APK 2105C Lecture Notes - Lecture 33: Brachial Artery, Muscle Tone, Arteriole
16 Jun 2018
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Chapter 14, Lecture 4
Vessels & Blood Pressure
• Arterioles
o Provide greatest amt of resistance to blood flow (>60% TPR)
▪ Resistance in capillaries vs. arterioles
• Capillaries have larger total area—spread out pressure/resistance
o Why they are not more resistant than the arterioles
o Very low resistance in capillaries
o BP drops off in capillaries
o Velocity of blood flow is very low in capillaries
o First part of microcirculation
o Very important for BP regulation and blood distribution to organs/capillary beds
o Through all the arteries—maintain BP very well/easily
▪ Why can measure BP at brachial artery
▪ When you get to arterioles = drop in pressure (large)
• Largest pressure at arterioles because very small radius, lots of
muscle tone
• Slows things down, decrease pressure in capillaries
o No pressure in capillaries
o No muscle in wall of capillaries
o Arteriolar tone
▪ Very very small arteries
▪ Small internal radius = some resistance
▪ LOTS of smooth muscle in walls compared to radius
• A little constricted at all times
o If fully relaxed—cannot vasodilate = cannot increase blood
flow
o Can get 50% more constricted or dilated
o Very common
▪ Regulatory mechanism
• Increased or decreased for 2 reasons
o Control of blood flow to capillary beds
▪ Intrinsic control mechanisms (from within organ
itself)
▪ Local factors (chemicals)
o Changing MAP
▪ Extrinsic control mechanisms
▪ ANS and hormones
• Autonomic NS
o Control of blood flow is more local, change in BP is more
global
• Intrinsic control of BF = distribution of blood to organs
o EXTRINSIC control mechanisms regulate MAP
o INTRINSIC control mechanisms regulate flow of blood
directed to individual capillary beds
▪ Based on metabolic needs of tissues
▪ Decreased demand → flow is decreased
▪ Increased demand → flow is increased
▪ Locally regulated
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