Arterial System Bioengineering 6000 CV Physiology

Arterial SystemLecture Block 10

Arterial System Bioengineering 6000 CV Physiology

Vascular Structure/Function

Bioengineering 6000 CV PhysiologyArterial System

Functional Overview

Bioengineering 6000 CV PhysiologyArterial System

Vessel Structure

Endothelium

Elastic tissue

Smooth Muscle

Fibrous Tissue

Arteriole30 µm

Capillary8 µm

Venule20 µm

6 µm 0.5 µm 1 µm

Diameter

Wall thickness

Aorta25 mm

Artery4 mm

Vein5 mm

Vena Cava30 mm

2 mm 1 mm 0.5 mm 1.5 mm

Bioengineering 6000 CV PhysiologyArterial System

Aortic Compliance• Factors:

– age– athersclerosis

• Effects– more pulsatile flow– more cardiac work– not hypertension

Pressure [mm Hg]100 200150

Blo

od V

olum

e [%

]

C = dVdP

20--24 yrs

30--40 yrs

50-60 yrs

70--75 yrs100

300

200

Laplace’s Law (thin-walled cylinder): T = wall tensionP = pressurer = radius

T = Pr

� =Pr

w

For thick wall cylinderP = pressureσ = wall stressr = radiusw = wall thickness Tension

[dyne/cm]Wall Stress[dyne/cm2]

Aorta 2 x 105 10 x 105

Capillary 15-70 1.5 x 105

Arterial System Bioengineering 6000 CV Physiology

Arterial Hydraulic Filter

Bioengineering 6000 CV PhysiologyArterial System

Arterial System as Hydraulic Filter

• Pulsatile --> smooth flow

• Cardiac energy conversion

• Reduces total cardiac work

CardiacOutput

ArterialPressure

tt

PhysiologicalIdeal

CardiacOutput Arterial

Pressure

t

t

Pulsatile Challenge

CardiacOutput

ArterialPressure

t

t

Filtered Reality

Bioengineering 6000 CV PhysiologyArterial System

Elastic Recoil in Arteries

Arterial System Bioengineering 6000 CV Physiology

Effects of Vascular Resistance and Compliance

Bioengineering 6000 CV PhysiologyArterial System

Cardiac Output vs. Runoff

Arterial Pressure/Flow

Bioengineering 6000 CV PhysiologyArterial System

Basic Pressure Equations

Mean arterial pressure:

which we can approximate as

with ! Ps = systolic pressure! Pd = diastolic pressure

Total peripheral resistance is

If we assume Pra=0

with ! Pa = mean arterial pressure! Pra = right atrial pressure! Qr = runoff flow into veins (=Qh at equilibrium)

Bioengineering 6000 CV PhysiologyArterial System

Time Course of Arterial Flow

We can estimate change in arterial volume as:Cardiac output Runoff flow

Arterial volume change

(1)

Which we differentiate w.r.t time to get (3)

Arterial compliance we define as (2)

Substituting (1) into (3), we get

or

(4)

(5)

Bioengineering 6000 CV PhysiologyArterial System

Arterial Pressure Response to Cardiac Output

• Stable pressure determined by flow and peripheral resistance

• Increase in CO or Rp both increase pressure

• Pressure always changes to force CO to equal runoff flow

• Compliance affects rate but not final values

Art

eria

l Pre

ssur

e

Time

Large Compliance

Small Compliance

P1

P2

Increase in Qh (CO) or Rp

Bioengineering 6000 CV PhysiologyArterial System

Pressure and Age (Compliance)

Pre

ssur

e

Mean Pressure

Bioengineering 6000 CV PhysiologyArterial System

Peripheral Pulse Pressure

• Pressure wave velocity– vp = k/C– vp increases along the

arteries and with age• Pressure wave pulse

amplitude grows with distance from heart– reflection/superposition– decrease in C– dispersion

Arterial System Bioengineering 6000 CV Physiology

Venous System

Bioengineering 6000 CV PhysiologyArterial System

Venous System

• Venous volume– Large volume, low pressure system– Reservoir of blood (50% of total volume)– Blood loss covered by venous system

• Vasoconstriction, drinking (blood doning)

• Venous flow– Skeletal muscle activity– Valves– Breathing– Paristaltic contractions in venules

Bioengineering 6000 CV PhysiologyArterial System

Venous Valves

• Muscle pump• Unidirectional flow• Vericose veins

Arterial System Bioengineering 6000 CV Physiology

Measurement

Bioengineering 6000 CV PhysiologyArterial System

Measuring Blood Pressure: Catheters

• Liquid column and external manometer– frequency response of

transducer and fluid column

– calibration and zeroing– motion artifacts

• Manometer-tipped catheters– higher frequency

response– less motion artifact

Bioengineering 6000 CV PhysiologyArterial System

Auscultatory Blood Pressure Method

• Effect of arm position• Alternate measurement

locations (leg)• Pressure varies during the

day (lowest during sleep)• Psychological bias in

measurements (in subject and operator)

Bioengineering 6000 CV PhysiologyArterial System

Automated Pressure Measurement

Ausculatory

Oscillometric

Bioengineering 6000 CV PhysiologyArterial System

Measuring Blood Flow

• Ultrasound flowmeter (velocity)

• Electromagnetic flowmeter (velocity)

• Thermal dilution• Functional MRI (diffusion

or oxygenation)

Bioengineering 6000 CV PhysiologyArterial System

Measuring Blood Flow II

• Bioelectric impedance (plethysmography)

• Light (pulse oxymetry)

Finger artery

Bone

Lightproofcasing

Elastomer ring

To device