3. Basic Theory of Circulatory
Function
• 1. The rate of blood flow to each tissue of the body
is almost always precisely controlled in relation to
the tissue need
• 2. The cardiac output is controlled mainly by the
sum of all the local tissue flows
• 3. In general the arterial pressure is controlled
independently of either local blood flow control or
cardiac output control
4. Pressure, Flow and Resistance
• Blood flow through a blood vessel is determined by
two factors:
• (1) pressure difference of the blood between the two
ends of the vessel, also sometimes called “pressure
gradient” along the vessel, which is the force that
pushes the blood through the vessel,
• (2) the inhibition to blood flow through the vessel,
which is called vascular resistance - occurs as a result
of friction between the flowing blood and the
intravascular endothelium
5. Pressure, Flow & Resistance
• Ohm’s law: F = ∆P / R
• F is blood flow,
• ∆P is the pressure difference between the two ends
of the vessel,
• R is the resistance
6. Turbulent Flow
• the flow may become turbulent
• When the rate of blood flow becomes too great,
• when it passes by an obstruction in a vessel,
• when it makes a sharp turn,
• when it passes over a rough surface,
• the blood flows with much greater resistance
7. Turbulent Flow
• Re = ρ v d / η
• where Re is Reynolds’ number and is the measure
of the tendency for turbulence to occur,
• ρ is density,
• v is the mean velocity of blood flow (in
centimeters/second),
• d is the vessel diameter (in centimeters),
• η is the viscosity (in poise).
8. Turbulent Flow
• When Reynolds’ number rises above 200 to 400, turbulent
flow will occur at some branches of vessels but will die out
along the smooth portions of the vessels.
• when Reynolds’ number rises above 2000, turbulence will
usually occur even in a straight, smooth vessel
• Reynolds’ number can rise to several thousand during the
rapid phase of ejection by the ventricles – causes turbulence
in the proximal aorta and pulmonary artery
• (1) high velocity of blood flow,
• (2) pulsatile nature of the flow,
• (3) sudden change in vessel diameter, and
• (4) large vessel diameter
9. Resistance to Blood Flow
• If the pressure difference between two points is 1 mm
Hg and the flow is 1 ml/sec, the resistance is said to be
1 peripheral resistance unit (PRU)
• the resistance of the entire systemic circulation, called
the total peripheral resistance, is about 1 PRU
• Widespread vasoconstriction – 4 PRU
• Widespread vasodilatation – 0.2 PRU
• Total pulmonary vascular resistance - 0.14 PRU
10. Conductance
• Conductance is a measure of the blood flow
through a vessel for a given pressure difference.
• This is generally expressed in terms of
ml/sec/mmHg
• Conductance = 1 / Resistance
• Slight changes in the diameter of a vessel cause
tremendous changes in the vessel’s ability to
conduct blood when the blood flow is streamlined.
11.
12. Conductance
• Conductance α Diameter4
• the blood that is near the wall of the vessel flows extremely
slowly, whereas that in the middle of the vessel flows
extremely rapidly
• Poiseuille’s law: F = ∏∆Pr4 / 8ηl
• F is the rate of blood flow,
• ∆P is the pressure difference between the ends of the vessel,
• r is the radius of the vessel,
• η is viscosity of the blood,
• l is length of the vessel
13. Fourth Power Law
• fourth power law makes it possible for the
arterioles, responding with only small changes in
diameter to nervous signals or local tissue
chemical signals,
• either to turn off almost completely the blood
flow to the tissue
• at the other extreme to cause a vast increase in
flow
14. Hematocrit and Viscosity
• The greater the viscosity, the less the flow
• the viscosity of normal blood is about 3 times as
great as the viscosity of water
• large numbers of suspended red cells in the blood
• The percentage of the blood that is cells is called
the hematocrit
• Male – 45, female - 40
15.
16. Hematocrit and Pressure
• The viscosity of blood increases considerably as
the hematocrit increases
• factors that affect blood viscosity are the plasma
protein concentration and types of proteins in the
plasma
• increase in arterial pressure not only increases the
force that pushes blood through the vessels but also
distends the vessels at the same time, which
decreases vascular resistance
17.
18. Vascular Distensibility
• the distensible nature of the arteries allows them to
accommodate the pulsatile output of the heart and to average
out the pressure pulsations.
• This provides smooth, continuous flow of blood through the
very small blood vessels
• The most distensible by far of all the vessels are the veins.
Even slight increases in venous pressure cause the veins to
store 0.5 to 1.0 liter of extra blood.
• The veins provide a reservoir function for storing large
quantities of extra blood that can be called into use whenever
required elsewhere
19.
20. Stress-Relaxation of Vessels
• a vessel exposed to increased volume at first exhibits a large
increase in pressure,
• but progressive delayed stretching of smooth muscle in the
vessel wall allows the pressure to return back toward
normal over a period of minutes to hours
• stress-relaxation is a valuable mechanism by which the
circulation can accommodate much extra blood when
necessary, such as after too large a transfusion.
• Reverse stress-relaxation is one of the ways in which the
circulation automatically adjusts itself over a period of
minutes or hours to diminished blood volume after serious
hemorrhage
