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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 http://stricker.jcsmr.anu.edu.au/ Blood_flow.pptx. Plan for System’s Part in Block 1.

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The australian national university

THE AUSTRALIAN NATIONAL UNIVERSITY

Overview of Blood Flow and Factors Affecting It.Christian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - [email protected]://stricker.jcsmr.anu.edu.au/Blood_flow.pptx


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 australian national university

Aims

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.


Contents

Contents

  • 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


1 determinants of resistance

1. Determinants of Resistance


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.


2 considerations for flow

2. Considerations for Flow


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


3 pressure and wall tension

3. Pressure and Wall Tension


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


4 implications for vascular beds

4. Implications for Vascular Beds


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.


The australian national university

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


That s it folks

That’s it folks…


The australian national university

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


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