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Flashcards in 4- Blood Vessels And Flow Deck (18):

What is the purpose of small and large arteries, vein and venues?

Large (elastic) arteries act as conduits and dampening vessels
Small arteries and arterioles have extensive smooth muscle in their walls to regulate their diameters and the resistance to blood flow.
Veins and venules are highly compliant and act as a reservoir for blood volume


Approx. What volume of blood is in the vein+venues?



What did Hales experiment show about blood flow

Blood flows due to a pressure difference.


Describe Darcy’s law and how can this be applied to BP

Change in pressure= volumetric flow x resistance
mean BP= cardiac output x resistance


Why does pressure fall across the circulation

Pressure falls across the circulation due to viscous (frictional) pressure losses. Small arteries and arterioles present most resistance to flow.


What are the three variables determining resistance to flow (Poiseullies equation).
And what happens when the radius in halved

1) Fluid viscosity (eta) - Not fixed but in most physiological conditions is constant
2) The length of the tube (L)- fixed, lengths of blood vessels remain constant
(3) Inner radius of the tube (r)-Variable – main determinant of resistance

R= 8L eta/ pi r^4

Increases resistance x16, or decrease flow x16


How much blood is pumped by the heart each minute during rest and exercise

5L/min. 25L/min


What is laminar and turbulent flow and why do these occur

Laminar- the flow of a fluid when each particle of the fluid follows a smooth path flowing in layers or streamlines, paths which never interfere with one another. Velocity of the fluid is constant at any one point.
In parts of vessels blood can exhibit turbulent flow:
this is irregular flow characterized by tiny whirlpool regions and associated with pathophysiological changes to the endothelial lining of the blood vessels. The velocity of this fluid is not constant at every point.


How is shear stress determined and what does it show

Adhesive forces between fluid and surface cause Velocity of layers to increase as distance from wall increases.
A parabolic velocity profile can be drawn
The shear rate (s) is the gradient of the velocity profile at any point
The shear rate multiplied by the viscosity is the shear stress (τ)

High shear stress, as found in laminar flow, promotes endothelial cell survival and quiescence, cell alignment in the direction of flow, and secretion of substances that promote vasodilation and anticoagulation.

Low shear stress, or changing shear stress direction as found in turbulent flow, promotes endothelial proliferation and apoptosis, shape change, and secretion of substances that promote vasoconstriction, coagulation, and platelet aggregation.


How can BP be measured

Most commonly measured with a cuff on the upper arm
Cuff contract, restricting blood flow in vein.
Cuff released until pressure is the same as the BP in the vein.
This causes blood to flow again in occluded vein, turbulently, which can be detected (Korotkoff sounds)


How is mean BP calculated

Pulse pressure (PP) = SBP - DBP
•Mean blood pressure ≈ DBP + 1/3 PP


Why do ventricular and aortic pressure differ?

Once the aortic valve closes, ventricular pressure falls rapidly but aortic pressure only falls slowly in diastole. This can be explained by the elasticity of the aorta and large arteries which act to “buffer” the change in pulse pressure. The elasticity of a vessel is related to its compliance.

When the aortic valve closes, ejection ceases but due to recoil of the elastic arteries, pressure falls slowly and there is diastolic flow in the downstream circulation.
•This damping effect is sometimes termed the “Windkessel”


Describe the effect of pressure on the walls of vessels

Transmural pressure causes a tension force (T) in the wall that can be described by Laplace’s relationship: T=Pr
Circumferential stress (σ) = tension force (T) / wall thickness (h)
Maintained high circumferential stress causes vessel distension.


Describe aneurysms using the law of Laplace

As a result of the Law of Laplace, if an aneurysm forms in a blood vessel wall, the radius of the vessel increases. This means that for the same internal pressure, the inward force exerted by the muscular wall must also increase. However, if the muscle fibres have weakened, the force needed cannot be produced and so the aneurysm will continue to expand ... often until it ruptures.


Compare compliance in arteries and veins

The relationship between the transmural pressure and the vessel volume is called the compliance and depends on vessel elasticity

Volume/ pressure graph can be plotted, in veins same increase in pressure requires a much greater change in volume than arteries.

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


Why does standing cause pressure problems

Standing increases hydrostatic pressure in leg to ~80mmHg as a result of gravity (h·ρ·g) . Height x density x gravitational force
•Blood transiently “pools” in the veins due to their compliance and reduces venous return to the heart.
•This would reduce cardiac output and blood pressure if there were no compensatory response.


Describe the compensatory response

Standing causes:
–Activation of the sympathetic nervous system to:
•constrict venous smooth muscle and ‘stiffen’ the veins.
•constrict arteries to increase resistance and maintain blood pressure
•increase heart rate + force of contraction and maintain cardiac output
–Myogenic venoconstriction (in response to elevated venous pressure) to ‘stiffen’ veins
–Use of muscle and respiratory ‘pumps’ to improve venous return
•Nevertheless cerebral blood flow falls on standing
•Failure of these mechanisms causes fainting (syncope)


Describe the respiratory pump

During inspiration, the intrathoracic pressure is negative (suction of air into the lungs), and abdominal pressure is positive (compression of abdominal organs by diaphragm). This makes a pressure gradient between the infra- and supradiaphragmatic parts of v. cava inferior, "pulling" the blood towards the right atrium and increasing venous return.