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Hemodynamics

Organisation of the Body

QuestionAnswer
Hemodynamics Applying physical principles to study the movement of blood Flow Pressure Tension Compliance Resistance Energy
Blood flow Volume in motion A number expressed as distance/time Has a precise physical definition Flow = change in V/change in t Already a rate
Cardiac output An example of flow CO = SV x HR Around 5 L/min Easy to measure - tells you about heart failure Thicker left wall - more pressure to overcome more resistance
Conservation of flow Blood does not disappear or spontaneously form Therefore flow from the lungs = flow to the body and flow from the body = flow to the lungs Flow must be equal (steady state) no matter where in the body Despite different sizes - flow is equal
Is the blood a closed circuit Volume can be lost or gained at exchange surfaced so the closed circuit analogy is only an approximation E.g. blood into kidneys is less than venous output
Is velocity the same as flow Flow = volume/time Velocity = distance/time flow has to be the same in all structures, whist velocity will be faster in smaller compartments Flow = velocity x cross sectional area Blood moves slower in capillaries but flow is same
Blood pressure A driving force for blood flow Pressure = force/area Changes with time E.g. pressure higher in systole Left ventricle assist device - flow with no pulse as produces constant flow
Pressure wave decays with distance Blood pressure taking in arm - allows low resistance so low pressure change from aorta Highest near heart Largest resistance to flow is in arterioles Higher resistance = low pressure - decrease with distance from heart
Does pulse velocity measure speed of blood Pulse represents vibration of vessel wall - ahead of blood Does not represent blood flow Elastic vasculature - compliant and health so velocity is slow - lots of effort to vibrate Stiff vasculature - faster velocity as easier to vibrate
Units of pressure mmHg or cmH2O Force = area x height x density x g Pressure = height x density x g Pressure is proportional to height Knowing height gives an idea of pressure
Measuring central venous pressure Patient lies in supine Tilted backwards Moved forwards until jugular is visible above the clavicle - this distance give a measurement of pressure Jugular normally behind heart and clavicle so not visible
Measuring arterial pressure High pressure to overcome resistance Sphygmomanometer Cuff around arm - inflate to apply resistance Decrease in resistance gives noise as vessels close Detected by stethoscope - sound appearing is systolic disappearing is diastolic
Vessel wall tension - Laplace's law Compares pressure inside a vessel with external tissue pressure Arteries experience higher pressure, so their walls need to develop greater tension Capillaries have a small lumen and only require a small tension to prevent bursting
Tension A force that keeps a vessel intact - tension running along vessel walls keeps it intact Tension = pressure change x radius/ thickness
Vessel compliance Expandability of vessels Measure of elasticity -how much you can expand a vessel per unit of force Compliance = volume/pressure Veins are more elastic - higher compliance
High compliance in veins For storage of blood Capacitance vessels Can expand or collapse to compensate for changes in blood volume changes A reservoir of moving blood - would clot if stagnant
Arterial compliance Affects the pressure pulse - difference in diastolic and systolic pressure Normal artery - stroke volume causes smaller pressure change Stiffer artery - stroke volume leads to a bigger pressure change This is a key problem in aging
Resistance Constant of proportionality between pressure and flow Same pressure gives more flow under lower resistance Flow = pressure/resistance
Lamina flow Occurs in most vessels - movement of blood in one direction with a parabolic shape Obeys ohms law - reflects lamina flow Double flow double pressure Flow is proportional to pressure
Turbulent flow Favoured in wide diameter, fast velocity vessels e.g. aorta Flow is proportional to the square root of pressure This is less effective - doubling pressure does not double flow
Poiseuille's law Resistance = (8 x viscosity x length)/(pi x radius^4) Length not used to modify resistance Viscosity could be used but would affect blood concentration Radius is a powerful regulator of resistance - doubling radius reduces resistance by factor of 16
Viscosity of blood Can vary Depends on how many red cells are present Small vessel - only fits one red cell surrounded by plasma - plasma is a high proportion - low viscosity and low resistance
Fahraeus effect Reduces resistance in micro-circulation
Resistance is higher in systemic circulation Length cannot be regulated physiologically However, systemic circulation is longer Flow must be balanced so has higher resistance 6 fold higher pressure - 6 fold higher resistance
Measuring total peripheral resistance TPR = (mean aortic pressure - central venous pressure)/cardiac output Ventricle to aorta has a small pressure drop due to small resistance Increased resistance in capillaries leads to reduced pressure
Site of greatest resistance Pressure = flow x resistance Drop in pressure is greatest in arteries Arterioles larger than capillaries - most resistance held here Where resistance is rate limiting
Resistance vessels Can dilate or constrict to change resistance to flow Innervated by ANS
Hydraulic energy A more complete model of haemodynamics Pressure at the feet can be up to 200 mmHg whilst only 90 mmHg in the heart - how does blood flow up a pressure gradient
Bernoulli's principle Energy = work = Integral of force dx Considers Forces acting on blood This accounts for how blood flows against pressure gradients - accounts for work by gravity, pressure, kinetic energy and friction
Stenosed vessels Narrowed vessels e.g. valve not fully opened Pressure around stenosed vessel goes down Leads to a momentary increase in velocity
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