Autoregulation - Cardiovascular

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