The
Venous System
185
As
above, flow
Q
through a segment of the collapsible tube is a
function of the pressure just upstream to the collapsible segment, p1, the
pressure just downstream to it, p2, and the external pressure within the
enclosed chamber, pe. The experimental results for the Penrose tube is
shown also in Fig.
6.3.1.
When the upstream pressure is greater than the
downstream pressure (P1>P2) and the downstream pressure exceeds the
external pressure, p2>pe, the vessel is simply open over its entire length.
The slope of the p1-p2 vs.
Q
is
determined by the flow resistance
of
the
cylindrical tube. The more interesting result occurs when p2<pe, i.e. the
tube no longer has a circular cross-section and is partially collapsed.
Holt observed that flow to be constant in this range and described it as
autoregulation, i.e. flow is now longer determined by the difference of
upstream and downstream pressures, pl-p2, but by pI-pe. Flow limitation
is said to be reached at this point.
AP
mm
Hg
pc
R,
J)
mmHg mmHg
1
7.5
0.04
2
14.5
0.07
3
22.0
0.10
4
29.5
0.14
5
37.0
0.17
6
44.0
0.20
cms
sz
Fig.
6.3.2:
Relationship of pressure gradient Ap (pI-p2) across the collapsible tube and
flow
(Q)
with increasing
(1
to
6)
external pressure pe and nonlinear resistance,
Rz.
Notice
the region when impedance
is
negative (decreasing Ap with increasing Q).
From
Conrad
(1
969).
