1128 
MONITORING 
ference exists between man and the dog, re- 
sulting in a major difference in the peripheral 
circulatory adjustments to exercise. 
One possibility, based on anatomical differ- 
ences, is that the role of the spleen is signifi- 
cantly different in man and the dog during exer- 
cise, and that this organ represents a major 
circulatory reserve mechanism in the dog. It is 
possible that contraction of the canine spleen 
augments the circulating blood supply suflfi- 
ciently to provide additional oxygen to the ac- 
tive muscular beds eliminating the necessity to 
divert additional flow from the inactive viscera. 
Accordingly, the peripheral vascular response 
to severe exercise was studied in another series 
of dogs before and after splenectomy.^^ Prior to 
splenectomy, exercise increased hematocrit 
from 40 to 48% while telemetered measure- 
ments of renal and mesenteric flow remained 
constant. Following splenectomy, exercise failed 
to increase hematocrit and compensatory reduc- 
tion and diversion of visceral flow was observed. 
Thus, the spleen has an important role as a 
reserve mechanism in the canine response to 
exercise and this factor of species difference 
may explain the discrepancy between results of 
visceral flow during exercise reported from 
studies in man and in the dog. 
To eliminate problems of species difference, it 
would be ideal to conduct experimental physio- 
logical studies in man. However, this is neither 
ethically possible nor are techniques currently 
available to directly measure cardiac dimen- 
sions, blood flows and pressures in man. The 
most attractive alternative is to directly meas- 
ure these variables in a primate model of man. 
To study circulatory dynamics in larger pri- 
mates, telemetry is almost essential since these 
animals are not domesticated as are the com- 
monly used laboratory animals, and have the 
capability of inflicting serious injury with their 
razorsharp teeth (Figure 3). Telemetry in this 
MESENTERIC BLOOD 
FLOW VELOCITY 
cm /sec 
MEAN MESENTERIC 
FLOW 
ml/min 
ARTERIAL 
BLOOD PRESSURE 
mmHg 
MEAN 
PRESSURE 
mmHg 
HEART RATE 
beats/min 
50-1 
75- 
jRUN {AV.SPEED = IOm.p.h.) j 
1 m^a+ir 
4tt4:ti-;W+i:i±-H-fl|-H- 
I sec 
I sec 
Imin 
Figure 3. — A typical response of the mesenteric bed to severe exercise in the presence of circulatory impairment. 
This animal was in complete heart block. The phasic and mean values for mesenteric blood flow and arterial 
pressure are shown along with heart rate. The marked reduction in flow is best shown by the comparison of 
the phasic waveforms at rest (left) and during severe exercise (center). (Reprinted with permission from 
J. Clin. Invest. 50:1950, 1971). 
