THE AUSTRALIAN NATIONAL UNIVERSITY

1. THE AUSTRALIAN NATIONAL UNIVERSITY Overview of Blood Flow and Factors Affecting It.Christian StrickerAssociate Professor for Systems PhysiologyANUMS/JCSMR - ANUChristian.Stricker@anu.edu.auhttp://stricker.jcsmr.anu.edu.au/Blood_flow.pptx

3. 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

4. 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.

5. 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

6. 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

7. 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.

8. 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).

9. 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

10. 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

11. 1. Determinants of Resistance

12. Resistances: Serial - Parallel • Kirchhoff’s laws apply: • Resistances in series:increase in Rtot. • Resistances in parallel:decrease in Rtot(total area for flow increases).

13. 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.

14. 2. Considerations for Flow

15. 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.

16. 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

17. 3. Pressure and Wall Tension

18. 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

19. 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

20. 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.

22. 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

23. 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

24. 4. Implications for Vascular Beds

25. 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

26. 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

27. 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.

28. 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

29. That’s it folks…

30. 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