276 EFFECTS OF VASO-MOTOR ACTIONS. [BOOK i. 



of the body are in a state of tonic, i. e. of moderate, contraction, 

 and it is the narrowing due to this contraction which forms 

 a large item of that peripheral resistance which we have seen 

 to be one of the great factors of blood pressure. The nor- 

 mal general blood pressure, and therefore the normal flow of 

 blood, is in fact dependent on the ' general tone ' of the minute 

 arteries. 



In the second place local vaso-motor changes in the condi- 

 tion of the minute arteries, changes, that is to say, of any par- 

 ticular vascular area, have very decided effects on the circulation. 

 These changes, though local themselves, may have effects which 

 are both local and general, as the following considerations will 

 shew. 



Let us suppose that the artery A is in a condition of normal 

 tone, is midway between extreme constriction and dilation. The 

 flow through A is determined by the resistance in A and in the 

 vascular tract which it supplies, in relation to the mean arterial 

 pressure, which again is dependent on the way in which the heart 

 is beating and on the peripheral resistance of all the small arteries 

 and capillaries, A included. If, while the heart and the rest of 

 the arteries remain unchanged, A be constricted, the peripheral 

 resistance in A will increase, and this increase of resistance will 

 lead to an increase of the general arterial pressure. Since, as we 

 have seen, 101, it is arterial pressure which is the immediate 

 cause of the flow from the arteries to the veins, this increase of 

 arterial pressure will tend to drive more blood from the arteries 

 into the veins. The constriction of A however, by increasing the 

 resistance, opposes any increase of the flow through A itself, in fact 

 will make the flow through A less than before. The whole increase 

 of discharge from the arterial into the venous system will take 

 place through the arteries in which the resistance remains un- 

 changed, that is, through channels other than A. Thus, as the 

 result of the constriction of any artery there occur, (1) diminished 

 flow through the artery itself, (2) increased general arterial 

 pressure, leading to (3) increased flow through the other arteries. 

 If, on the other hand, A be dilated, while the heart and other 

 arteries remain unchanged, the peripheral resistance in A is 

 diminished. This leads to a lowering of the general arterial 

 pressure, which in turn tends to drive less blood from the arteries 

 into the veins. The dilation of A however, by diminishing the 

 resistance, permits, even with the lowered pressure, more blood to 

 pass through A itself than before. Hence the diminished flow 

 tells all the more on the rest of the arteries in which the resistance 

 remains unchanged. Thus, as the result of the dilation of any 

 artery, there occur (1) increased flow of blood through the artery 

 itself, (2) diminished general pressure, and (3) diminished flow 

 through the other arteries. Where the artery thus constricted or 

 dilated is small, the local effect, the diminution or increase of flow 



