MSK – Muscle

Master answer

• Normal blood flow to skeletal muscle ~1l/min
• Can increase 20-fold during exercise

Local factors

Acute (Autoregulation)

• Myogenic
  • Stretch of vessel walls causes constriction due to stretch mediated Ca²⁺ release
    • Prominent in small arterioles throughout body
    • Prevents excessive increases in blood flow in response to pressure
  • Sheer stress on vessels due to increased flow causes release of Endothelial Derived Relaxing Factors such as Nitric Oxide which dilates the vessel, increasing vessel diameter and reducing sheer stress
• Metabolic
  • Increased concentration of metabolic byproducts such as CO₂, lactate, adenosine and H⁺, K⁺, cause upstream vasodilation
    • This diffuse through the interstitium to upstream pre-capillary sphincters, arterioles and meta-arterioles
    • Due to alteration of vessel radius, flow increases, according to the Poisuille-Hagen Equation
  • Conversely, presence of excess nutrients (and O₂) leads to vasoconstriction and decreased flow
• Reactive hyperaemia
  • During muscle contraction, interstitial pressure increases compressing blood vessels and decreasing flow
    • On restoration of flow to the tissue, flow can increase 4~7 fold due to buildup of metabolites and autoregulatory mechanisms
• Active hyperaemia
  • During exercise, due to the consumption of nutrients and production of metabolites, blood flow increases due to autoregulation

Long term

• Arterioles and vessels increase in size and number due to increased metabolic requirements of a tissue and chronic hypoxia
  • Important factors
    • VEGF
    • Angiogenin

Neural factors

• Sympathetic
  • Sympathetic α adrenergic stimulation causes vasoconstriction (integral in baroreceptor response)
    • Resting sympathetic tone contributes significantly to peripheral vascular resistance
  • Sympathetic cholinergic vasodilator supply under cortical control

Hormonal factors

• β₂ mediated vasodilation in response to adrenaline
• α₁ mediated vasocontriction at higher concentrations

Sakurai 2016

Answer by Sakurai