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

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

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

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

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

Physical Characteristics (cont.) Pressures in Different Portions of the Circulatory System Fig. 14.2 Normal blood pressures in the different portions of the circulatory system.

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.

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

Pressure, Flow and Resistance (cont.) Fig. 14.3 Interrelationships of pressure, resistance, and blood flow

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

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

Fig. 14. 6 A. Two fluids (one red and one clear) before flow begins; B Fig. 14.6 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.

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

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)

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

Pressure, Flow, and Resistance (cont.) Slight Changes in Diameter Has a Great Impact on Conductance Fig. 14.8 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

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

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

Pressure, Flow, and Resistance (cont.) Resistance to Blood Flow in Series and Parallel Vascular Circuits Fig. 14.9 Vascular Resistances in A: in series, and B: in parallel

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

Fig. 14.11 Effect of viscosity on hematocrit

Effects of pressure on flow and resistance Autoregulation attenuates the effect of arterial pressure on tissue blood flow Fig. 14.12 Effects of changes in arterial pressure on tissue

Pressure-Flow Relationship in Passive Vascular Beds Fig. 14.13 Effect of arterial pressure on blood flow through a passive vessel caused by increased or decreased sympathetic stimulation.