Blood vessels

Question 1

Explain the stages of the vessels in the body

Answer 1

Arteries, arterioles, capillaries, venules and veins.

Question 2

Describe how to total surface area, mean pressure ad proportion of systemic blood changes over the course of these vesseles?

Answer 2

Total surface area:
-Bell shaped, peak at capillaries.

Mean pressure:
-highest at arteries and starts to go down after arterioles and lowest at veins

Proportion of systemic blood volume:
-low and gradually increases, highest at veins (capacitance)

Question 3

What is Darcy’s Law/Fluid circuit

Answer 3

Pressure difference= volumetric flow x resistence

Question 4

How to work out blood pressure and is this the true value?

Answer 4

Blood pressure = cardiac output x resistance
This relation is an approximation because it assumes:
-steady flow (which does not occur due to the intermittent pumping of the heart)
-rigid vessels
-right atrial pressure is negligible.

Question 5

What is poiseuille’s equation and what can it be simplified to in regards to the heart

Answer 5

R= 8Ln/ π r^4

resistance depends on:

• Fluid viscosity ( n)
• the length of the tube (L)
• Inner radius of the tube ( r)

Normal physiology:
viscosity stays the same
-length is defined and doesn’t change

therefore:
R= 1/ r^ 4
therefore if you halve the radius you decrease the flow by 16 times.
-for a very small amount of change in radius you can change the flow by large amount

Question 6

How does blood flow normally and abnormally?

Answer 6

• Laminar flow ( in a linear way)
• blood flows in layers and flows fastest closest to the centre of the lumen.
• velocity of the blood increases near the middle
• slow near the edges due to adhesive forces between fluid and surfaces

Turbulent flow (less healthy):
-blood flows erratically.

Question 7

What is shear rate and shear stress?

Answer 7

Shear rate: it is the gradient of the velocity profile at any point
Shear stress: shear rate multiplied by the viscosity.

Question 8

Why is shear stress important?

Answer 8

Shear stress governs how well endothelial cells work.
Endothelial lining relies on high shear stress and high shear stress relies on laminar flow.

If there is high shear stress:

• promotes endothelial cell survival and quiescence.
• cells aligned in direction of flow
• secretions promote vasodilation and anticoagulation.

Low shear stress/ disturbed shear stress:

• low shear stress
• promotes endothelial proliferation
• promotes endothelial proliferation, apoptosis and shape change
• secretions promotes vasoconstriction, coagulation and platelet aggregation (NOT good)

Question 9

When is turbulent flow useful?

Answer 9

Blood pressure measurement uses transitions between laminar and turbulent flow to denote systolic and diastolic blood pressure

You can hear turbulent flow NOT laminar flow

• useful to measure blood pressure.
• can hear the systolic pressure.

Question 10

How to work out pulse pressure and Mean atrial pressure and systolic blood pressure ?

Answer 10

Pulse pressure= systolic blood pressure - diastolic pressure.
Mean atrial pressure= diastolic pressure + 1/3 pulse pressure
SBP= DBP + PP

Question 11

What is windkessel effect?

Answer 11

In the aorta:
The pressure in the aorta doesn’t follow what happens in the ventricles. it is more maintained, the elasticity maintains the systole.
The arteries elasticity allows the flow to be maintained.

Question 12

Describe the effect of pressure on vessel walls using equations?

Answer 12

As the blood flows through the vessels, there is transmural pressure which causes tension.

Law of LaPlace: T= P x r
Circumferential stress = P x r / h
h= wall thickness

Question 13

What happens if there is persistent high circumferential stress?

Answer 13

causes vessel distention.

• over time vessels can weaken, causing distention and balloons
• these distention increases the radius, and according the law of Laplace the force should increase but the muscles fibres may have weakened so the force needed cannot be produces so the aneurysm will continue to expand, often until it ruptures.

Question 14

what special feature allows veins to hold lots of blood?

Answer 14

Veins are very compliant, so they can dilate a lot and hold lots of blood.

Question 15

what is compliance

Answer 15

the relationship between the transmural pressure and the vessel volume is called the compliance and depends on vessel elasticity.
-veins are very compliant

Question 16

How can we increase storage of blood in veins

Answer 16

we can change the volume of blood veins hold .

• increasing smooth muscle contraction decreases venous volume and increases venous pressure.
• relatively small changes in venous pressure distend veins and increase the volume of blood stored in them.

Question 17

what effect does gravity have and mainly where does it effect and wht is this due to?

Answer 17

-due to very compliant (Compliance is propensity for a vessel to stretch under pressure
) veins, causes pooling of blood.
-standing up increases hydrostatic pressure in the legs due to gravity.
-Blood transiently pools in the veins due to their high compliance and there is reduced venous return, this would reduce cardiac output and blood pressure( which causes fainting)
-Major effect of gravity is on distensible veins in legs and the blood within them ( can cause people to faint).

Question 18

Explain the response to standing etc

Answer 18

• activates the skeletal muscle pump, pushing the blood back into the veins to the heart.—– positive pressure
• valves withing the veins so skeletal pump pushes blood through the valves

Respiratory pump:—negative pressure from respiratory pump
diaphragm drops, causes negative intrathoracic pressure and this allows blood to flow back to the heart more easily.

Question 19

what are the consequences of incompetent valves?

Answer 19

• varicose veins (pooling of blood in the veins)

- oedema (prolonged elevation of venous pressure)

Question 20

Compare what happens when you are standing up compared to sitting on the pressure difference?

Answer 20

-pressure difference stays the same, but it is raised positive to the y axis so lots more pressure in ankle, this is due to hydrostatic pressure.

Question 21

how to work out pulse pressure

Answer 21

Pulse pressure = systolic minus diastolic blood pressure