Control of Blood Flow During Exercise

• 2 formulas for cardiac output:

  • Q = HR x SV
  • Q = MAP / TPR
• Rise in HR during exercise:

  • due to innervation of the SA node by SNS and PSNS
  • to increase HR --> FIRST! decrease PSNS
  • to increase HR --> NEXT! increase SNS
  • PSNS always active at rest
  • SNS reserved for stressful situations
  • with exercise --> inhibit PSNS vagal nerve, no more slow contraction, HR increases, then SNS takes over
• Increase in contractility during exercise:

  • inotropy
  • = alters force of muscle contraction
  • positive inotropes = NE and Epi
  • increase inotrope concentration = increase cardiac muscle tension for EDV/preload
  • during exercise, increase inotropy by... 1) increasing SNS; 2) increasing catecholamines
• The muscle pump:

  • central effects
  • muscle contraction helps increase venous return
  • propels blood from muscles to heart
  • increase VR = increase SV
• How do we increase Q with higher exercise intensity?

  there are 5 ways:

  • decrease PSNS
  • increase SNS
  • increase inotropy
  • increase catecholamines
  • mechanical muscle pump
• Distribution of Q:

  • mainly goes to muscle during exercise
• Muscle pump effect:

  • peripheral effect
  • muscle contraction increases arterial flow
  • when muscles contract, they pinch the venous vessels
  • when muscles relax, venous system opens up again; causes negative pressure
  • vacuum effect = opening up of vessels causes blood in capillaries to be sucked up into the venous system
• Diversion of blood flow:

  • at any pressure, flow through vessels is regulated by modifications in local vascular resistance
  • resistance = 8 n L / pi r4
• Vasoconstriction:

  • due to sympathetic vasoconstriction (SNS --> NE)
  • due to circulating vasoconstrictors
• Vasodilation:

  • due to vasodilator formation in skeletal muscles
  • due to flow-induced vasodilation
  • due to myogenic vasodilation
• Vasoconstrictor control:

  • SNS innervates smooth muscle in arterioles
  • increase SNS = increase constriction
  • decrease SNS = decrease constriction
• Vasoconstrictor control:

  • SNS innervates adrenal medulla
  • increase SNS = increase catecholamines
  • increase catecholamines = increase constriction of non-active tissues
  • increase catecholamines = increase HR and contractility
  • increase catecholamines = increase glycogenolysis and gluconeogenesis
• NE and Epi (vasoconstriction):

  • NE and Epi constrict non-active vessels
  • NE and Epi dilate active vessels
• SNS diverts Q to muscle:

  • splanchnic and renal blood flow = decreases
  • plasma NE = increases (has constricting effect on kidneys and organs in the splanchnic region)
• Vasodilator control:

  three types:

  1. metabolic --> chemical agents released; cause smooth muscle relaxation (CO2, K+, H+, lactic acid)
  2. endothelial --> dilator substances within endothelium of arterioles (nitric oxide, etc.)
  3. myogenic --> pressure changes within vessels cause smooth muscle constriction or relaxation (increase pressure outside = decrease pressure inside (dilation); decrease pressure outside = increase pressure inside (constriction))
• Factors influencing Q:

  • neural factors
  • mechanical factors
  • vascular factors