The autonomic section of the PNS consists of two systems, which are antagonists: the sympathetic and parasympathetic systems.
The sympathetic system is associated with ‘fight or flight’; when in an emergency, a quick supply of oxygen and glucose is needed. The sympathetic system increases both the heart rate and the stroke volume of the heart. The release of adrenaline in the body causes the heart to pump faster. The feedback stimulus for the increase of heart rate would be the increase of respiration, and therefore the increase of carbon dioxide in the blood. The carbon dioxide needs to be excreted quickly so the blood needs to be pumped around faster.
The sympathetic system is also responsible for dilating the bronchi to provide more oxygen. Digestion in this state is not necessary, therefore, flow of blood to the gut is restricted by contraction of the smooth muscle of the blood vessels carrying blood to the digestive system – it causes the diameter of the blood vessels to narrow.
The sympathetic system is also responsible for dilating the bronchi to provide more oxygen. Digestion in this state is not necessary, therefore, flow of blood to the gut is restricted by contraction of the smooth muscle of the blood vessels carrying blood to the digestive system – it causes the diameter of the blood vessels to narrow.
When not in an emergency situation and in a relaxed state, the parasympathetic system is in use. Parasympathetic nerves return the system to normal. The heart rate slows and stroke volume is released. Blood flow also returns to the digestive system as the smooth muscles of the blood vessels relax and the diameters of the blood vessels become wider.
Source: Damon, A. McGonel R. Tosto P. and Ward W., 2007, Higher Level Biology, Essex: Pearson Education Limited
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