Introduction to haemodynamics L1

Question 1

What does haemodynamics mean?

Answer 1

• The relationship between blood pressure, blood flow & the resistance to blood flow

Question 2

What is the perfusion pressure?

Answer 2

• the difference in pressure between the arteries that supply the region (part of body) and the veins that drain it

Question 3

What does resistance mean?

Answer 3

• it is the friction that delays blood flow
• how difficult it is for the blood to flow between 2 points at any given pressure difference

Question 4

What is systolic pressure?

Answer 4

• the pressure at the peak of ejection of blood from the heart - highest point

Question 5

What is diastolic pressure?

Answer 5

• the pressure during the ventricular relaxation - lowest point

Question 6

What 2 things does vascular resistance depend on?

Answer 6

1. Vessel geometry - radius & length of vessel (note - radius changes depending on relaxation/contraction & length does not change too much)
2. Viscosity of the blood (friction between the molecules of a flowing fluid) - normally stays in a range unless hematocrit changes

Question 7

What is Darcy’s law in CV physiology? What are the main inputs to this equation?

Answer 7

• defines blood flow against resistance
• main inputs: Mean Arterial pressure (mm Hg) & total peripheral resistance
• CO =(MAP - CVP/TPR)

Question 8

What is TPR - total peripheral resistance?

Answer 8

• the sum of all the vascular resistances within the systemic circulation
• determined by the viscosity of the blood and the total CSA (cross sectional area)

Question 9

What is MAP - mean arterial pressure?

Answer 9

• the average arterial pressure throughout one cardiac cycle - systole & diastole

Question 10

What is Poiseuille’s law?

Answer 10

• the resistance to flow of liquid through a straight rigid tube is directly proportional to the viscosity of the liquid & the length of the tube
• but is inversely proportional to the 4th power of the radius

Question 11

What does Poiseuille’s law describe?

Answer 11

• EG - if there is an decrease in the diameter of the blood vessel and the viscosity of the blood, then there is an increase in the resistance and decrease in the flow rate. However, if there is an increase in the blood vessel diameter, there will be a higher flow rate and thus a lower resistance

Question 12

What is laminar/streamlined flow?

Answer 12

• this is the normal state of blood flow in the CV system
• the blood flows in layers/laminae
• the highest point of blood viscosity is in the middle of the vessel
• NB - flow is invers proportional to pressure difference, so if there is an increase in the pressure gradient along the vessel, the flow will eventually become turbulent

Question 13

What is turbulent flow?

Answer 13

• turbulent flow is blood flow that does not occur in layers - it is chaotic
• it happens in circumstances in which there is a high flow - eg the aorta/ branch points or a distorted artery due to plaque

Question 14

What is Reynold’s number?

Answer 14

• a number that predicts when turbulent flow will occur
• when the Reynolds number is exceeded (Re greater than 2000) , turbulent flow will occur

Question 15

Why is there a fall in pressure from large arteries to veins and capillaries?

Answer 15

• As arteries offer greater resistance - they have a thick media layer containing smooth muscle layers and elastic layers and they also have a narrow lumen

Question 16

what is the cross-sectional area?

Answer 16

• the area of a sliced 3d object
• NB - if the cross-sectional area is large - then there is less resistance and slower flow rate (eg venules) & if the CSA is small, then there is more resistance & higher flow rate
• think of it like a stream that flows fast through narrow opening but slowly through broad opening

Question 17

Why do capillaries offer little resistance to blood flow?

Answer 17

• the resistance to blood flow depends on the diameter of the vessel & the CSA available for blood flow
• the CSA of the capillaries is very large, but diameter is small, and they also lack any smooth muscle to allow constriction - so they offer very little resistance to flow

Question 18

RECAP - describe the wind Kessel effect of arteries

Answer 18

• the media layer of the arteries contains elastin
• during systole (bp is highest), the arterial walls expand and stretch to accept the stroke volume of blood pumped by the left ventricle
• The arterial walls then recoil back to their original state during diastole

Question 19

What is blood vessel compliance?

Answer 19

• How easily a blood vessel expands when it is filled with a volume of blood
• defined as a change in volume over a change in pressure
• C = delta V/ delta P

Question 20

What is arterial compliance C_a?

Answer 20

• the ability of the arterial wall to expand to allow the large volume of blood

Question 21

What is pulse pressure?

Answer 21

• the difference between the systolic and diastolic pressure
• when Bp is measured, you have 2 numbers - systolic bp and diastolic bp
• pulse pressure is important to measure to detect a risk of heart disease

Question 22

Why would there be an increase in pulse pressure?

Answer 22

• there would be an increase in pulse pressure if the compliance of the artery is decreased (reduction in the elasticity/hardening of the arteries) - atherosclerosis

Question 23

Why would there be a decrease in the pulse pressure?

Answer 23

• there may be a decrease in the pulse pressure due to a reduced SV - ie your heart isn’t pumping enough blood

Question 24

Why is there an increase in pressure during systole?

Answer 24

• the pressure rises as the rate of the blood being pumped from the left ventricle into the arterial tree is higher than the rate at which blood is being distributed