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Heart as a Pump

Physiology and Pharmacology

QuestionAnswer
Vascular system Arteries - rapid conduits Arterioles - resistance vessels Capillaries - exchange vessels Veins - capacitance vessels
Designing a pump to circulate blood Source of energy to propel blood - cardiac muscle Restricting flow to one direction - valves Refilling after contraction - diastole and a refilling pump Coordinating pumps - combine into one organ as an electrical syncytium
Four chambers of the heart Left atrium Right atrium Left ventricle Right ventricle This is a conserved layout Valves between chambers
Apex beat Heart twists and taps on the chest wall at the 5th intercostal space Most direct way of measuring heart rate
Organisation of cardiac muscle Alternates sarcomere direction between longitudinal and circular Wrings the heart to move blood out
Cascade to produce blood flow Electricity - tension - pressure - flow Uses ECC, Laplaces law and Ohms law
Functional readout of cardiac activity Cardiac output = heart rate x stroke volume 5 litres per minute 200 million litres in a life
Laplace's law Pressure = (2 x tension)/radius A larger heart needs greater tension to generate a threshold pressure In hypertrophy the heart needs to contract with a greater force The pressure generated determines if a valve opens, so a threshold must be reached
Ohm's law Flow = pressure/resistance This is 0 when valves are closed Left and right valves close at the same time in healthy individuals
Healthy valves Either in an open or closed state Open = allows flow Closed = 0 flow
Diseased valves Incompetent - cannot fully close - allows reflux which reduces efficiency of the heart Stenosis - cant fully open - increases resistance so heart has to create a larger pressure
Valves open in one direction Valves only allow blood flow unidirectionally Can flow down pressure gradient only if in the correct direction Chordae tendineae and papillary muscles prevent opening in the wrong direction
When are both valves closed There is a range of pressures when ventricular pressure is sufficient to close the inlet valve but not open the outlet valve During this time the volume is constant Known as isovolumetric
Isovolumetric phase Secures stroke volume - avoids backflow Generates the pressure required to pump blood without any change in volume Blood only released when a sufficient pressure is reached
The cardiac cycle Split into ventricular diastole and systole Muscle is excited in systole and relaxed in diastole Atria prompts ventricular phases
Pressure driven flow Initially - pressure low in all chambers AP arrives at atrium - they contract and pressure rises, AV valve opens and flow begins AP reaches ventricle - contract to give increase in pressure, flow into aorta Blood in artery gives an increase in pressure
Ventricular diastole - isovolumetric relaxation All valves are closed Isovolumetric as pressure decreases but there is no change in volume of the ventricle
Ventricular diastole - filling Ventricle returns to a normal shape - end diastolic volume of 120ml Has a suction effect which draws blood from the atria Pressure lowest in ventricle
Atrial systole - ventricular filling SAN spontaneously activates and atria contract Increased pressure forces blood into the ventricle
Ventricular systole - isovolumetric contraction Excitation jumps to the ventricle Shuts inlet valve but does not open outlet valve Constant volume with increasing pressure
Ventricular systole - ejection Pressure of ventricle opens semilunar valve Begins ejection phase causing blood to leave the heart
Timing of the cardiac cycle Isovolumetric phases are short - 1/10 s Longest phase is ventricular filling Need to look for diastolic dysfunction - not enough time to fully relax
Opening and closing of valves Isovol relaxation - All valves closed Ventricular filling - outlet closed and inlet open Isovol contraction - both valves closed Ejection - outlet valve open, inlet valve closed
Heart sounds Closing of valves generated heart sounds 1st - end of ventricular filling due to closure of AV valves 2nd - end of ejection due to closure of semilunar valves
Role of atria Assist in ventricular filling By 10-20% in young - more elastic ventricles 50% in elderly - stiffer ventricles More important when filling time is shortened like in exercise Atrial fibrillation can cause large problems
Pressure and volume in left ventricle Small increase due to atrial contraction Large increase in pressure during ventricular systole Volume increases in ventricular filling and rapidly decreases in ejection Volume changes slightly delayed from pressure changes - due to isovolumetric phase
Pressure in aorta When valve open aorta and ventricles communicate and have the same pressure When valve closes ventricular pressure decreases while aorta pressure stays high Dicrotic notch - increase in pressure due to valve closing
Central venous pressure Rises when atrium contracts, AV valve shuts and atrium fills with blood Falls when atrium relaxes and AV valve opens A measure of how well the heart is working Good reflection of the atria as no valve is present between them
Jugular venous pressure Three waves per cycle Increases in atrial systole Begins to fall and rises again when tricuspid valve closes and bulges into atrium Falls as atrium relaxes Increases as atria fill Falls as tricuspid valve opens to release pressure
Pressure-volume loop Ventricle filling-rapid increase in vol, small increase in pressure Isovolumetric contraction-increase in pressure no change in volume Ejection-volume decreases, pressure increases then falls Isovolumetric relaxation-fall in pressure no change in vol
Work done by the heart Area under the pressure volume graph is work done by the heart Work done = distance (volume) x Force (pressure) Working out heart respiratory rate can allow calculation of efficiency Width = stroke volume
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