– more pulsatile flow – more cardiac work – not hypertension Laplace’s Law (thin-walled cylinder): T = wall tension P = pressure r = radius For thick wall cylinder P = pressure σ = wall stress r = radius w = wall thickness
T = Pr
Blood Volume [%]
• Effects 200
C=
dV
30--40 yrs
dP 50-60 yrs
70--75 yrs
100
100
Pr = w
150
200
Pressure [mm Hg] 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
Arterial System
Bioengineering 6000 CV Physiology
Arterial System as Hydraulic Filter • Pulsatile --> smooth flow • Cardiac energy conversion • Reduces total cardiac work
Arterial Pressure
Cardiac Output
t
Physiological Ideal
t
Cardiac Output
Arterial Pressure
t t
Cardiac Output
Arterial Pressure
t t
Arterial System
Pulsatile Challenge
Filtered Reality
Bioengineering 6000 CV Physiology
Elastic Recoil in Arteries
Arterial System
Bioengineering 6000 CV Physiology
Effects of Vascular Resistance and Compliance
Arterial System
Bioengineering 6000 CV Physiology
Cardiac Output vs. Runoff
Arterial Pressure/Flow
Arterial System
Bioengineering 6000 CV Physiology
Basic Pressure Equations Mean arterial pressure: which we can approximate as with ! !
Ps = systolic pressure Pd = diastolic pressure
Total peripheral resistance is with !
Pa = mean arterial pressure
!
Pra = right atrial pressure
!
Qr = runoff flow into veins (=Qh at equilibrium)
If we assume Pra=0
Arterial System
Bioengineering 6000 CV Physiology
Time Course of Arterial Flow We can estimate change in arterial volume as:
Cardiac output
Runoff flow (1)
Arterial volume change
Arterial compliance we define as
(2)
Which we differentiate w.r.t time to get
(3)
Substituting (1) into (3), we get
(4) (5)
or Arterial System
Bioengineering 6000 CV Physiology
Arterial Pressure Response to Cardiac Output
Increase in Qh (CO) or Rp
Small Compliance Arterial Pressure
• 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
P2
P1
Large Compliance
Time Arterial System
Bioengineering 6000 CV Physiology
Pressure and Age (Compliance)
Pressure
Mean Pressure
Arterial System
Bioengineering 6000 CV Physiology
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
Arterial System
Bioengineering 6000 CV Physiology
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)
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
Arterial System
Bioengineering 6000 CV Physiology
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)
