Sun21November0437PM 6

Muscle blood flow can increase 20- to 50-fold during exercise.

Which is the single most important mechanism of increased blood flow?

(Please select 1 option)

Sympathetic cholinergic stimulation Incorrect answer selected

Beta-2 adrenoreceptor stimulation

Skeletal muscle pump

Alpha-1 and alpha-2 adrenoreceptor stimulation

Local autoregulation This is the correct answer

Explanation

At rest skeletal muscle blood flow is in the order of 1-4 ml/min per 100 g. During exercise blood flow may reach 50-100 ml/min per 100 g. Therefore, blood flow can increase 20- to 50-fold with maximal vasodilation.

During exercise there is a rise in neural sympathetic activity and catecholamine release from the adrenal medulla. In normal circumstances the alpha-1 and alpha-2 would produce vasoconstriction in the muscle groups that are being used but vasodilatory metabolites override these effects, a so-called functional sympathectomy. These stimuli for vasodilatation include local hypoxia and hypercarbia, nitric oxide, K+ ions, adenosine and lactate.

However, blood vessels supplying inactive muscles, the splanchnic and cutaneous circulations, vasoconstrict.

Some species (such as cats and dogs, but not humans) have sympathetic cholinergic innervation of skeletal muscle arteries. Stimulation of smooth muscle beta-2 adrenoreceptors produces vasodilatation but the alpha-adrenoreceptor effects of adrenaline and noradrenaline predominate at rest. The skeletal muscle pump promotes venous emptying during exercise but does not necessarily increase blood flow in these circumstances.

Answer Statistics

1

12%

2

17%

3

14%

4

10%

5

50%

Times answered: 230