Applied Cardiac Physiology Lecture 1 PDF

MSc PA Studies Applied Cardiac Physiology Module Lecture 1 Prof S Ghosh  Flow flow of blood through blood vessels driven by gradient of pressure flow is proportional to the High Flow Low...

MSc PA Studies Applied Cardiac Physiology Module Lecture 1 Prof S Ghosh  Flow flow of blood through blood vessels driven by gradient of pressure flow is proportional to the High Flow Low Pressure pressure difference Pressure between the ends of a vessel the higher the pressure difference the greater the flow  Flow resistance the flow for a given pressure gradient is determined by the resistance of the vessel resistance is determined by the nature of the fluid and the vessel  Definitions Flow: the volume of fluid passing a given point per unit time Velocity: the rate of movement of fluid particles along the tube Relationship between flow and velocity flow must be the same Flow Flow Flow Constant Constant Constant at all points along a vessel velocity can vary along the length if the radius of the tube changes at a given flow velocity is inversely proportional Velocity Velocity Velocity to surface area High Low High  Laminar flow in most blood vessels flow is laminar there is a gradient of velocity from the middle to the edge of the vessel velocity is highest in the centre fluid is stationary at the edge  Turbulent flow as the mean velocity increases flow eventually becomes turbulent the velocity gradient breaks down fluid tumbles over flow resistance greatly increased Now, consider a vessel with a constant pressure driving flow the flow will be determined by the mean velocity the mean velocity depends upon: – the viscosity of the fluid – the radius of the tube  Viscosity in laminar flow the fluid moves in concentric layers like layers of an onion the middle layers move faster than the edge layers so fluid layers must slide over one another  Viscosity the extent to which fluid layers resist sliding over one another is known as Low Viscosity viscosity the higher the viscosity the slower the central layers will flow, and the lower the average High Viscosity velocity  Effects of radius viscosity determines the slope of the gradient of velocity at a constant gradient, Low Cross sectional area the wider the tube the faster the middle layers move, so mean velocity is proportional to the cross sectional area of the tube High Cross sectional area  So mean velocity is – inversely proportional to viscosity – directly proportional to surface area (r 2) but, flow is the product of mean velocity and surface area n.b. this is not inconsistent with earlier slide, as flow is not fixed  Put mathematically flow   P x r2 x r2/(viscosity x length) this is Poiseulles Law  Put another way it is very much more difficult to push blood through small vessels than big ones the ‘thicker’ the blood the harder it is to push it through blood vessels What happens if blood vessels are connected together? Series resistances combine just like electrical R1 R2 resistances R=R1+R2 for vessels in series, Parallel resistances add R1 for vessels in parallel the effective R2 resistance is lower R=(R1xR2)/(R1+R2) Pressure, flow and resistance if flow is fixed – the higher the resistance the greater the pressure change from one end of the vessel to the other if pressure is fixed – the higher the resistance the lower the flow  To understand the CVS you must understand the relationships between flow, resistance and pressure, be able to say what happens to pressures if resistance changes at a constant flow and what happens to flow if resistance changes at a constant pressure  The whole circulation - 1 over the whole circulation flow is same at all points arteries are low resistance – pressure drop over arteries is small arterioles are high resistance – pressure drop over arterioles is large  The whole circulation - 2 Heart individual capillaries P P are high resistance Low but there are many Veins Arteries P resistance connected in parallel P P so the overall resistance is low Venules Arterioles High resistance – pressure drop over P capillaries small P P Capillaries  The whole circulation - 3 Heart 3 venules and veins 100 are low resistance Veins Arteries P Low resistance – pressure drop over venules & veins low 8 100 Venules Arterioles High resistance 35 10 35 Capillaries Pressures in mm Hg  The whole circulation - 4 the pressure within arteries is high because of the high resistance of the arterioles for a given total flow, the higher the resistance of the arterioles, the higher the arterial pressure  The whole circulation - 5 if the heart pumps more blood and the resistance of arterioles remains the same the arterial pressure will rise  Special problems of flow in blood vessels - 1 in some vessels eg aorta flow may become turbulent – resistance much increased flow also turbulent if a vessel is narrowed (eg atheroschlerosis) turbulent flow generates sound  Special problems of flow in blood vessels - 2 blood vessels have distensible walls the pressure within the vessel generates a transmural pressure between inside and outside which tends to stretch the tube  Distensible vessels as the vessel stretches, so Distensible Tube resistance falls so the higher the Flow pressure in a vessel, Rigid Tube the easier it is for blood to flow through it Pressure  Distensible vessels as vessels widen with increasing pressure more blood transiently flows in than out (and vice-versa) distensible vessels ‘store’ blood they have capacitance veins are most distensible  Funny things about blood blood cells congregate in the middle of the flow so apparent viscosity increased and cells go round faster than the plasma!  The Heart two pumps in series each side consists of – thin walled atrium – muscular ventricle flows into and out of ventricle controlled by valves – atrioventricular valves (mitral & tricuspid) – outflow valves (aortic and pulmonary)  Heart Muscle a specialised form discrete cells connected electrically cells contract when action potential in membrane  Heart Muscle action potential causes a rise in intracellular calcium action potential long - single contraction lasts 280 ms - systole action potentials triggered by spread of excitation from cell to cell  Pacemakers an action potential generated in a small group of cells will spread over the whole heart and produce a coordinated contraction pacemakers generate one action potential at regular intervals  Phases of the Cardiac Cycle each action potential produces one beat - systole interval between beats known as diastole  Spread of excitation -1 pacemaker in sino- atrial node - right 1 atrium activity first spreads over the atria - atrial systole to reach the atrio- ventricular node, where delayed for about 120 ms  Spread of excitation - 2 from a-v node spreads down the septum between the ventricles then spreads through 4 the ventricular myocardium from inner 2 (endocardial) to outer (epicardial) surface ventricle contracts from the apex up, forces blood towards the 3 outflow valves  The Cardiac Cycle at rest the SA node generates an action potential about once a second this produces a short atrial systole followed by a longer ventricular systole ventricular systole last about 280 ms followed by a relaxation lasting about 700 ms before the next systole  The Left Ventricle inflow valve - the mitral valve – allows blood from the atrium to ventricle, but not vice-versa opens when atrial pressure exceeds intraventricular pressure closes when ventricular pressure exceeds atrial pressure  The Left Ventricle outflow valve - the aortic valve – allows blood from the ventricle to the aorta, but not vice-versa opens when intra-ventricular pressure exceeds aortic pressure closes when aortic pressure exceeds ventricular pressure  The Cardiac Cycle - 1 start towards the end of ventricular systole – ventricles contracted Pressure – intra-ventricular pressure high – outflow valves open – blood flowing into the arteries – ventricular pressure > Volume atrial pressure so a/v Aorta Ventricle valves closed Atrium  The Cardiac Cycle - 2 ventricles begin to relax – intra ventricular pressure falls Pressure – intra-ventricular pressure becomes < arterial – brief backflow closes outflow valves – all valves now closed – get isovolumetric Volume relaxation Aorta Ventricle Atrium  The Cardiac Cycle - 3 during systole blood has continued to return to the atria Pressure – atrial pressure relatively high – as intra-ventricular pressure falls, eventually, – atrial pressure > intra- venticular pressure Volume – so a/v valves open Aorta Ventricle Atrium  The Cardiac Cycle - 4 with the a/v valves open ventricles – fill rapidly - ‘rapid Pressure filling phase’ – lasts about 200-300 ms – most filling of ventricles occurs in Volume Aorta this phase Ventricle Atrium  The Cardiac Cycle - 5 as diastole continues the ventricles fill more slowly Pressure – intraventricular pressure rises as the ventricular walls stretch – until intra-ventricular pressure matches atrial, and filling stops Volume Aorta Ventricle Atrium  The Cardiac Cycle - 6 atrial systole – forces a small extra amount of blood into Pressure the ventricles – but the heart pumps perfectly well without atrial systole Volume Aorta Ventricle Atrium  The Cardiac Cycle - 7 ventricular systole – intraventricular pressure rises very rapidly Pressure – quickly exceeds atrial pressure – so after brief backflow a/v valves close – all valves closed – get isovolumetric Volume contraction Aorta Ventricle Atrium  The Cardiac Cycle - 8 intraventricular pressure rises very rapidly Pressure – until intra-ventricular pressure > arterial pressure – which has been falling in diastole – so outflow valves Volume Aorta open Ventricle Atrium  The Cardiac Cycle - 9 as outflow valves open – blood is ejected Pressure rapidly into the arteries – rapid ejection phase – arterial pressure rises rapidly Volume Aorta Ventricle Atrium  The Cardiac Cycle - 10 as arterial pressure rises – the rate of ejection of Pressure blood falls – both arterial and intraventricular pressures peak towards the end of systole – outflow eventually Volume ceases wih blood still in Aorta ventricle Ventricle Atrium  The Cardiac Cycle - 11 eventually, systole ends – and back to 1 Pressure Volume Aorta Ventricle Atrium  Heart Sounds two main sounds associated with valves closing – first sound - ‘lup’ - closure of a/v valves – second sound - ‘dup’- closure of outflow valves  Heart Sounds first sound at onset of ventricular systole second sound at the end of ventricular systole at rest, interval from 1st to 2nd about 280 ms interval second to next first 700 ms  The cardiac cycle Pressure Volume Aorta Ventricle Atrium LUP DUP  Heart Sounds occasionally hear extra sounds in normal hearts – third sound early in diastole – fourth sound - atrial systole  Heart Murmurs turbulent flow of blood generates murmurs – narrowed valve - stenosis – valve not closing properly - incompetence murmurs occur when blood flow highest so can predict when in the cardiac cycle they should occur  Heart Murmurs example – aortic stenosis produces murmur in the rapid ejection phase  Cardiac output the heart ejects a stroke volume with each beat cardiac output is stroke volume times heart rate at rest 80ml x 60 ie c. 5l.min-1