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Organisation of the Body

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Hemodynamics   Applying physical principles to study the movement of blood Flow Pressure Tension Compliance Resistance Energy  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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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  
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Tension   A force that keeps a vessel intact - tension running along vessel walls keeps it intact Tension = pressure change x radius/ thickness  
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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  
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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  
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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  
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Resistance   Constant of proportionality between pressure and flow Same pressure gives more flow under lower resistance Flow = pressure/resistance  
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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  
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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  
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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  
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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  
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Fahraeus effect   Reduces resistance in micro-circulation  
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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  
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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  
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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  
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Resistance vessels   Can dilate or constrict to change resistance to flow Innervated by ANS  
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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  
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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  
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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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