Canonical Question

Regional Circulations

V5 D4.v Historical V4 G4.ii 1 appearance

Master answer

Autoregulation: “intrinsic ability of an organ to maintain a constant blood flow despite changes in perfusion pressure“
if perfusion pressure is decreased to an organ, blood flow initially falls, then returns toward normal levels over the next few minutes
This autoregulatory response occurs in the absence of neural and hormonal influences and therefore is intrinsic to the organ, although these influences can modify the response

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 occlusion to blood flow to a tissue, metabolic products build-up
  - On restoration of flow to the tissue, flow can increase 4~7 fold due to buildup of metabolites and autoregulatory mechanisms
Active hyperaemia
- During periods of increased metabolic work, due to the consumption of nutrients and production of metabolites, blood flow increases due to autoregulation

System	Blood Flow	Circulation & Autoregulation
Heart	250ml/min	80% of perfusion occurs during diastole – Maintenance of diastolic pressure is imperative – Tachycardia reduces perfusion Coronary blood flow increases in response to myocardial O₂ consumption approx. 5 fold Autoregulated between 60~180mmHg
Skin	500ml/min	Can increase 30x or decrease 10x Glomus bodies (A-V anastamosis allowing shunting away from skin)
Brain	750ml/min	Autoregulated between 50~150mmHg Munroe-Kellie Doctrine CO₂, O₂ play role in regulation
Skeletal muscle	1000ml/min	Can increase 20 fold during exercise and decreases to 20% during severe hypovolaemia β₂innervation causing vasodilation at low concentrations of adrenalin (α predominates at higher) Skeletal muscle pump
Kidneys	1250ml/min	Autoregulated between 70~170mmHg Tubuloglomerular feedback – Adenosine produced in response to increased perfusion pressure causing vasoconstriction and reduction in flow Sympathetic constriction Angiotensin II constriction Prostaglandins
Liver	1500ml/min	In GI tract, counter-current conformation of the A-V system allows nutrient uptake however predisposes to necrosis with hypoxia Gastrin and CCK increase blood flow
Uterus	100ml/min → 700ml/min at term	NOT AUTOREGULATED

Sympathetic
- Sympathetic α adrenergic stimulation causes vasoconstriction (integral in baroreceptor response)
  - Resting sympathetic tone contributes significantly to peripheral vascular resistance
  - Most tissues except coronary and cerebral circulations
- Sympathetic cholinergic vasodilator supply under cortical control to skeletal muscle

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

Sakurai 2016

Answer by Sakurai

Exam	Exact wording	Relationship	Success
2014A Q12	Describe autoregulation within peripheral circulations.	historical_member	—