Vasculature: Arterial Blood Flow and Peripheral Resistance - PDF

5. Vasculature: arterial blood flow and peripheral resistance MD3001 Dr Alun Hughes 1 Lecture overview Fluid flow and turbulence Regulation of tissue blood flow Capillaries 2 Factors affecting resistance to blood flow Arterial pressure = cardiac output x total peripheral resistance Guyton p1...

5. Vasculature: arterial blood flow and peripheral resistance MD3001 Dr Alun Hughes 1 Lecture overview Fluid flow and turbulence Regulation of tissue blood flow Capillaries 2 Factors affecting resistance to blood flow Arterial pressure = cardiac output x total peripheral resistance Guyton p163 12th ed, p175 13th ed Fluid flow Parabolic velocity profile during laminar flow Laminar fluid flow Vessels lined with endothelial cells Fluid molecules touching wall adhere move slowly – Next layer slips over these • Next layer over these… –Middle most layers move the most rapid Guyton p161 12th ed, p173 13th ed Fluid flow and turbulence Turbulent flow Turbulence disrupts flow, increases resistance. • Poiseuille’s law doesn’t hold true during turbulence Reynold’s number (Re) is used to indicate whether flow is laminar or turbulent. Reynold’s number Reynold’s number (Re) is used to indicate whether flow is likely to be laminar or turbulent. – For a given system, there will be a “critical value” for Re, above which turbulence is highly likely. Re = (velocity of flow) x (radius of vessel) viscosity Turbulence is therefore likely with (because Re increases with): – – – – High velocity flow Large diameter vessels Low blood viscosity Abnormal vessel wall Thixotropic fluids Flow affects viscosity – Static blood has 100x the viscosity of flowing blood Rhoades p223 Figure 11.7 Korotkoff sounds Artificially generated turbulence – Ausculatory measurements using a sphygnomanometer cuff Guyton p170 12th ed, p183 13th ed LaPlace’s Law Distending pressure (P) produces an opposing force or tension (T) in the vessel wall, proportional to the radius (R) of the vessel: T = PR Think about pressure and vessel radius in: – Aorta – Arteriole – Capillary Rhoades p218 Figure 11.4 Practical consequences of LaPlace’s Law 1) Control of blood flow – Low tension required to oppose blood pressure in arterioles – Smooth muscle control of arteriole and precapillary sphincters are the sites of tissue blood flow regulation 2) Capillaries – Can be extremely thin and still withstand the pressure – Thin walls essential for exchange processes 3) Aneurysm Regulation of blood flow Arterioles – Control regional distribution (Local and extrinsic controls) Metarterioles – Links arterioles to venules, discontinuous smooth muscle Precapillary sphincters – When a true capillary branches from a metarteriole – Vasodilation produced by local factors Regulation of tissue blood flow Active and reactive hyperemia – Local factors associated with metabolic activity of tissues Flow autoregulation – In response to changes in arterial pressure • Arterial pressure ↑, arterioles constrict to reduce flow • Arterial pressure ↓, arterioles dilate to increase flow – Myogenic response • Stretch-activated Ca2+ channels Vasomotion – Spontaneous oscillating contraction of blood vessels Response to injury – E.g. endothelin-1 released from endothelial cells • Potent vasoconstriction Active and reactive hyperaemia Active hyperaemia – If tissue is highly active, the rate of flow will increase – E.g. by up to 20x in skeletal muscle Reactive hyperaemia – When blood supply blocked (few s to h) – Blood flow increases to 4-7x normal Naish p609 Role of the endothelial cells in regulating vascular tone Guyton p196 12th ed, p208 13th ed Regulation of arteriolar radius Flow through the microcirculation Rhoades p280 Figure 15.1 Regulation of local blood flow Blood flow through capillaries is intermittent, turning on/off every few seconds or minutes – At rest, only ~5% of total cardiac output is in the capillaries Acute regulation of local blood flow – Rapid changes within seconds or minutes – Vasodilator theory widely accepted (“local factors”) • E.g. ↑ PCO2, ↓ PO2, ↑ H+, ↑ K+, ↑ lacKc acid, ↑ adenosine, ↑ histamine Long term regulation of local blood flow – Change in physical size or number of blood vessels Capillary flow and pressure Large number of capillaries  large cross-sectional area Velocity of blood flow through capillaries is the slowest – Allows time for diffusion and exchange of nutrients and waste Basic principles of circulatory function (main points) The rate of blood flow to each tissue of the body is almost always precisely controlled in relation to the tissue need. The cardiac output is controlled mainly by the sum of all the local tissue flows. Arterial pressure regulation is generally independent of either local blood flow control or cardiac output control. Specialised flow (skin) Guyton p868 12th ed, p912 13th ed Specialised flow (lungs) Decreased alveolar O2 reduces local alveolar blood flow – Opposite to effect observed in systemic vessels – Mediator unknown Specialised flow (kidney) Learning outcomes To explain how physical factors affecting the cardiovascular system (e.g. flow, pressure, tension, vessel radius, blood viscosity, and the velocity of flow) are interrelated and the practical haemodynamic implications of these. To describe how blood flow through the microcirculation is regulated at the tissue level, via both short-term (acute) and long-term mechanisms. To identify systems with specialised blood flow requirements and describe the function of these specialised flows.