Hemodynamics

Question 1

Peripheral circulation resistance

Answer 1

• blood flow through an individual organ can be considered to be flow through an series network
• dramatic differences in resistance in different organs, thus allowing changes control of blood flow even though the mean arterial blood pressure is relatively constant
• arterioles have the largest compared to other vessels so the overall resistance of any organ determined by R arterioles
• overall resistance to flow through circulation is total peripheral resistance

Question 2

TPR and cardiac output

Answer 2

• aortic valve and the right atrium are at the same height, therefore any pressure differences come only from frictional losses from circulation
• flow equation relates rate of blood flow and the resistance of the vessels
• total flow is equal to cardiac output and the resistance of the entire system is TPR

Question 3

Viscosity

Answer 3

• measure of intermolecular attractions in the liquid, determines the steepness of the velocity gradient
• distinguished from density, which is specific gravity and which is defined as the mass per unit volume

Question 4

Laminar flow

Answer 4

• where flow is proportional to change in pressure, breaks down when velocity reaches a critical point
• above this velocity, the flow depends less strongly on pressure gradient rater the square root of pressure, this is because effective resistance increases
• this region is turbulent flow which causes a significant losses of kinetic energy
• in turbulent flow, Q is proportional to the square root of change in pressure
• for the same change in pressure there is less flow when flow is turbulent than when laminar

Question 5

Murmurs

Answer 5

• in turbulent flow, the surfaces of constant velocity are spirals, swirls and eddies, noisy auscultatory sounds
• murmurs are audible sounds due to vibrations in heart or vessel walls, also called bruits, not usually occur under resting conditions
• innocent systolic murmers occur when cardiac output increases during exercise, causing turbulent aortic flow during systole
• Korotkoff sounds of sphygmomanometry (blood pressure measurement with inflatable cuff) are examples of the sound of turbulent flow
• aortic or mitral stenosis may also cause murmers, aortic regurgitation generates turbulence and characteristic sounds
• arterial wall may be damaged by turbulence and by development of thrombi

Question 6

Effect of gravity on blood pressure

Answer 6

• mean circulatory pressure is the equilibrium pressure that would result throughout the cardivascular system if the heart stopped beating and is equal to 7 mm Hg. Cardiac contractions establish an arterial pressure gradient in a supine subject
• gravity affects the lateral pressure through a gravitational pressure term, which either adds or subtracts from the pressure generated by the heart
• gravity does not affect the flow of blood in a circuit of distensible vessels because gravitational pressure in the arteries is exactly counterbalanced by the same gravitational pressure at the same level by corresponding veins
• does affect the distribution of blood throughout the system of distensible vessels and therefore it affects the transmural pressure PTM= (PL + pgh) -Pext

Question 7

Total pressure in circulatory system

Answer 7

• energy changes from applied pressures (ex heart contraction), changes in gravitational energy (height) and changes in velocity (pulsatile flow or changes in velocity or viscosity)
• the pressure at any point in the circulatory system is the sum of static and dynamic pressures
• static pressure- composed of the applied or lateral pressure and force of gravity
• dynamic pressure is due to kinetic energy
• as velocity decreases, dynamic pressure becomes a small faction of the total pressure
• as vessel radius narrows the dynamic component increases significantly

Question 8

Dynamic pressures in different vessels

Answer 8

• in aorta dynamic pressure is small in resting state but may be important when cardiac output increases
• in smaller arteries, capillaries and small veins, it is negligible
• in the vena cava, it is very important at high cardiac output, while in the atria and pulmonary arteries is significant and of great importance at elevated cardiac output

Question 9

Direct measurement of blood pressure

Answer 9

• use cannulation or catheterization into a large vessel
• a solid state pressure transducer (a stiff diaphragm on a strain gauge) converts mechanical displacement into an electrical signal
• the other end of the tube is open to air. Catheterizations are commonly used in measuring pressures in the heart
• less invasive, indirect use electromagnetic or ultrasound flow meters which measure the movement of electrolytes in the blood or Doppler effect on emitted sound waves, respectively
• clearance methods measure regional blood flow as dilution of metabolite or injected dye. Most common method is auscultatory method

Question 10

Sheer stress on blood vessel walls

Answer 10

• sheer stress is created by flowing blood on endothelial wall directed along the long axis of the vessel
• sheer stress on vessel wall is proportional to the viscosity and the shear rate, which is the rate at which the axial velocity changes from the wall to the lumen
• for poiseuille flow, sheer stress is directly proportional to viscosity and flow rate, and inversely proportional to the cute of the vessel radium. Sheer stress is measured in units of pressure

Question 11

Velocity profile

Answer 11

• in a rigid cylindrical tube is parabolic
• the outermost layer has zero velocity because friction is greatest at the wall
• axial layer in the center is farthest from the wall, has minimal frictional retardation and therefore maximal velocity
• the axial velocity is twice the mean velocity as averaged over the cross-section of the tube
• on thruway the cars may travel with a velocity of 50 mph; the traffic flow is how many cars pass the tollbooth per hour

Question 12

Sheer forces and turbulence

Answer 12

• Stenosis model- different patterns of post stenotic recirculation in models having the same stenosis severity but different stenosis geometries. Since such flow patterns may provide ideal conditions for the formation of the blood clots that cause many strokes
• carotid model- pulsatile flow in an idealized normal carotid bifurcation- particles go both ways, this is where many atherosclerotic

Question 13

Pressure and stress in a stenosis

Answer 13

• constriction of an artery
• can be congenital or acquired and occur in the aorta and carotid, coronary renal and femoral arteries
• causes a local increase in velocity leading to an increase in dynamic pressure and decrease in lateral pressure
• turbulence and flow separation can occur leading to changes in sheer stress and strain
• if PL decreases below the external pressure, the artery will collapse and flow stops momentarily whereupon the energy is reconverted to lateral pressure, and the artery opens again
• this oscillation of pressure may produce a flutter, which could result in rupture of a plaque and the formation of emboil
• whenever blood velocity increases, a greater proportion of the blood pressure is converted from lateral into dynamic pressure
• thus lateral pressure decreases while dynamic pressure increases