CVS 4 - Blood vessels and blood flow Flashcards
Structure of vessels is highly appropriate for their function, how are:
- large arteries appropriate?
- veins and venules?
- small arteries and arterioles?
- capillaries?
- 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
What divisions of blood vessels has the largest cross-sectional area?
Capillaries
-> exchange function
What division of blood vessels has the largest relative volume of blood contained within the vessel?
Venules and veins
-> reservoir function
What drives blood flow? Who first measured it?
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
State the equation relating blood flow (Q), pressure difference (P) and resistance (R).
P = Q x R
How does the amount of blood stored in the veins change during exercise?
Exercise causes VENOCONSTRICTION leading to a decrease in the amount of stored blood, so there is more venous return.
Where does the biggest drop in blood pressure take place?
In the arterioles
What is the equation for MBP?
MBP = CO x TPR
(this is an approximation since blood flow is not steady (intermittent pumping) and blood vessels are not rigid.
What three variables determine resistance to blood flow?
Fluid viscosity (n) Vessel radius (r) Length of tube (L)
Recall Poiseuille’s equation. What is its significance?
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)
Describe how blood normally flows in vessels.
Normal blood flow is laminar - blood flows fastest in the middle and slowest around the outside.
What is shear rate?
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.
What is shear stress?
Shear rate x viscosity
What are the effects of a) high, and b) low shear stress?
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
What are the equations for pulse pressure and mean arterial blood pressure?
Pulse Pressure = SBP - DBP
Mean Arterial Blood Pressure = DBP + 1/3 PP