Autoregulation

Introduction

Autoregulation is a mechanism that ensures constant blood flow to organs, despite changing resistance (perfusion pressures) in the blood vessels 2 hypotheses explain autoregulation myogenic hypothesis metabolic hypothesis recall that blood flow (Q) = (change in pressure [P])/resistance (R)

Hypotheses of Autoregulation

Myogenic hypothesis when the vascular smooth muscle is stretched (like when arterial pressure is increased), the smooth muscle contracts (increases resistance) when there is an increase in blood pressure, the myogenic reflex causes smooth muscle contraction in order to maintain flow Metabolic hypothesis O2 delivery to a tissue is matched to O2 consumption of that tissue this is accomplished by changing the resistance (and blood flow) of the arterioles metabolic activity causes tissues to produce metabolites, including vasodilators CO2, H+, K+, lactate, and adenosine ↑ metabolic demand → ↑ O2 demand → vasodilation → ↓ resistance this results in ↑ blood flow

Autoregulation of Organs Organ Factors Determining Autoregulation Heart Local metabolites pO2, adenosine, pCO2, and NO   Brain Local metabolites pCO2 and pH Kidneys Myogenic hypothesis tubuloglomerular feedback Lungs Local metabolites pO2 → vasoconstriction Skeletal muscles Local metabolites lactate, K+, and adenosine Skin Sympathetic innervation

Organ-Specific Autoregulation

Heart most sensitive to pO2, adenosine, pCO2, and NO adenosine results in coronary vasodilation  ↑ myocardial contractility → ↑ O2 demand and consumption → vasodilation → ↑ blood flow Brain most sensitive to pCO2 and pH ↑ pCO2 → ↓ pH → vasodilation→ ↑ blood flow to remove excess CO2 Kidneys myogenic hypothesis tubuloglomerular feedback ↑ renal arteriole pressure → ↑ blood flow ↑ glomerular filtration rate (GFR) ↑ GFR increases delivery of solute and water to juxtaglomerular apparatus (JGA) JGA secretes vasoactive substance constriction of afferent arterioles returns renal blood flow and GFR back to normal Lungs most sensitive to low pO2, which causes vasoconstriction NOT vasodilation the only organ in which low pO2 causes vasoconstriction causes only well-ventilated areas to be perfused to maximize areas of gas exchange Skeletal muscles most sensitive to vasoactive metabolites during exercise (e.g., lactate, K+, and adenosine) sympathetic innervation at rest recall that α1 receptors cause vasoconstrict and β2 receptors cause vasodilation Skin most sensitive to sympathetic innervation ↓ body temperature → α1 receptors vasoconstrict ↓ blood flow → retention of heat ↑ body temperature  → inhibition of cutaneous vasoconstriction ↑ blood flow from warm core to cutaneous vessels → dissipation of heat plays a major role in the regulation of body temperature