45. Local control of the vascular smooth muscle

From greek.doctor

Most organs and tissues can regulate their own blood flow in response to their own needs. These needs can be:

  • They need more oxygen
  • They need more nutrients
  • They need to remove more CO2
  • They need to remove more H+
  • etc.

The way tissues regulate their own blood flow is called local regulation. This regulation is important to ensure that blood flow is sufficient to meet the tissues’ metabolic needs, but it is also important to ensure that the blood flow is not so high that the workload of the heart becomes abnormally high.

Local regulation is also important in the autoregulation of the coronary, cerebral and renal circulations. In most tissues an increase in arterial blood pressure means an increase in blood flow. However, autoregulated tissues will quickly reduce their blood flow back to normal even if the blood pressure remains elevated.

There are many mechanisms with which are involved in regulating local blood flow:

  • The Bayliss effect
  • Release of metabolites
  • Release of vasoactive substances (vasodilators and vasoconstrictors)
  • Oxygen delivery

The Bayliss effect:

The Bayliss effect is an effect where a stretch of blood vessels causes them to vasoconstrict automatically. So, when the blood pressure increases the blood vessels will stretch as the blood flow increases, but the vessels will automatically vasoconstrict, returning blood flow back to normal.

Metabolites:

As tissues increase their metabolism their oxygen and blood requirement increase. This causes a local increase in metabolites and a decrease in oxygen and glucose. These changes induce vasodilation.

Vasoactive substances:

As tissues increase their metabolism, they produce more vasodilator substances like adenosine, NO, prostacyclin, CO2, H+ and K+. These substances will cause vasodilation, which increases local blood flow.

Oxygen delivery:

When a tissue increases its metabolism, they use up more oxygen. As the local availability of oxygen decreases, the vascular smooth muscle doesn’t have enough oxygen to maintain vasoconstriction, causing the level of vasoconstriction to decrease. This increases blood flow and increases the oxygen supply.