Guyton ch 14

Chapter 14:Overview of the Circulation;

Medical Physics of Pressure, Flow, and Resistance

Slides by J. P. Granger, Ph.D.

Overall Objectives

Physical characteristics of the circulation: distribution of blood volume total cross sectional area velocity blood pressure

Determinants of blood flow

Define and calculate blood flow, resistance, and pressure

Define and calculate conductance

Know Poiseulle’s law

The Circulatory System Services the Needs of the Tissues

• Transporting nutrients to the tissues• Transporting waste products away from the tissues• Transporting hormones

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Components of the Circulation

Figure 14-1; Guyton and Hall

The Circulatory System is Composed of the Systemic and Cardiopulmonary Circulation

Systemic Circulation - Serves all tissues except the lungs

- Contains 84% of blood volume - Also called the peripheral circulation

Pulmonary Circulation - Serves the lungs - Lungs contain 9% of blood volume and heart 7%

The Capillaries Have the Largest Total Cross-sectional Area of the Circulation

cm

Aorta 2.5Small Arterioles 20Arterioles 40Capillaries 2500Venules 250Small Veins 80Venae Cavae 8

Velocity of Blood Flow is Greatest in the Aorta

Velocity of Blood Flow = Blood Flow Cross sectional area

Aorta > Arterioles > Small veins > Capillaries

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The Majority of Blood Volume is in the Veins

Figure 14-1; Guyton and Hall

Blood Pressure Profile in the Circulatory System

Systemic Pulmonary

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• High pressures in the arterial tree• Low pressures in the venous side of the circulation• Large pressure drop across the arteriolar-capillary junction

Basic Theory of Circulatory Function Blood flow to tissues is controlled in

relation to tissue needs. Cardiac output is mainly controlled by

local tissue flow. Arterial pressure is controlled independent

of either local blood flow control or cardiac output control.

Variations in Tissue Blood Flow

Brain 14 700 50Heart 4 200 70Bronchi 2 100 25Kidneys 22 1100 360Liver 27 1350 95 Portal (21) (1050) Arterial (6) (300)Muscle (inactive state) 15 750 4Bone 5 250 3Skin (cool weather) 6 300 3 Thyroid gland 1 50 160 Adrenal glands 0.5 25 300 Other tissues 3.5 175 1.3

Total 100.0 5000 ---

Per cent ml/minml/min/100 gm

Relationship between Pressure, Flow, and Resistance Q=P/R Flow (Q) through a blood vessel is

determined by: 1) The pressure difference (P)

between the two ends of the vessel 2) Resistance (R) of the vessel

Determinants of Blood FlowFLOW = arterial - venous pressure (P)

resistance (R)

FLOW = 100 - 0 mmHg.1 mmHg/ml/min

FLOW = 100 - 20 mmHg.1 mmHg/ml/min

FLOW = 1000 ml/min FLOW = 800 ml/min

100 mmHg

0 mmHg 20 mmHg

100 mmHg

R = .1mmHg/ml/min R = .1mmHg/ml/min

A B

How Would a Decrease in Vascular Resistance Affect Blood Flow?

FLOW = P RESISTANCE

FLOW = P RESISTANCE

Conversely,

Blood Flow Blood flow is the quantity of blood that passes a given

point in the circulation in a given period of time.

Unit of blood flow is usually expressed as milliliters (ml) or Liters (L) per minute.

Overall flow in the circulation of an adult is 5 liters/min which is the cardiac output.

Blood Vessel

Characteristics of Blood Flow

Blood usually flows in streamlines with each layer of blood remaining the same distance from the wall, this type of flow is called laminar flow.

– When laminar flow occurs, the velocity of blood in the center of the vessel is greater than that toward the outer edge creating a parabolic profile.

Blood Vessel

Laminar flow

Laminar Vs. Turbulent Blood Flow

Turbulent flow

Causes of turbulent blood flow:• high velocities• sharp turns in the circulation• rough surfaces in the circulation• rapid narrowing of blood vessels

• Laminar flow is silent, whereas turbulent flow tend to cause murmurs.

• Murmurs or bruits are important in diagnosing vessels stenosis, vessel shunts, and cardiac valvular lesions.

Aortic Aneurysm Atherosclerosis

Effect of Wall Stress on Blood Vessels

Turbulent flow increases wall stress

Blood Pressure Blood pressure is the force exerted by the blood

against any unit area of vessel wall.

Measured in millimeters of mercury (mmHg). A pressure of 100 mmHg means the force of blood was sufficient to push a column of mercury 100mm high.

Low pressures are sometimes reported in units of mm of water.

1mmHg = 13.6 mm of water

High-Fidelity Methods for Measuring Blood Pressure. Themercury cannot rise and fall rapidly. so is excellent for recordingsteady pressures only cannot respond to pressure changesthat occur more rapidly than about one cycle every 2 to3 seconds. Whenever it is desired to record rapidlychanging pressures, High-Fidelity Methods are used

Resistance

Resistance is the impediment to blood flow in a vessel.

Resistance can be calculated by dividing the pressure difference between two points in a vessel by the vessel blood flow

R = P = mmHg

Q ml/min

Parallel and Serial Resistance Sites in the Circulation

Conductance Conductance is a measure of the blood flow through

a vessel for a given pressure difference. Units ml/min per mmHg

Poiseulle’s Law = Q = Pr4

8l

Conductance = 1 .

Resistance

Effect of Vessel Diameter on Blood Flow• Conductance is very sensitive to change in diameter of vessel.

• The conductance of a vessel increases in proportion to the fourth power of the radius.

Figure 14-9; Guyton and Hall

Hematocrit and Viscosity Effects on Blood Flow

Figure 14-11; Guyton and Hall Figure 14-12; Guyton and Hall