810 
ANATOMY AND PATHOLOGY 
"RECRUITMENT" 
RED CEU 
VELOCITY 
500 
400 
- O PERCENTAGE OF OPEN 
CAPILLARIES 
• MEAN RCV IN 
CAPILLARIES 
300 
200 
100 
OPEN 
CAPIUARIES 
t 
100 
80 
60 
40 
20 
WO 200 300 
PERFUSION PRESSURE mmHg 
Figure 3. 
for the oxygenation of heart muscle is "recruit- 
ment of capillaries." When the perfusion pres- 
sure is raised in the perfused arrested heart, the 
number of capillaries with discernible red cell 
movement increases (Figure 3). At the same 
time, the red cell velocity in individual capil- 
laries remains constant. Similar observations 
were also made in the beating heart in situ. 
Apparently recruitment and autoregulation 
can occur simultaneously. Autoregulation is 
here defined as the intrinsic tendency of an or- 
gan to maintain constant blood flow despite 
changes in arterial perfusion pressure. It has 
been observed in the coronary circulation by 
Cross and Olsson.^*'^^ Many investigators have 
recognized that autoregulation occurs only over 
a limited range of pressures. Frequently we 
have observed that when perfusion pressure in- 
creases, red cell velocity in individual capillaries 
remains constant; but because of recruitment. 
the total oxygen supply to a unit of heart muscle 
increases. 
The cause of recruitment is as yet unknown. 
It may be a purely mechanical phenomenon. It 
is more likely, however, that recruitment as 
well as autoregulation are functions of the 
metarteriolar muscles and the precapillary 
sphincters. Direct observations to substantiate 
this are, however, lacking. 
Since our observations were made on the 
capillary circulation of the left atrium only, no 
conclusions can be drawn on phasic flow in the 
coronary microcirculation of ventricular mus- 
cle. In addition, red cell velocity varies some- 
what between individual capillaries; and, be- 
cause of movements of the heart, a number of 
frames may be out of focus, making determina- 
tions of phasic flow difficult. 
However, a definite pattern for red cell veloc- 
ity in the capillaries of the left atrium of the cat 
has emerged (Figure 4). Usually two major 
CONTROL 
Figure 4. 
