Blood Pressure Fundamentals Lecture

Blood flow: ◦Pressure gradient drives blood flow ◦Major pressure drop in arteries leads to a drop in pressure by the time the blood reaches the capillaries ◦Low pressure in venule system ◦Flow is directly proportional to change in pressure, and inversely proportional to...

Blood flow: ◦Pressure gradient drives blood flow ◦Major pressure drop in arteries leads to a drop in pressure by the time the blood reaches the capillaries ◦Low pressure in venule system ◦Flow is directly proportional to change in pressure, and inversely proportional to resistance. ◦There is a drop in blood pressure in arterioles as the diameter of the vessels decreases from the large arteries. ◦Blood flows from high to low pressure. Blood pressure: ◦Pressure difference = CO x TPR ‣ CO: Cardiac output ‣ TPR: total peripheral resistance - the resistance felt by blood as it moves through blood vessels (predominantly systemic resistance) ◦Cardiac output: heart rate x stroke volume ‣ Stroke volume = the total volume of blood in ml pumped out of the ventricles for every 1 beat ‣ ~4900ml ‣ ~5L/minute ◦Cardiac output is the total volume of blood pumped out of the ventricles per minute Cardiac output; flow, velocity and area: ◦Cardiac output = ml/minute ◦Flow rate = cm3/minute = ml/minute ◦Velocity = cm/min or (Flow/area) ◦Velocity is directly proportional to flow, which is directly proportional to CO ◦Velocity is directly proportional to 1/Area ‣ When the area of the capillaries is high, the velocity of blood travelling through them is low. It is the opposite way for the arteries. Resistance: ◦Blood pressure (pressure difference) = CO x TPR ◦Resistance = pressure difference/CO ◦Resistance = pressure difference/Flow ◦Poiseuille’s equation:  ‣ n = viscosity, l = length of the vessel, r = radius ‣ Viscosity: increased in: Polycythemia (increase in RBC) Dehydration ‣ Viscosity: decreased in: Anaemia (decrease in RBC) ‣ Length in vessel: increased with: Weight Height ‣ Radius: increased in: Vasodilation ‣ Radius: decreased in: Vasoconstriction Resistance: in series and parallel vascular circuits: ◦Normal blood pressure is 120/80 ◦Series and parallel circuits are needed to maintain blood pressure ◦Capillaries have low resistance because they are arranged in parallel, and all the blood splits into a parallel circuit. ◦Arterioles have the highest resistance because they have the ability to change the size of their diameter (vasodilation and vasoconstriction) Resistance: flow: ◦Types of flow: ‣ Laminar - smooth and in a straight line: Silent Velocity highest at the centre Low resistance ‣ Turbulent - all over the place: Not silent High resistance ‣ Differences in properties allow blood pressure management Blood pressure:  ◦Pressure = force/unit area ◦Blood pressure: force exerted by blood on the vessels ◦Systolic blood pressure (SBP): pressure in arteries during systole: 120 mmHg ◦Diastolic blood pressure (DBP): pressure in arteries during diastole: 80 mmHg ◦Pulse pressure (PP): systolic-diastolic blood pressure: 120-80 = 40 mmHg ◦Mean arterial pressure (MAP): ‣ 2/3 DBP + 1/3 SBP ‣ 2/3 DBP + 1/3 DBP + 1/3 PP ‣ DBP + 1/3 PP ‣ = 93.3 mmHg Pulse: ◦Expansion and recoiling of arteries creates pulse ◦Pulse indicates heart rate ◦Most readily measured at the radial artery, but can be measured at any of the pulse points ◦Recorded as beats per minute ◦Both the rate and the strength of the pulse are important clinically Blood pressure measurement: