6 Low-Frequency Blood Volume Rhythms in the Skin Perfusion …
109
in particular the O 2 exchange from blood to the interstitium and the related CO 2
balance. This would be particularly relevant for the brain, which reacts very sensitive to O 2 deficiencies: for example, it is known that a complete blocking of blood flow
to the brain leads to noticeable effects in 4 to 6 s and unconciousness after about 15 s.
Normally, a deficiency due to whatever causes will be autoregulated by widening of
the arterioles and enhanced blood flow. However, there are time constants involved
in the control processes which may easily be in the order of magnitude of seconds.
A naive estimate for capillary action in the brain based on literature values [7]
for blood flow to the brain of 715 ml/min, ~ 11.9 ml/sec, and a blood volume of
300 ml in the head, with ~ 15% in the capillary system, i.e. ~ 45 ml would result in
a crossing time of ~ 3.8 s. A similar value is obtained by estimate of the capillary
length: with an overall cross-section of the capillary system of 3500 cm
2 , a diameter
of 0.0009 cm [7] together with a value of 5% of the total blood volume, would result
in an average capillary length of around 0.086 cm and a flow velocity of
v ∼ 0.026 cm/s.
This is to be corrected for non-constant velocity profile across the capillary
”tubes”, as the first estimate with laminar flow and parabolic distribution a factor
of π /4, hence
v ∼ 0.021 cm/s.
and a total time for crossing the capillary system t ~ 4.1 s. This in no way can describe
the detailed and complicated blood flow in the capillaries, which partially may even
be shut at times, but it is believed to give a reasonably correct order of magnitude
of gross effects. In this purely speculative picture, the relaxation after some stress
inducing enforced physical or mental activity would lead to a lowering of the blood
pressure in the brain, particularly also at the hypothalamus, which controls a. o.
the circulatory parameters. This would lead to a gradually increasing O 2 deficiency
due to reduced blood flow. By autoregulation, this in turn would lead gradually to
a widening of arterioles via sympathic nerves tending to counteract the decrease in
blood flow. However, until the original blood flow rate has been reached, there will
still remain an overall O 2 -deficiency, so blood perfusion would continue to rise for
some time until stopped by counteracting nervous control, possibly even associated
with some reactive hyperaemia. Thus, a rhythmic oscillation with a period of roughly
2 × t ~ 8.2 s could be set up, Fig. 6.4. The control process involves the action of
both, the (slow) sympathic and the (fast) parasympathic nerves, which ”stray” into
the entire circulatory system.
In this picture, the low frequency rhythm appears as a centrally excited oscillation,
which by nervous coupling can be observed in all different locations on the skin and
also in impedance plethysmography [8] and in correlated small variations in the
ECG, etc. [9]. In a sense, the model implies a slight dynamic oscillation around a
”rest state” which may be advantageous compared to a fixed, constant value. This
109
in particular the O 2 exchange from blood to the interstitium and the related CO 2
balance. This would be particularly relevant for the brain, which reacts very sensitive to O 2 deficiencies: for example, it is known that a complete blocking of blood flow
to the brain leads to noticeable effects in 4 to 6 s and unconciousness after about 15 s.
Normally, a deficiency due to whatever causes will be autoregulated by widening of
the arterioles and enhanced blood flow. However, there are time constants involved
in the control processes which may easily be in the order of magnitude of seconds.
A naive estimate for capillary action in the brain based on literature values [7]
for blood flow to the brain of 715 ml/min, ~ 11.9 ml/sec, and a blood volume of
300 ml in the head, with ~ 15% in the capillary system, i.e. ~ 45 ml would result in
a crossing time of ~ 3.8 s. A similar value is obtained by estimate of the capillary
length: with an overall cross-section of the capillary system of 3500 cm
2 , a diameter
of 0.0009 cm [7] together with a value of 5% of the total blood volume, would result
in an average capillary length of around 0.086 cm and a flow velocity of
v ∼ 0.026 cm/s.
This is to be corrected for non-constant velocity profile across the capillary
”tubes”, as the first estimate with laminar flow and parabolic distribution a factor
of π /4, hence
v ∼ 0.021 cm/s.
and a total time for crossing the capillary system t ~ 4.1 s. This in no way can describe
the detailed and complicated blood flow in the capillaries, which partially may even
be shut at times, but it is believed to give a reasonably correct order of magnitude
of gross effects. In this purely speculative picture, the relaxation after some stress
inducing enforced physical or mental activity would lead to a lowering of the blood
pressure in the brain, particularly also at the hypothalamus, which controls a. o.
the circulatory parameters. This would lead to a gradually increasing O 2 deficiency
due to reduced blood flow. By autoregulation, this in turn would lead gradually to
a widening of arterioles via sympathic nerves tending to counteract the decrease in
blood flow. However, until the original blood flow rate has been reached, there will
still remain an overall O 2 -deficiency, so blood perfusion would continue to rise for
some time until stopped by counteracting nervous control, possibly even associated
with some reactive hyperaemia. Thus, a rhythmic oscillation with a period of roughly
2 × t ~ 8.2 s could be set up, Fig. 6.4. The control process involves the action of
both, the (slow) sympathic and the (fast) parasympathic nerves, which ”stray” into
the entire circulatory system.
In this picture, the low frequency rhythm appears as a centrally excited oscillation,
which by nervous coupling can be observed in all different locations on the skin and
also in impedance plethysmography [8] and in correlated small variations in the
ECG, etc. [9]. In a sense, the model implies a slight dynamic oscillation around a
”rest state” which may be advantageous compared to a fixed, constant value. This