Hemodynamics Flashcards
What happens to pressure as we move through circulation?
It decreases. Biggest drop occurs in arterioles.
How do flow, pressure and resistance relate?
Flow = change in hydrostatic pressure / Resistance Q=P/R
How does velocity relate to flow and vessel area?
Flow = velocity X cross sectional area
Poiseuille’s Equation for Vascular Resistance
Blood viscosity increases with…
Increased hematocrit! More force needed to move fluid.
Vessel compliance
Compliance = Volume/Pressure
Low compliance: stiff, difficult to fill
High Compliance: loose, easy to fill (veins)
Compliance is like capacitance, the ability for a vessel to store fluid.
Flow in series
Q1=Q2=Q3
Flow through aorta = flow through all capillaries = flow through all veins
Flow in parallel
Q total = Q1 + Q2 + Q3
Resistance in series
R total = R1 + R2 + R3
If one resistance is much greater than other two, the total resistance is effectively that number.
Resistance in series
1/Rtotal = 1/R1 + 1/R2 + 1/R3
Overall resistance is less than each individual resistance.
Why do capillaries have such low resistance if they have such tiny radii?
Because there are 3 billion of them wired in parallel. The total resistance is less than each individual resistance.
Which vascular section presents the greatest resistance?
Arterioles! Greatest pressure drop equals greatest resistance because flow doesn’t change.
Is blood flow through capillaries slow or fast?
Because v=Q/A, we’d suspect that the flow would be fast, but not so. It’s very slow. Why? Because need to consider total area, or need to divide flow by three billion.
How does the body control blood flow to different organs?
By increasing the resistance in some regions, diverting flow to others. Organs are wired in parallel, so increased resistance at one means reduced resistance at the other!
Why do arterioles have such high resistance?
Not due to the presence of smooth muscles, more due to the fact that they’re not much bigger than capillaries, and there aren’t nearly as many.