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THE AUSTRALIAN NATIONAL UNIVERSITY. Overview of Blood Flow and Factors Affecting It. Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU [email protected] au Blood_flow.pptx. Plan for System’s Part in Block 1.

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Overview of Blood Flow and Factors Affecting It.Christian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - [email protected]://

plan for system s part in block 1
Plan for System’s Part in Block 1
  • 6 May 2014 3 PM: Overview of Blood Flow
  • 13 May 2014 3 PM: Vascular Filtration
  • 27 May 2014 2 PM: Introduction Kidney Function
  • 28 May 2014 2 PM: Pulmonary Pressures and Volumes
  • 3 Jun 2014 3 PM: Partial Pressures and Blood Gasses
  • 10 Jun 2014 2 PM: Oxygen Delivery to Tissue
  • 11 Jun 2014 3 PM: Introduction to Block 2

The students should

  • be cognisant of a few physical principles that relate flow, pressure and velocity; among them Ohm’s law;
  • realise that arteries are cardiofugal and veins cardiopetal vessels;
  • know the notion of blood pressure;
  • appreciate factors determining resistance, pressure, flow, and its characteristics;
  • understand the distal impact of a resistance change; and
  • recognise why some vascular beds display different characteristics.
  • Role and properties of circulation
  • Haemodynamic principles
    • Ohm’s law
    • Resistance
    • Flow / Volume
    • Pressure
    • Wall tension
    • Impact of changes in R on distal P
  • Implications for circulation
    • Pressure and flow
    • Volumes
systemic pulmonary circulation
Systemic & Pulmonary Circulation
  • More or less continuous flow of blood through all tissues.
  • Systemic circulation: oxygenated blood to (artery) and hypoxige-nated from (veins) tissues.
  • Pulmonary circulation: hypoxygenated blood to (artery) and oxygenated from (vein) lung.
  • O2concentration is best expressed as .
  • Not all venous blood is low inand not all arterial blood is high in.

Rhoades & Pflanzer 2003

parts of systemic circulation
Parts of Systemic Circulation
  • Arterial system: high P on systemic side (MAP ~95 torr).
    • Cardiofugalvessels
    • To capillaries
  • Venous system: low P on systemic side (PMSF ~7 torr) .
    • Cardiopetalvessels
    • From capillaries
  • Lymphatic system: very low pressure (a few torr).
    • Drains lymph into big veins
  • Naming of vessel has nothing to do with.
physiological role of circulation
Physiological Role of Circulation
  • Purpose: continuous flow of blood through all tissues
  • Transport of
    • O2 and CO2,
    • nutrients and metabolites between different compartments (uptake, consumption, processing, storage),
    • water, electrolytes and buffers,
    • cells (host defence),
    • proteins (transport vehicles, immunoglobulins, etc.),
    • hormones and other signalling molecules, and
    • heat (dissipation).
flow pressure difference
Flow - Pressure Difference
  • Flow = volume (V) / time unit.
  • Net flow is constant: cardiac output = venous return.
  • Without a pressure difference, flow is zero (V = 0).
  • Flow is result of pressure difference along vessel (∆P).
  • Pressure = Force / Area= Energy per volume.
  • Pressure cannot be absolutely measured; only relative. In medicine, reference point is atmospheric pressure.

Rhoades & Pflanzer 2003

flow pressure relationship
Flow - Pressure Relationship
  • What do you know from hose?
  • Resistance relates flow (F) to pressure difference (ΔP).
  • The effect of R↑ is to dissipate energy per volume, i.e. P↓ distally (see later).
  • Ohm’s law (Darcy’s law).
    • The only law that you have to formally know (applies only to what I teach).
  • Only applies to time-invariant conditions (steady-state).
  • Rewritten specifically for circulationwhere MAP is mean arterial pressure, TPRtotal peripheral resistance and CO cardiac output.

G.S. Ohm, 1789-1854

H. Darcy, 1803-1858

resistances serial parallel
Resistances: Serial - Parallel
  • Kirchhoff’s laws apply:
    • Resistances in series:increase in Rtot.
    • Resistances in parallel:decrease in Rtot(total area for flow increases).
length and diameter
Length and Diameter
  • R is determined by L, r and η as follows: where L is vessel length, r is radius and ηis blood viscosity (dependent on haematocrit).
  • Resistance is proportional to total length, viscosity, but indirectly proportional to 4th power of vessel radius (r).
    • Every unit length imposes a small amount of R against flow.
    • P drops along vessels.
    • Smallest vessels determine biggest part of total resistance.
flow velocity and diameter
Flow Velocity and Diameter
  • What you know from the garden hose?… what you put in, is what you get out (conservation of volume and energy).
  • For constant throughput: v(velocity [cm/s]) ~ F/A, where F is flow and A is cross-sectional area; i.e. velocity is inversely proportional to cross-sectional area.
  • For example: as diameter of vena cava is bigger than that of aorta, flow velocity in vena cava must be smaller.
flow types in vessels
Flow Types in Vessels
  • Two forms: laminar and turbulent.
  • Velocity fastest in centre and close to 0 near vessel walls.
  • Blood flow is laminar below and turbulent above a critical velocity, which iswhere Re is Reynold’s number (< 1200 laminar; > 3000 turbulent), η viscosity,ρ fluid densityand r vessel radius.
  • vc small in aorta, larger in small vessels.
  • Laminar: F ~ ΔP; turbulent: F ~ √ΔP (large energy dissipation; uneconomical).
  • Clinically: rapid changes in diameter (stenosis, aneurism), valves (stenosis) and low viscosity (anaemia) can cause vibrations/sounds (palpation/auscultation).

Modified from Schmidt & Thews, 1977

what generates p
What Generates ΔP?
  • Heart, in particular muscle.
  • Corresponds to a force per unit area in Pa [N/m2].
  • Measured in kPa (body fluids typically in mmHg, i.e. torr).
  • Blood pressure: typically 120/80 torr.
  • Determinants of blood pressure in Block 2.
  • What does P represent?

Rhoades & Pflanzer 2003

physical nature of pressure
Physical Nature of Pressure
  • Energy (W) = ΔP · V
  • P is energy per unit volume.
  • Mechanical energy has 3 parts:
    • Pressure energy: ΔP · V
    • Gravit. energy: ρ · V · g · h
      • BP measurement at level of heart.
    • Kinetic energy: ρ · V · v2 / 2
  • Pressure raised by heart = const
    • Energy for speed-up from pressure.
    • P↓over stenosis as v↑ (problem).
    • Measurement of P with catheters.
  • Pressure is “versatile”; i.e. can drive different phenomena.

Modified from Boron & Boulpaep, 2002

Modified from Schmidt & Thews, 1977

pressure and wall tension
Pressure and Wall Tension
  • Pressure (∆P) is the same in all directions:
    • Longitudinal (driving force for flow).
    • Transmural (“stiffness”/tension of vessel): circular “force” needed to counter it; i.e. to hold vessel together.
  • Wall tension (T) is related toPaccording to Laplace’ law:
  • Large vessels are exposed to biggest wall tension (histological specialisation required).
  • Larger force required to contract dilated vessels than partially contracted ones.
functional specialisations
Functional Specialisations
  • Vessel wall tensions are matched by thickness of smooth muscle and connective/elastic fibres (see histology).
  • Tension of big arterial vessels is biggest; even more so of vessels, which are pathologically extended (aneurysms).

Modified from Berne et al., 2004

change in tpr and distal p
Change in TPR and Distal P
  • Over R, “energy” is lost (E dissipated): distal P↓.
  • Vasoconstriction: R↑ → P↓ in cap./venous bed (less P “gets through”).
  • Vasodilation: R↓ → P↑ in capillary/venous bed (more P “gets through”).
  • Changes in TPR have consequences on capil-lary/venous bed.

Levick, 5. ed., 2010

p and v in vascular beds
P and v in Vascular Beds
  • P highest in systemic arteries.
  • P lowest in large systemic veins.
  • P drops sharply in precapillary areas.
  • P in pulm. bed < systemic circ.
  • Cross-sectional area in capilla-ries very large (syst. & pulm.):
    • v is small.
  • v in pulmon. bed < syst. vessels
    • Larger cross-sectional area in lung.
  • v in aorta > than vena cava.
  • v continuous in capillaries but pulsatile in large vessels and pulm. bed.

Modified from Boron & Boulpaep, 2002

flow volumes in vascular beds
Flow / Volumes in Vascular Beds
  • Most blood in syst. vessels.
  • Very little is in syst. arteries.
  • Most blood is in syst. veins.
  • 80% of blood is in low pressure part of circulation.
  • v in veins < arteries.
  • Very little blood is in heart.

Modified from Boron & Boulpaep, 2002

take home messages
Take-Home Messages
  • A few physical principles describe F, P and v.
  • Arterioles determine peripheral resistance (~50%; resistive vessels).
  • Pressure causes wall tension; histological specialisations (potential for rupture).
  • P↓ after R↑: distally less P “gets through”.
  • Blood is primarily in venous system (~70%; capacitive vessels due to larger diameters).
  • Flow in capillaries is slowest and continuous.
Which vessel(s) determine the biggest amount of resistance in the circulation?
  • Aorta
  • Arteries
  • Arterioles
  • Capillaries
  • Veins

At which location occurs the biggest change in resistance?

  • Aortic valve
  • Arterial bifurcations
  • Precapillaryareas
  • Postcapillaryvenules
  • Venous valves
Which vessel(s) determine the biggest amount of resistance in the circulation?
  • Aorta
  • Arteries
  • Arterioles
  • Capillaries
  • Veins

At which location occurs the biggest change in resistance?

  • Aortic valve
  • Arterial bifurcations
  • Precapillaryarea
  • Postcapillaryvenules
  • Venous valves