l66 THE BLOOD-PRESSURE IN THE ARTERIES. 



Within the large arteries the blood-pressure undergoes relatively 

 slight diminution toward the periphery, because the differences in the 

 resistance in various sections of the large tubes are inconsiderable. 

 As soon, however, as the arteries undergo frequent division and their 

 caliber accordingly becomes greatly diminished, the blood-pressure 

 rapidly diminishes, because the propulsive power of the blood is 

 weakened by the effort to overcome the increased resistances produced 

 in this way. 



The arterial pressure increases directly with the quantity of 

 blood present in the arteries, and conversely. The pressure, therefore, 

 Increases Diminishes 



1. As the heart's action becomes stronger i. As the heart's action becomes feebler 



and more rapid. and slower. 



2. In plethoric individuals. 2. In anemic individuals. 



3. After considerable increase in the 3. After profuse hemorrhage or loss from 



quantity of blood by the direct in- the blood in some other way, as 



jection of blood, and also after cop- for example, by profuse sweating 



ious ingestion of food. or copious diarrhea. 



The increase and decrease in blood-pressure is not directly proportional to 

 the increase and decrease in the quantity of blood. By virtue of their muscular 

 libers the blood-vessels possess the faculty of adapting themselves within fairly 

 wide limits to the variable volume of blood. The blood-pressure, therefore, does 

 not rise at once when the quantity of blood is moderately increased. The cir- 

 cumstance that fluid rapidly transudes from the blood into the tissues also assists 

 in maintaining a constant blood-pressure. Moderate venesection, in the dog up 

 to 28 per cent, of the body-weight, is not followed by any noteworthy diminution 

 in the blood-pressure. After slight hemorrhages the pressure may even rise, but 

 the removal of a large quantity of blood is followed by a considerable fall in the 

 blood-pressure, and the loss of from 4 to 6 per cent, of the body-weight reduces 

 it to zero. Increased pressure within the vessels produced by engorgement tends 

 to dilate the cutaneous and muscular vessels, especially those of the extremities, 

 and affects the arteries in the viscera but little. After the pressure has fallen, the 

 visceral blood-vessels return to their original caliber much more promptly than 

 do the cutaneous and muscular blood-vessels. 



The arterial pressure rises as the capacity of the arteries is 

 diminished, and conversely. This is accomplished by contraction or 

 relaxation of the unstriated muscle-fibers of the arterial wall. 



The pressure within a certain area of the arterial system rises 

 or falls accordingly as the blood-vessels in neighboring areas undergo 

 contraction or even become impermeable from compression or ligation 

 or dilatation. The application of heat or cold to a circumscribed portion 

 of the body, also of pressure or diminution of pressure (the latter by 

 introducing an extremity into a closed space containing rarefied air, as, 

 for example, Junod's cupping boot), and the effect of stimulation or 

 paralysis of certain vasomotor areas, furnish striking proofs of the cor- 

 rectness of this statement. 



The respiratory movements produce regular variations in the 

 arterial pressure, known as respiratory pressure- variations the 

 pressure falling with each deep inspiration and rising with each expira- 

 tion. These variations are readily explained by the fact that at each 

 expiration the blood in the aorta is subjected to the increased pressure 

 of the compressed air in the thorax, while with each inspiration the blood 

 undergoes a diminution in pressure, in consequence of the influence of 

 the rarefaction of the air in the lungs, on the aorta. In addition, the in- 

 spiratory expansion of the thorax tends to draw the blood from the venae 

 cavae into the heart, while during expiration the blood stagnates, and 



