Hemodynamics Purpose of blood flow Maintain homeostasis Nutrient

Hemodynamics Purpose of blood flow? Maintain homeostasis Nutrient and waste exchange Purpose of control mechanisms of blood flow? Blood flow must match metabolic needs of tissue Blood flow to brain and heart must be maintained Insufficient blood volume to perfuse all tissue simultaneously AJ Davidoff

Important to maintain adequate perfusion pressure in order to control blood flow MAP = CO x TPR HR x SV MAP = mean arterial pressure TPR = total peripheral resistance CO = cardiac output Sherwood Fig 10 -1

Blood flow depends on pressure gradients and vascular resistance Capillary exchange is the sole purpose of the circulatory system Sherwood

Relationship between blood flow, pressure and resistance Ohm's Law: V = I*R or I = V/R V = voltage (potential difference) I = current R = resistance Blood Flow: P = Q*R or Q = P/R Q = flow (m. L/min) P = pressure gradient (mm Hg) R = resistance (mm Hg/m. L/min) The major mechanism for changing blood flow is by changing arterial resistance (e. g. , TPR or in a single organ)

Pressure gradients Pressure difference affects flow not absolute pressure Sherwood Fig 10 -3

Resistance to Blood Flow Poiseuille equation R = 8 L r 4 R = resistance = viscosity of blood L = length of blood vessel r 4 = radius of blood vessel raised to the fourth power If radius decreases by one half, resistance increases by 16 -fold (= 24)!!! ( r 4 = area)

Radius profoundly affects blood flow R~ 1/r 4 Q = P/R Sherwood Fig 10 -4 Flow ~ r 4

Series Resistance (93 mm Hg) (4 mm Hg) Total resistance equals the sum of the individual resistances Total flow is the same at each level, but pressure decreases progressively Q = P/R Why? Costanzo Fig 4 -5

Parallel Resistance Needs work 5 L/min Flow in aorta is equal to the flow in the vena cave (steady state) Flow to each organ is a fraction of the total blood flow Total resistance is less then any of the individual resistances, therefore no significant loss of arterial pressure to each organ

Velocity of Blood Flow v = Q/A Costanzo Fig 4 -4 v = velocity of flow (cm/sec) Q = flow (ml/sec) A = cross-sectional area (cm 2)

Total cross sectional area of systemic blood vessels Costanzo Fig 4 -3 v = Q/A

Laminar flow and Turbulence quiet noisy Costanzo Fig 4 -6 Laminar flow is parabolic, highest velocity in center (least resistance), lowest adjacent to vessel walls Turbulent flow is disoriented, no longer parabolic, energy wasted, thus more pressure required to drive blood flow.

Turbulence is velocity of blood flow diameter of blood vessel 1/ viscosity of blood Ganong Fig 30 -8 Mohrman and Heller Fig 6 -6

Bernouilles Principle (in a single vessel) Total energy = distending pressure (PD) + kinetic energy (KE) Higher velocity through a constriction KE PD Bad for plaque regions Why? Total energy is actually not conserved completely because of heat loss

Bernouilles Principle Bad for aneurysms Why? PD KE

Compliance of blood vessels • Compliance is a slope • At low pressures, veins have a greater compliance than arteries C = V/ P C = compliance (m. L/mm Hg) V = volume (m. L) P = pressure (mm Hg) • At high pressures, compliance is similar in veins and arteries (but volume is much greater in veins) Cardiovascular Physiology Concepts http: //www. cvphysiology. com/Blood%20 Pressure/BP 004. htm

Compliance changes related to vasocontraction or aging With vasocontraction: • Venous volume decreases and pressure increases • Venous compliance decreases Similar effects in arteries with aging

Blood Vessels Arteries Conduits Martini Fig 21 -2

Pressure reservoir Elastic recoil continues to drive blood toward arterioles during diastole Sherwood Fig 10 -6 & -7

B&B Fig 17 -11

MAP = diastolic pressure + 1/3 pulse pressure (at rest) 80 mph for 40 min 120 mph for 20 min Sherwood Fig 10 -7 2/3 time in diastole 1/3 time in systole

Dampening pulse pressures Arterial pulse pressure influenced by: elasticity rigidity resistance, pulse pressure G&H Fig 15 -6

Cardiac Output (CO) = MAP TPR What does systolic pressure tell you? What does diastolic pressure tell you? Sherwood Fig 10 -9 CO & TPR

Aortic pressure changes rigid G&H Fig 15 -4 and B&B

Aortic pressure changes G&H Fig 15 -4

G&H Fig 23 -4

Mean arterial pressure (MAP) is the main driving force for blood flow through capillaries G&H Fig 14 -2

Basis of auscultatory method for measuring BP (Sounds of Korotkoff) Turbulent flow is noisy Mohrman and Heller Fig 6 -9

Why should cuff be placed at heart level? What effects on BP measurement would the presence of obesity cause?