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Hemodynamics
Organisation of the Body
| Question | Answer |
|---|---|
| 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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