Thus the blood flows with great velocity in the
great arteries, less rapidly in their branches, and
very slowly indeed in the capillaries, the com-



250 THE CIRCULATION OF THE BLOOD

bined width of which is so great compared to
that of the arteries. And as the capillaries unite
into the smaller veins, and these into the larger
veins, the combined cross-section or bed becomes
ever smaller and the blood moves ever more
swiftly. Were the slow passage of the blood in
the capillaries due simply to friction, the blood
would move still more slowly in the veins be-
cause the retarding influence of the friction in
the veins would be added to that of the capillaries.
There is an inverse relation between the rate of
flow and the area of bed.
The Blood-Peessuee

The Relation of Peripheral Resistance to Blood-
Pressure. — Compress the bulb at a rate that will
produce a continuous outflow.
With each successive stroke the portion of
liquid unable to pass the resistance during the
stroke and the succeeding interval is added to
that left behind from preceding strokes. The
arteries become more and more full. The arte-
rial manometer registers a higher and higher
pressure. At length the pressure ceases to rise.
The mercury remains at a mean level broken by
a slight accession at each stroke. The pump
now merely maintains the constant high arterial
pressure. This pressure suffices to drive through



THE MECHANICS OF THE CIRCULATION 251

the resistance during each stroke and the suc-
ceeding interval all the liquid received from the
pump during the stroke.
