Unit 3 WK 10 Blood vessels pt. 2 Arterioles and flow control

arterioles

• resistance vessels

• radius can be adjusted independently to

  • distribute cardiac output among systemic organs

  • help regulate arterial blood pressure

local chemicals influence of arteriola radius: intrinsic

• metabolic control mechanism matches blood flow to local tissue needs

• histamine release: injured organs, allergic reactions

  • vasodilation causes inflammatory response

active hyperemia

• high metabolism → vasodilation

  • decreased O2

  • increased CO2

  • decreased pH

  • increased K+

endothelium

• paracrine regulators promote vasodilation (vasoactive mediator)

  • nitric oxide (NO) sets resting “tone” of vessels

  • vasodilator drugs: nitroglycerin or viagra

  • endothelin 1: vasoconstrictor produced by endothelium

local physical influences on arteriolar radius

• local application of heat or cold

• shear stress: blood flow causes friction on endothelial lining. Cells release NO

• myogenic response to stretch

  • vasoconstriction when stretched

  • vasodilation when not stretched

  • reactive hyperemia

  • autoregulation

Reactive hyperemia

• bloackage (occlusion causes less blood flow → less stretch → dilation

• causes a buildup of metabolites → dilation

autoregulation

• MAP decreases → blood flow decreases → dilation

• MAP increases → blood flow increases → constriction

• both exacerbate (make worth) pressure problem, however

Extrinsic Regulation of blood flow at arterioles

• ANS, Endocrine

• Sympathoadrenal activation: increased CO and total peripheral resistance

  • blood flow to viscera is decreased

  • blood flow to skeletal muscles in increased

  • blood is shunted away from viscera and skin toward muscles

Capillaries

• thin-walled, small-radius, extensively branched

• sites of exchange

  • maximized surface area; minimized diffusion distance

  • velocity of blood flow through capillaries relatively slow

• 2 types of passive exchanges

  • diffusion

  • bulk flow