Review Note

Answer B:  During exercise, skeletal muscle blood flow increases significantly (“exercise-induced hyperemia”). This increase in blood flow is driven by local factors in skeletal muscle – it is not a coordinated response by the nervous system. Numerous local substances produced by skeletal muscle tissue cause vasodilation which increases blood flow. Of these, three that are worth remembering for your exams include adenosine, potassium, and lactic acid. There are many others. 

Answer A (incorrect answer): Acetylcholine is a neurotransmitter used by the autonomic nervous systems. Acetylcholine can indirectly cause vasodilation – this is how the parasympathetic system works. To cause vasodilation, acetylcholine induces nitric oxide release from vascular endothelium. This mechanism is not critical to the fall in resistance with exercise.

Answer C (incorrect answer): Histamine is released by mast cells as part of hypersensitivity (allergic) reactions.

Answer D (incorrect answer): Prostaglandin E2 is important in utero for maintaining patency of the ductus arteriosus. It also has a role in causing fever with infection, in addition to other effects.