Vascular Control by Ions and Other Chemical Factors
Many different ions and other chemical factors can either dilate or constrict local blood vessels. Most of them have little function in overall regulation of the circulation, but some specific effects are:
1. An increase in calcium ion concentration causes vasoconstriction. This results from the generaleffect of calcium to stimulate smooth muscle contraction.
2. An increase in potassium ion concentration causes vasodilation. This results from the ability ofpotassium ions to inhibit smooth muscle contraction.
3. An increase in magnesium ion concentration causes powerful vasodilation because magnesium ions inhibit smooth muscle contraction.
4. An increase in hydrogen ion concentration (decrease in pH) causes dilation of the arterioles.
Conversely, slight decrease in hydrogen ion concentration causes arteriolar constriction.
5. Anions that have significant effects on bloodvessels are acetate and citrate, both of which cause mild degrees of vasodilation.
6. An increase in carbon dioxide concentration causes moderate vasodilation in most tissues, but marked vasodilation in the brain. Also, carbon dioxide in the blood, acting on the brain vasomotor center, has an extremely powerful indirect effect, transmitted through the sympathetic nervous vasoconstrictor system, to cause widespread vasoconstriction throughout the body.