CVS 4 - Blood vessels and blood flow

Question 1

Structure of vessels is highly appropriate for their function, how are:

• large arteries appropriate?
• veins and venules?
• small arteries and arterioles?
• capillaries?

Answer 1

• large arteries act as conduits dans dampening vessels
• veins and venules are highly compliant and act as a reservoir
• small arteries and arterioles have extensive smooth muscle in their walls to regulate their diameter and the resistance to blood flow
• capillaries have very thin walls to facilitate transport and diffusion

Question 2

What divisions of blood vessels has the largest cross-sectional area?

Answer 2

Capillaries

-> exchange function

Question 3

What division of blood vessels has the largest relative volume of blood contained within the vessel?

Answer 3

Venules and veins

-> reservoir function

Question 4

What drives blood flow? Who first measured it?

Answer 4

Blood pressure drives blood flow
Steven hales - model of the tank like aorta that exerts a pressure, the pressure difference at the start and end of tube drives the flow Q

Question 5

State the equation relating blood flow (Q), pressure difference (P) and resistance (R).

Answer 5

P = Q x R

Question 6

How does the amount of blood stored in the veins change during exercise?

Answer 6

Exercise causes VENOCONSTRICTION leading to a decrease in the amount of stored blood, so there is more venous return.

Question 7

Where does the biggest drop in blood pressure take place?

Answer 7

In the arterioles

Question 8

What is the equation for MBP?

Answer 8

MBP = CO x TPR

(this is an approximation since blood flow is not steady (intermittent pumping) and blood vessels are not rigid.

Question 9

What three variables determine resistance to blood flow?

Answer 9

Fluid viscosity (n)
Vessel radius (r)
Length of tube (L)

Question 10

Recall Poiseuille’s equation. What is its significance?

Answer 10

R = (8Ln)/(πr^4)
This emphasises the importance of arterial diameter as a determinant of resistance. Relatively small change in vascular tone (eg. constriction) can produce marked changes in flow.
-> Halving the radius decreases the flow 16 times.
-> useful during exercise: dilatation of the arteries and arterioles feeding skeletal muscles results in 30 fold increase in blood flow (also by constricting blood flow to non useful organs)

Question 11

Describe how blood normally flows in vessels.

Answer 11

Normal blood flow is laminar - blood flows fastest in the middle and slowest around the outside.

Question 12

What is shear rate?

Answer 12

The gradient of the velocity profile at any point.
The velocity profile is parabolic, it is how the velocities in the vessels differ with laminar flow: blood flows fastest in the middle and slowest around the outside.

Question 13

What is shear stress?

Answer 13

Shear rate x viscosity

Question 14

What are the effects of a) high, and b) low shear stress?

Answer 14

High (as found in laminar flow) = promotes endothelial survival, cell alignment, secretion of substances promoting vasodilation and anticoagulation
Low (turbulent flow) = promotes endothelial proliferation/apoptosis, which promotes vasoconstriction, platelet aggregation, coagulation

Question 15

What are the equations for pulse pressure and mean arterial blood pressure?

Answer 15

Pulse Pressure = SBP - DBP

Mean Arterial Blood Pressure = DBP + 1/3 PP

Question 16

How do aortic and ventricular pressure differ and what is the reason for this difference?

Answer 16

Once the aortic valves close, ventricular pressure falls but aortic pressure only falls slowly during diastole because the elasticity of the aorta buffers the pressure.

Question 17

What is the name given to the dampening effect of the aorta?

Answer 17

Windkessel Effect

Question 18

What are the consequences of a decrease in arterial compliance?

Answer 18

Decrease in arterial compliance = arteries becomes stiffer so windkessel effect decreases so pulse pressure will increase.

Question 19

State the Law of Laplace.

Answer 19

When the pressure within a vessel is kept constant, tension is directly proportional to the radius.
T = PR
T determines the distension of the vessel.

Question 20

What does circumferential stress equal?

Answer 20

Circumferential stress = tension/wall thickness
And T = PR so:
Circumferential stress = PR/h
High Circumferential stress causes vessel distention. This can cause aneurysms.

Question 21

What is compliance?

Answer 21

It is the relationship between transmural pressure and vessel volume
Depends on wall elasticity. The less elastic, the more compliant, ie. it can sustain higher pressure for the same volume. Elastance is the opposite of compliance.
Venous compliance is 10 to 20 times greater than arterial at low pressures and arteries are compliant over a wider pressure range.

This physically means that blood vessels with a higher compliance deform easier (veins) than lower compliance blood vessels (arteries) under the same pressure and volume conditions.

“Compliance is the ability of a hollow organ (vessel) to distend and increase volume with increasing transmural pressure or the tendency of a hollow organ to resist recoil toward its original dimensions on application of a distending or compressing force.”

By dilating/constricting you modify your vessel’s compliance.

Question 22

Describe how aneurysms are linked to the Law of Laplace.

Answer 22

The vessel radius increases meaning that greater wall tension is needed to withstand the internal pressure. If the structure of the wall is damaged and it can’t have the required tension, then the vessel will continue to dilate.

Question 23

What is the difference between the blood pressure at heart level compared to the hand?

Answer 23

Blood pressure in the ankle is much greater than at heart level.

Question 24

Why don’t we faint every time we stand up due to blood pooling in our leg veins?

Answer 24

Standing causes activation of the sympathetic nervous system, which stiffens and constricts the veins resulting in greater venous return to the heart.
Constricted arteries means there is an increase in TPR to maintain blood pressure.
There is also a slight increase in heart rate and force of contraction.

Question 25

What are the two pumps that facilitate the movement of venous blood back to the heart?

Answer 25

Skeletal muscle pump

Respiratory pump

Question 26

What is the major effect of gravity?

Answer 26

it is on the distensible veins (high compliance) of the leg -> blood ‘pools’ in the veins and reduces venous return to heart