cardiovascular mechanics 3

Question 1

what are the main aspects of design of the system?

Answer 1

• circulation is designed to transport blood around the body - regulate temperature - deliver nutrients and hormones - two circulations present - and a muscular pump that generates a pressure gradient - the diffusion is only effective over short distances

Question 2

how do arteries and arterioles respond to resistance?

Answer 2

• extensive smooth muscle - regulates diameter and the resistance to blood flow

Question 3

how do veins and venuoles respond to resistance?

Answer 3

• they are very compliant - they act as a reservoir for blood volume

Question 4

why does blood flow?

Answer 4

• it is blood pressure that drives the circulation - the pressure difference between the aorta and the vessels

Question 5

what is the equation for resistance in fluids?

Answer 5

• ΔP = Q x R Q = volumetric flow, R = resistance

Question 6

how can we replace the terms of - ΔP = Q x R ?

Answer 6

blood pressure = cardiac output x resistance

Question 7

what assumptions does blood pressure = cardiac output x resistance make?

Answer 7

• steady flow - rigid vessels - the right atrial pressure is negligible

Question 8

how is regulation of flow achieved?

Answer 8

• variation in resistance in the vessels - the blood pressure remains constant through

Question 9

how does pressure change across the circulation?

Answer 9

• the pressure falls across the circulation - due to frictional pressure losses - small arteries and arterioles are most resistant to flow

Question 10

what factors decide the resistance of a tube to flow?

Answer 10

1. Fluid viscosity (, eta) 2. The length of the tube (L). 3. Inner radius of the tube (r)

Question 11

what is poiseulles equation?

Answer 11

• small changes in vascular tone results in large changes to flow halving the radius decreases by 16 times

Question 12

what is the cardiac output at rest? what is the cardiac output with exercise?

Answer 12

• rest = 5L/min - exercise = 20L/min

Question 13

what is laminar flow?

Answer 13

• the velocity of fluid is constant - flows in layers - fastest flow closer to the lumen

Question 14

what is turbulent flow?

Answer 14

• blood flows erratically and is prone to pooling - Associated with pathophysiological changes to the endothelial lining of blood vessels

Question 15

why is the flow fastest in the lumen?

Answer 15

• there are adhesive forces between the fluid and the surface - the velocity of layers increases as the distance form the wall increases

Question 16

sketch a velocity profile:

Answer 16

Question 17

what is the shear rate

what is shear stress?

Answer 17

shear rate is the gradient of the velocity profile at any point

shear stress is the shear rate multiplied by the viscosity

Question 18

what does laminar shear stress do?

Answer 18

• high shear stress
• promotes endothelial cell survival
• the cells are aligned in direction flow
• Secretions promote vasodilation and anticoagulation

Question 19

what does disturbed shear stress do?

Answer 19

• low shear stress
• promotes endothelial proliferation
• apoptosis and shape change
• Secretions promote vasoconstriction, coagulation and platelet aggregation

Question 20

how is blood pressure measured?

Answer 20

• measured on the upper arm
• slow deflation of the cuff causes turbulent flow
• this is heard with a stethoscope

Question 21

how to work out pulse pressure?

how to work at MAP?

Answer 21

pulse pressure = SBP - DBP

MAP = DBP + 1/3 pulse pressure

Question 22

why do ventricular and aortic pressures differ?

Answer 22

• when the aortic valve shuts the ventricular pressure falls rapidly the aortic pressure falls slowly
• we can explain this by the elasticity of the aorta and the large arteries which try to buffer the change in pulse pressure

Question 23

how does the aorta manage blood from the ventricles?

Answer 23

• during ejection, blood enters the aorta faster than it leaves the aorta
• 40% of SV is stored in arteries
• when the aortic valve closes the ejection ceases and due to the recoil of elastic arteries the pressure falls slowly and there is diastolic flow
• If arterial compliance decreases , arteries become stiffer , the pulse pressure gets higher

Question 24

what is the law of laplace?

Answer 24

tension = pressure x radius

Question 25

what does high circumferential stress cause?

Answer 25

vessel distension

Question 26

how do aneurysms form?

Answer 26

• vessel walls can weaken causing a balloon-like distension

Question 27

how does the law of laplace relate to aneurysms?

Answer 27

• vascular aneurysms increase the radius of the vessel
• this means that for the same internal vessel there is a greater radius therefore greater wall tension
• so more force is needed
• however the muscle is too weak to provide the force

so the aneurysm will continue to expand till it ruptures

Question 28

how does venous compliance compare to arterial compliance?

Answer 28

Venous compliance is 10 to 20 times greater than arterial compliance at low pressures.

Question 29

what does increases smooth muscle contraction do the vein?

Answer 29

decreases venous volume and increases venous pressure.

so with contraction, the compliance goes down

Question 29

what does standing do?

Answer 29

increases hydrostatic pressure in legs

• Blood transiently pools in the veins due to their high compliance -> reduced venous return
• reduce cardiac output and blood pressure

might cause fainting as not enough blood is going to the head

Question 30

what are problems with standing?

Answer 30

• incompetent valves cause dilated superficial veins in the leg
• Prolonged elevation of venous pressure causes oedema in feet