THE AUSTRALIAN NATIONAL UNIVERSITY Overview of Blood Flow and Factors Affecting It. Christian Stricker Associate Professor for Systems Physiology ANUMS/JCSMR - ANU Christian.Stricker@anu.edu.au http://stricker.jcsmr.anu.edu.au/Blood_flow.pptx

Plan for System’s Part in Block 1 14 Mar 2017 3 PM: Overview of Blood Flow 15 Mar 2017 3 PM: Vascular Filtration 28 Mar 2017 3 PM: Pulmonary Pressures and Volumes 3 April 2017 10:30 AM: Partial Pressures and Blood Gasses 5 April 2017 4 PM: Oxygen Delivery to Tissue 12 April 2017 3 PM: Introduction to Kidney Function Jun 2017 10 AM: Introduction to Block 2

At the end of this lecture students should be able to Aims At 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; and argue why some vascular beds display different characteristics.

Contents Role and properties of circulation Haemodynamic principles Ohm’s law Resistance Flow / Volume Pressure Wall tension Implications for circulation Pressure and flow Volumes

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

Parts of Systemic Circulation Arterial system: high P on systemic side (mean ~95 torr). cardiofugal vessels to capillaries Venous system: low P on syste-mic side (mean initial P ~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 .

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, immu-noglobulins, etc.), hormones and other signalling molecules, and heat (dissipation).

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

Flow - Pressure Relationship What do you know from hose? Resistance relates flow (F) to pressure difference (ΔP). The effect of R↑ is to use up 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 (steady-state). Rewritten for circulation MAP = mean arterial pressure TPR = total peripheral resistance CO = cardiac output. G.S. Ohm, 1789-1854 H. Darcy, 1803-1858

1. Determinants of Resistance

Resistances: in Series vs. 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 R is determined by L, r and η as follows: where L is vessel length, r radius and η 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

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.

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 the 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

3. Pressure and Wall Tension

What Generates ΔP? Heart, in particular muscle. Corresponds to a force per unit area. Normally measured in kPa (but body fluids typically in mmHg, i.e. torr). Blood pressure: ~120/80 torr. Determinants of blood pressure in Block 2. What does P represent? Rhoades & Pflanzer 2003

Physical Nature of Pressure Energy (W) = ΔP · V P ≡ 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 Boron & Boulpaep, 2002 Modified from Schmidt & Thews, 1977

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.

4. Implications for Vascular Beds

Functional Specialisations Modified from Berne et al., 2004 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).

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. Modified from Boron & Boulpaep, 2002

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. Modified from Boron & Boulpaep, 2002

Take-Home Messages A few physical principles describe F, P and v. Arterioles determine peripheral resistance (~50%; resistive vessels). R↑: uses up P and less “gets through”. Pressure causes wall tension; histological specialisations (potential for rupture). Blood is primarily in venous system (~70%; capacitive vessels due to larger diameters). Flow in capillaries is slowest and continuous.

At which location occurs the biggest change in resistance? 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 Precapillary areas Postcapillary venules Venous valves

That’s it folks…

At which location occurs the biggest change in resistance? 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 Precapillary area Postcapillary venules Venous valves