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Haemodynamics Notes

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Haemodynamics Hemodynamics describes the relationship between blood flow, blood pressure, hydraulic resistance. The sole function of the circulatory system is transportation. Flow

Difference in volume/difference in time-Units: Litres/min Flow is a rate : another way of measuring flow is by velocity x area = cm3/secondsso branching increases the cross sectional area but decreases the velocity
-Darcy's law: Flow is proportional to pressure difference, the proportionality coefficient is hydraulic conductance (ease of blood flow) the reciprocal of hydraulic conductance is hydraulic resistnace The flow through systemic circulation is measured by the cardiac output
-one way of measuring cardiac output = stroke volume (70ml) x Heart rate (70bpm)
- also calculated by the pressure difference (mean aortic pressure minus central venous pressure) and systemic resistance is total peripheral resistance According to darcy's law, Cardiac output (roughly 5l / minute) CO = (Mean arterial pressure - central venous pressure ) / Total peripheral resistance (MAP - CVP) = (CO x SVR) if rearranged CO = [(MAP - CVP) / SVR]

-central venous blood pressure is close to atmospheric pressure = 0 so equation can be simplified to C0=Arterial pressure/total peripheral resistance
-mean arterial pressure is determined by cardiac output x total peripheral resistance Blood pressure- driving force for constant blood flow

-blood pressure is measured by the height it can drive a coloumn of liquid-

-Force = Area x height x density of the liquid in the coloumn (p) x gravitational

acceleration (g) Volume x density = mass

Pressure = area x height x density x gravity / area
= Height (h) x gravity (g) x density (p)
-units of blood pressure mmHg (millimeters of mercury)- if measuring aterial pressure or cmH20- used to measure venous pressure and kPA Measuring arterial blood pressure
-blood pressure is measured using a blood pressure gauge- Sphygomomanometerthis measures aterial blood pressure in millimeters Hg
-inextensible cuff containing an inflatable bag is wrapped around the arm and the bag is inflated to a pressure level above the expected systolic pressure
-this occludes (squeezes the vessel shut) the underlying brachial artery and halts blood flow downstream and the pressure in the cuff is measured by mercury
-a stethoscope is placed over the brachial artery in the hollow of the elbow- when the brachial artery is fully occluded- no sound is heard as no blood is flowing but when the cuff pressure is gradually lowered a sequence of sounds are found
-when the cuff pressure falls below systolic pressure the artery open briefly during each systole- there are spurts of blood and these vibrate the artery wall- creating a dull tapping sound Korotkoff sound- the pressure at which the Korotkoff sound first appears is the systolic pressure
-as the cuff pressure is lowered the Korotoff sounds grow louder because the spurts of blood grow stronger
-when the cuff pressure is lowered further the Korotkoff sounds diminish- diastolic pressure
-blood pressure is not constant over time
-Normal systemic aterial pressure: systolic 120mmHg, diastolic 80mmHg Mean arterial pressure
-mean pressure is halfway between systolic and diastolic pressures in the brachial artery

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