Should blood pressure fall too low, the heart would receive signals to increase its rate, and blood vessels would receive signals to contract. Sympathetic systems are dominant when activity levels increase The autonomic nervous system is further divided into the sympathetic and parasympathetic divisions. These divisions operate antagonistically to each other. For example, the excitation of heart muscle causing an increase in heart , ate is brought about by sympathetic action. The inhibition, or slowing, of the heart rate is caused by parasympathetic activity.
Response excited by one division is inhibited by the other, and vice versa.
In a stressful situation the sympathetic division is dominant. These responses alert the body to a need for action. The stimulation of the adrenal glands to secrete adrenaline is a common emergency response. In non stressful situations the parasympathetic division is in control. Parasympathetic responses act to conserve the resources of the body and to maintain its internal equilibrium.