MMED1005 Lecture Notes - Lecture 11: Arteriovenous Fistula, Capillary, Vascular Resistance
7 Jun 2018
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Arterioles control the overall resistance of blood flow. Does this mean it controls the blood
flow in a circulation too?
- Flow = P/R
- Flow = MAP/R
- MAP = Flow x R
- MAP = CO x SVR
- Note: overall difference in pressure is pressure in the arterial side minus pressure in
the venous side. But venous pressure is negligible pressure gradient is approx.
equal to mean arterial blood pressure (MAP). MAP = Flow x R, but Flow = CO and R =
SVR (systemic vascular resistance = resistance from all of the arterioles)
- If the diameter of the arterioles (SVR ), then MAP
- The in pressure is detected by the baroreceptors that adjust the CO and SVR to
return the MAP to normal
Innervated parts of the system?
- Parasympathetic innovation:
o Parasympathetic nerve supply to the heart comes from the brain via the
vagus nerve
o Parasympathetic nerves innervate the SA and AV nodes
o parasympathetic activity → rate of depolarisations from SA and AV nodes
o i.e. heart rate
- Sympathetic innervation:
o Sympathetic nerves innervate the SA and AV nodes from spinal cord
o Sympathetic nerves also innervate the atrial and ventricular myocytes (affect
force of contraction), arterioles ( constriction resistance) and veins
(constrict push blood back to heart)
o sympathetic activity → rate of depolarisations from SA and AV nodes
o i.e. heart rate and force of contractions
About the equations: what do we need to learn?
- Flow = P/R
- But P = Parterial - Pvenous, and Pvenous = negligible P = Parterial = MAP
- Flow = MAP/R
- Rearrange: MAP = Flow x R
- Now, Flow = CO and R = SVR
- MAP = CO x SVR
How much do we need to know about the types of leukocytes?
- You need to know what the different types are and their characteristics
If SVR is the overall resistance of a circulation, which is contributed by the capillaries, why is
the arterioles the main regulator of resistance in blood flow?
- Overall resistance DOES NOT come from CAPILLARIES
- Capillaries have a low resistance compared to arterioles
- It is because the capillaries have a network that is parallel (compared to arteries and
veins which are arranged in series with each other). Remember in y11 physics: for
network in series is much larger than that in parallel
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Document Summary
Arterioles control the overall resistance of blood flow. Note: overall difference in pressure is pressure in the arterial side minus pressure in the venous side. But venous pressure is negligible pressure gradient is approx. equal to mean arterial blood pressure (map). Map = flow x r, but flow = co and r = Svr (systemic vascular resistance = resistance from all of the arterioles) If the diameter of the arterioles (svr ), then map . The in pressure is detected by the baroreceptors that adjust the co and svr to return the map to normal. But p = parterial - pvenous, and pvenous = negligible p = parterial = map. Now, flow = co and r = svr. You need to know what the different types are and their characteristics. Overall resistance does not come from capillaries. Capillaries have a low resistance compared to arterioles.
