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Overview of Blood Flow and Factors Affecting It.

Christian StrickerAssociate Professor for Systems Physiology

ANUMS/JCSMR - ANU

Christian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Blood_flow.pptx

THE AUSTRALIAN NATIONAL UNIVERSITY

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Plan for System’s Part in Block 1

• 12 May 2015 3 PM: Overview of Blood Flow

• 18 May 2015 11:30 AM: Vascular Filtration

• 26 May 2015 2 PM: Introduction Kidney Function

• 27 May 2015 2 PM: Pulmonary Pressures and Volumes

• 2 Jun 2015 3 PM: Partial Pressures and Blood Gasses

• 9 Jun 2015 2 PM: Oxygen Delivery to Tissue

• 12 Jun 2015 1 PM: Introduction to Block 2

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AimsAt the end of this lecture students should be able to

• apply a few physical principles that relate to flow, pressure

and velocity; among them Ohm’s law;

• recognise that arteries are cardiofugal and veins

cardiopetal vessels;

• outline the notion of blood pressure;

• identify factors determining resistance, pressure, flow, and

its characteristics;

• point out the distal impact of a resistance change; and

• argue why some vascular beds display different

characteristics.

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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

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Systemic & Pulmonary Circulation

• More or less continuous flow of blood

through all tissues.

• Systemic circulation: oxygenated blood

to (artery) and hypoxygenated from

(veins) tissues.

• Pulmonary circulation: hypoxygenated

blood to (artery) and oxygenated from

(vein) lung.

• O2 concentration is best expressed as

.

• Not all venous blood is low in and

not all arterial blood is high in .

Rhoades & Pflanzer 2003

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Parts of Systemic Circulation• Arterial system: high P on

systemic side (MAP ~95 torr).– Cardiofugal vessels

– To capillaries

• Venous system: low P on

systemic side (PMSF ~7 torr) .

– Cardiopetal vessels

– From capillaries

• Lymphatic system: very low

pressure (a few torr).– Drains lymph into big veins

• Naming of vessel has nothing to

do with .

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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).

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Flow - Pressure Difference• Flow (F) = volume (V) / time unit.

• Net flow is constant:

cardiac output = venous return.

• Without a pressure difference, flow

is zero (F = 0 → V = 0).

• Flow is result of pressure difference

along vessel (∆P).

• Pressure = Force / Area = Energy

per volume.

• Pressure can’t be absolutely measu-

red; only relative. In medicine, refe-

rence point atmospheric pressure.Rhoades & Pflanzer 2003

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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 const. conditions (SS).

• Rewritten for circulation

MAP = mean arterial pressure

TPR = total peripheral resistance

CO = cardiac output.G.S

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1. Determinants of Resistance

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Resistances: Serial - Parallel

• Kirchhoff’s laws apply:– Resistances in series:

increase in Rtot.

– Resistances in parallel:

decrease in Rtot (total area

for flow increases).

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Length and Diameter• R is determined by L, r and η as follows:

where L is vessel length, r is radius and η is 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.

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2. Considerations for Flow

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Flow Velocity and Diameter

• What do 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.

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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 is

where Re is Reynold’s number (< 1200 laminar; > 3000 turbulent), η viscosity,ρ fluid density and 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 (ste-nosis, aneurism), valves (stenosis) and low viscosity (anaemia) can cause vibrations/sounds (palpation/auscultation).

Modified from Schmidt & Thews, 1977

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3. Pressure and Wall Tension

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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

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Physical Nature of Pressure• Energy (W) = ΔP · V

• P is energy per unit volume.

• Mechanical energy has 3 parts:– Pressure energy: ΔP · V

– Gravitational 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 Schmidt & Thews, 1977

Mod

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from

Bor

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Bou

lpae

p, 2

002

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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 to P according 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.

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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

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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

capillary/venous bed.Levick, 5. ed., 2010

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4. Implications for Vascular Beds

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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 capillaries very

large (syst. & pulm.):– v is small.

• v in pulmon. bed < syst. vessels– Larger cross-sectional area in lung.

• v in aorta > in vena cava.

• v continuous in capillaries but pulsatile

in large vessels and pulm. bed.

Mod

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Bor

on &

Bou

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002

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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 < in arteries.

• Very little blood is in heart.

Mod

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from

Bor

on &

Bou

lpae

p, 2

002

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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.

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Which vessel(s) determine the biggest amount of resistance in the circulation?

A. AortaB. ArteriesC. ArteriolesD. CapillariesE. Veins

At which location occurs the biggest change in resistance?

F. Aortic valveG. Arterial bifurcationsH. Precapillary areasI. Postcapillary venulesJ. Venous valves

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That’s it folks…

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Which vessel(s) determine the biggest amount of resistance in the circulation?

A. AortaB. ArteriesC. ArteriolesD. CapillariesE. Veins

At which location occurs the biggest change in resistance?

F. Aortic valveG. Arterial bifurcationsH. Precapillary areaI. Postcapillary venulesJ. Venous valves