4 - Cardiovascular Mechanics 3

Question 1

What is the purpose of the circulation?

Answer 1

• transport blood around body

- regulate temperature

Question 2

What 3 factors influence resistance to blood flow?

Answer 2

• fluid viscosity
• length of tube
• inner radius of tube

Question 3

What is Poiseuille’s equation?

Answer 3

• resistance = k/r^4

- resistance inversely proportional to radius^4

Question 4

What is the equation to calculate MAP?

Answer 4

MAP = CO x R

Question 5

What are the features of arteries and veins in the systemic circulation?

Answer 5

• arteries and arterioles: extensive smooth muscle (regulates diameter and resistance to blood flow)
• veins and venules: highly compliant and reservoir for blood volume

Question 6

What is laminar flow?

Answer 6

• high shear stress
• velocity of fluid constant and flows in layers
• blood flows fastest closest to lumen centre
• promotes endothelial survival, cell alignment and secretes molecules leading to vasodilation and anti-coagulation

Question 7

What is turbulent flow?

Answer 7

• low shear stress
• blood flows erratically
• eddys form (prone to pooling)
• promotes endothelial proliferation, apoptosis and shape change

Question 8

What is the equation for shear stress?

Answer 8

shear stress = shear rate x viscosity

Question 9

What is shear stress and shear rate?

Answer 9

• shear stress: velocity of layers increases as distance from wall increases
• shear rate: gradient of velocity profile

Question 10

What is circumferential stress?

Answer 10

• tension force / wall thickness

- stress experienced by wall at certain circumference at certain point

Question 11

What can persistent high circumferential stress cause?

Answer 11

vessel distension leading to ballooning of vessel (aneurysm)

Question 12

What are the differences between aortic and ventricular pressures?

Answer 12

• after ejection of blood when aortic valve closes, ventricular pressure falls rapidly
• aortic pressure falls slowly
• due to elasticity of aorta allowing it to dampen pressure changes

Question 13

What is the Windkessel effect?

Answer 13

• as ejection occurs, blood enters aorta faster than it leaves it
• 40% of SV stored by elastic arteries
• at end of ejection, aortic valve closes, elastic arteries recoil and send off all stored blood
• aortic pressure falls slowly
• dampening effect of pressure change = windkessel effect
• observed as dichrotic notch
• if arterial compliance decreases (arteries stiffen), dampening effect reduced

Question 14

What is compliance and its features?

Answer 14

• relationship between transmural pressure and vessel volume
• dependent on elasticity
• venous compliance greater than arterial compliance at low pressures
• small pressure changes distend veins and increase blood volume stored

Question 15

What are the features of the skeletal and respiratory pumps?

Answer 15

• skeletal pump: contraction of muscles squeezes veins and pushes blood back to heart
• respiratory pump: chest expands and intrathoracic pressure drops when breathe in (increases venous return)

Question 16

When the SNS is activated upon standing, what happens?

Answer 16

• venous smooth muscle constricted and veins stiffen
• arteries constricted to increase resistance and maintain BP
• HR and force of contraction increased
• CO maintained

Question 17

What happens to blood flow when you stand up?

Answer 17

• increases hydrostatic pressure in legs
• blood transiently pools in veins (due to high compliance and reduced venous return)
• CO and BP reduced without compensatory mechanisms
• incompetent valves can cause varicose veins
• prolonged elevation of venous pressure causes feet oedema