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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



FIG. 29. RLF and LLF = 



blood flow in ml/min through 



right and left external iliac 

 arteries. 



600 



venous oxygen difference also plays an important 

 role in achieving the maximal oxygen intake. The 

 arteriovenous oxygen difference during heavy exer- 

 cise is about 14.5 ml per 100 ml (6, 41, 66) and is ob- 

 tained by encroachment on the mixed venous oxygen 

 content. This encroachment on the mixed venous 

 content is, in turn, made possible by two mecha- 

 nisms: one is the extent to which blood perfusing 

 working tissue can surrender its oxygen (7); the other 

 is a proportional shunting of blood away from 

 inactive areas, and is discussed below. 



B. Peripheral I'ascidar Control 



An important means of widening the AV oxygen 

 difference during exercise is the ability propor- 

 tionally to shunt blood from inactive areas (34, 66). 

 Relevant to this, and central to the understanding of 

 circulatory regulation in varying states, is the demon- 

 stration that in an area of increased activity the 

 vascular bed becomes functionally sympathectomized 

 (78a). The existence of this phenomenon allows the 

 central nervous system to distribute a general increase 

 in the sympathetic outflow to the peripheral vascular 

 bed, as lor example during carotid hypotension, with- 

 out impairing blood flow to an area of increased 

 activity, since the vascular bed of the latter is unre- 

 sponsive to such stimulation. This has been demon- 

 strated to be so whether the sympathetic stimulation 

 to an extremity is accomplished by the intra-arterial 

 injection of norepinephrine, direct stimulation of the 

 sympathetic nerves supplying the active area, or 

 reflexly increased sympathetic activity induced either 

 by lowering carotid pressure or by stimulation of the 

 central cut end of the vagus nerve (78a). Figure 29 

 shows one example of this phenomenon. In the left 

 panel, with both legs at rest, stimulation of the 

 central cut end of one vagus nerve (indicated by bar 

 in the upper channel) produces an intense vasocon- 



striction in both lower extremities as evidenced by 

 the relation between pressure and flow in both during 

 the stimulation. In the right panel, at the arrow, 

 exercise is begun in the left lower extremity. Then 

 afferent vagal nerve stimulation is repeated. The 

 right lower extremity, still at rest, again responds 

 with vasoconstriction as it did previously. Flow 

 through the exercising left lower extremity is, how- 

 ever, now pressure dependent. This type of experi- 

 ment indicates that functional sympathectomy 

 occurs when local muscular activity is increased. As 

 might be expected, it was also observed that the 

 degree of immunity from the influence of sympathetic 

 impulses varies with the intensity of the activity. The 

 biochemical means by which local vessels are rendered 

 nonreactive to svmpathetic stimulation during in- 

 creased activity of the organ they supply is one of the 

 most important, vmanswered questions in the physi- 

 olog\' of the circulation. 



V. THE ARCHITECTURE OF CIRCULATORY REGUL.'SlTION 



The circulation is comprised of: /) two input sensi- 

 tive pumps, conduits through which each propels 

 blood to the tissues, permeable conduits through 

 which chemical exchange with the tissues is effected, 

 and conduits through which the blood is returned to 

 the other pump. There are, of course, also the conduits 

 of the Kmphatic s\stem. 2) Central activation ele- 

 ments in the circulatory centers which, by means of 

 evoking a variety of efferent nerve impulses, can \ary 

 the rate of the pumps, can \ary their stroke work 

 and power (both by a direct effect on the pumps and 

 by varying the input into them), can vary the re- 

 sistance to flow through the efferent conduits, and 

 can vary the pressure-volume relation of the conduits 

 returning blood, j) A local peripheral vascular 

 mechanism wherein- the conduits leading blood to an 



