Many fluctuations in blood flow occur as a result of alterations in signals the circulatory system receives from the nervous system. The brain communicates with various organs in the circulatory system through the autonomic nervous system.

This system is comprised of two separate systems called the sympathetic nervous system and the parasympathetic nervous system. The former directs what is frequently called the ‘fight-flight’ response, and the latter directs what has been called the ‘relaxation response.’

Although initially it was thought that these two systems were interconnected (as sympathetic nervous system activity increases, parasympathetic activity decreases), it is now known that they can operate independently.

In this regard, sympathetic and parasympathetic influences can work simultaneously to affect the target organs in the body. The sympathetic nervous system employs two distinct neural systems that affect blood pressure, known as the alpha-adrenergic and beta-adrenergic systems.

The alpha-adrenergic neurotransmitters and receptor systems affect blood vessels by causing them to constrict, whereas the beta-adrenergic system affects both the heart and the blood vessels. Beta-adrenergic activity leads to increased heart pumping action (increased heart rate) as well as vasodilation of blood vessels.

This combination of neural influences represents an adaptive response, as the increased blood flow caused by the increase in heart rate needs more space in the vasculature in order for blood pressure to be properly regulated.

The parasympathetic nervous system influences only the heart via the vagal nerve, which results in slowed heart rate. In sum, these components of the autonomic nervous system interact to regulate blood pressure with the aim of keeping it within adaptive limits.

During the type of exercise described above, heart rate will increase accompanied by vasodilation of the blood vessels in the leg muscles mediated by the beta-adrenergic system. This response pattern permits increased delivery of oxygen to the leg muscles without a concomitant alteration in local diastolic blood pressure.

At the same time, blood flow to the gastrointestinal system is likely reduced via vasoconstriction, as digestion is not an important use of the body’s resources during about of exercise.