1. Unit Four: The Circulation
Chapter 14: Overview of the Circulation; Biophysics of Pressure, Flow, and Resistance
Guyton and Hall, Textbook of Medical Physiology, 12 edition

2. Physical Characteristics of the Circulation
Functional Parts of the Circulation
Arteries
Arterioles
Capillaries
Venules
Veins
Volumes of Blood in the Different Parts of
Circulation

3. Fig. 14.1 Distribution of blood (in percentage of total blood)
in the different parts of the circulatory system

4. Physical Characteristics (cont.)
Cross-sectional Areas of Different Vessels
Aorta 2.5
Small arteries 20
Arterioles 40
Capillaries 2500
Venules 250
Small veins 80
Vena cavae 8

5. Physical Characteristics (cont.)
Velocity of Blood Flow-because the same volume of
blood flow (F) must pass thru each segment of
circulation each minute, the velocity (V) is
inversely proportional to vascular cross-
sectional area
V = F/A

6. Physical Characteristics (cont.)
Pressures in Different Portions of the Circulatory
System
Fig Normal blood pressures in the different portions of the circulatory system.

7. Basic Principles of Circulatory Function
The rate of blood flow to each tissue of the body is
almost always precisely controlled in relation to
the tissue need.
The cardiac output is controlled mainly by the sum of
all the local tissue flows.
Arterial pressure regulation is generally independent
of either local blood flow control or cardiac
output control.

8. Interrelationships of Pressure, Flow, and Resistance
Blood flow through a blood vessel is determined by
two factors: (1) pressure difference (gradient)
and (2) impediment to blood flow (resistance).
Ohm’s Law

9. Pressure, Flow and Resistance (cont.)
Fig Interrelationships of pressure, resistance, and blood flow

10. Pressure, Flow and Resistance (cont.)
Ohm’s Law-blood flow is directly proportional to the
pressure difference but inversely proportional
to the resistance.
Note that it is the difference in pressure that
determines blood flow not the actual pressure
Blood Flow-quantity of blood that passes a given point
in the circulation in a given period of time;
expressed in mm/min or l/min

11. Pressure, Flow and Resistance (cont.)
Turbulent Flow
When rate of blood flow becomes too great
When it passes an obstruction in a vessel
When it makes a sharp turn
When it passes over a rough surface
Eddy Currents Form-blood flows with greater
resistance

12. Fig. 14. 6 A. Two fluids (one red and one clear) before flow begins; B
Fig A. Two fluids (one red and one clear) before flow begins; B. The same fluids 1 second
after flow begins; and C. Turbulent flow, with elements of the fluid moving in a
disorderly pattern.

13. Pressure, Flow, and Resistance (cont.)
Blood Pressure (mm Hg)- the force exerted by the
blood against any unit area of the vessel wall
Resistance to Blood Flow- impediment of blood
flow in a vessel

14. Pressure, Flow, and Resistance (cont.)
Total Peripheral Vascular Resistance and Total
Pulmonary Vascular Resistance –
Rate of blood flow through the entire circulatory system is equal to cardiac output
Resistance of the entire systemic circulation
is 100 mm Hg or 1 peripheral resistance unit (PRU)
In the pulmonary system, it is 16 mm Hg and
in the right ventricle it is 2 mm, a net difference of 14 mm Hg (total is 0.14 PRU)

15. Pressure, Flow, and Resistance (cont.)
Conductance of Blood in a Vessel and Its Relation to
Resistance
Measure of the blood flow through a vessel at a
given pressure difference
b. Exact reciprocal of resistance

16. Pressure, Flow, and Resistance (cont.)
Slight Changes in Diameter Has a Great Impact on
Conductance
Fig A. Demonstration of the effect of vessel diameter on blood flow;
B. Concentric rings of blood flowing at different velocities; the
further away from the wall, the faster the flow

17. Pressure, Flow, and Resistance (cont.)
Conductance of the vessel increases in
proportion to the 4th power of the diameter
(Poiseuille’s Law)
Importance of the Vessel Diameter (4th power)-
About 2/3 of the total resistance in the systemic
circulation is due to arteriolar resistance

18. Pressure, Flow, and Resistance (cont.)
With only small changes in diameter, blood flow
can be completely shut off or increased
dramatically
Ranges of 100 fold have been observed in the
systemic circulation

19. Pressure, Flow, and Resistance (cont.)
Resistance to Blood Flow in Series and Parallel
Vascular Circuits
Fig Vascular Resistances in A: in series, and B: in parallel

20. Total peripheral vascular resistance is equal to the
sum of resistances of vessels.
Effect of blood hematocrit and viscosity on flow
and resistance.
The viscosity of normal blood is 3X that of water
Hematocrit-portion of blood that is rbcs
Viscosity increases as the hematocrit increases

21. Fig. 14.11 Effect of viscosity on hematocrit

22. Effects of pressure on flow and resistance
Autoregulation attenuates the effect of arterial pressure on tissue blood flow
Fig Effects of changes in
arterial pressure on
tissue

23. Pressure-Flow Relationship in Passive Vascular Beds
Fig Effect of arterial pressure on blood flow through a passive vessel caused by
increased or decreased sympathetic stimulation.