30. Local factors of the cardiovascular regulation

Question 1

Regulation of circulation and the satisfaction of the demand for material exchange.

Answer 1

Regulation of circulation, and the satisfaction of the demand for material exchange is based on two principles:

• The demands of material transport to and from tissues should be met.
• The collapse of normal circulation should be avoided by compensating for the inproportional blood supply to different organs during different physiological activities.

The local demand on oxygen and on nutrients is the major determinant of the intensity of blood flow to a certain organ or to a part of that.

• This is regulated by fast, local regulatory mechanisms.
• Achieved by an automatism ensured by the accumulation of metabolites and paracrine signals.

The continuous change of the intensity of tissue activities requires that the blood is directed towards more active tissues.

• This demand is met by the central regulation of the circulation.
• The major element of this central mechanism is the adjustment of the actual state of vessel contraction on the arterial (resistance) and venous (capacitance) segment of circulation by the CNS.

Both local and central regulation may appear in short or long run forms.

Question 2

Short run

Answer 2

• Tissues adjust their own perfusion by local factors.
• The actual tone of vessels in a tissue is adjusted by the intensity of metabolite formation of the tissue. Endothelial cells of the small vessels mediate between tissue demand and blood supply by paracrine signals.
• Autoregulation and myogenic control:
  
  • A tissue working at constant metabolic rate needs constant blood supply. Accordingly transient changes in the mean peripheral arterial blood pressure are counterregulated at the level of the microcirculatory bed. Partly ensured by the local myogenic control of arteriolar and metarteriolar diameter.
  • Increased pressure elicits contraction, decreased pressure elicits relaxation of the small arterioles and metarterioles. This automatic response is independent of CNS, it is of myogenic origin = Bayliss effect.
• Effects of the endothelium:
• Effects of metabolites:

Question 3

Short run

• Effects of the endothelium:

Answer 3

• The blood pressure mechanically deforms endothelial cells. This and many plasma factors prompt the endothels to produce humoral signals that influence the contraction state of smooth muscles in the wall of the vessel itself. Two major classes of such factors:
  
  • EDRF (endothel derived relaxing factor): The most important component beside other factors is nitric oxide (a gas), that is continuously produced by the endothel cells and causes relaxation of smooth muscles (dilation of the vessel).
  • EDCF (endothel derived contracting factor): Many factors that cause the contraction of the smooth muscle on the vessels (contraction). The major such signal are the endothelins.
• Acethylcholine has different effects on the diameter of vessels, depending on the route of application.
  
  • ACh liberated from nerve ending of the smooth muscle causes contraction.
  • ACh injected into the lumen of the vessel causes vasodilation.

Question 4

Short run

• Effects of metabolites

Answer 4

• The autoregulative adjustment of the perfusion of a tissue to its metabolic need is based on its oxygen consumption and metabolite production.
• Increased tissue activity decreases the partial pressure of oxygen and increases the tissue concentration of metabolites.
  
  • This is a stimulus for endothelial cells to produce signals which then elicit the dilation of smooth muscles leading to higher perfusion.
  • Increased local perfusion is called hyperaemia.
• Active hyperaemia changes parallel to the metabolic activity of the tissue. E.g. increased physical activity of muscles.
• Reactive hyperaemia is a secondary increase of the perfusion of an organ due to many possible factors. E.g. inflammation, compression of artery causes hyperaemia after releasing the pressure.

Question 5

Long run

Answer 5

• Intensive and sustained work of an organ causes morphological changes that contribute to the better perfusion at the local level.
• Growth factors, enzyme induction etc.
• Increased perfusion via angiogenesis (rearrangement and formation on new vessels)