and of about 4 mm. internal di- 

 ameter. Into the vein there is passed on the proximal side of the clamp and 

 in the direction of the capillaries a second cannula, to which is connected a 

 similar tube, though of less height. If the two clamps are removed at the 

 same time, the blood will mount in both tubes simultaneously. In the 

 arterial tube the blood will ascend by leaps corresponding to the heart-beats 

 until a certain height is reached, when the column becomes relatively 

 stationary, being kept in equilibrium by the blood-pressure within the 

 vessel and the atmospheric pressure without. Though stationary in a 

 general sense, nevertheless the blood-column oscillates, rising and falling 

 with each contraction and relaxation of the heart. Not infrequently larger 

 excursions of the column are seen which correspond in a general way to the 

 respiratory movements. This experiment was originally performed on the 

 horse, by the Rev. Stephen Hales (1732). 



In the venous tube the blood also rises to a certain height, after which it 

 remains quite stationary, as the effect of the cardiac contraction is not 



