In this way, the sympathetic nervous system plays an important part in the thermoregulation of the body. Sweating plays an essential role in the cooling of the body and it dissipates the excess heat during hyperthermia.

The sympathetic nerves release acetylcholine which then triggers sweating. When the sympathetic nervous system is activated, it acts on the muscarinic receptors present in the sweat glands. Sweat glands are present in almost every part of the body. This whole system is called the renin-angiotensin-aldosterone system and it influences vascular resistance and cardiac output. It stimulates the formation of angiotensin, which in turn stimulates the release of aldosterone. Renin is a proteolytic enzyme released by the kidneys into the bloodstream. The sympathetic nervous system increases the secretion of renin by acting through beta-1 adrenoreceptors. On the other hand, the blood flow is increased in the heart, the lungs, and the brain because these organs require more blood flow as they are involved in intense physical activity.

Vasoconstriction occurs when alpha-1 adrenergic receptors in vessels are activated by the sympathetic nervous system through the release of norepinephrine. The blood flow to these organs is reduced because it is not required in emergency situations. The sympathetic nervous system causes vasoconstriction of the blood vessels present in the skin, the kidneys, and the digestive tract. The supply of oxygen to tissues and the brain is increased which helps in threatening situations. The smooth muscles relax, and the bronchi dilate which allows for greater alveolar oxygen exchange. The sympathetic nervous system dilates the bronchioles by acting on the beta-2 adrenergic receptors present in the smooth muscles of bronchioles. In this way, the blood supply to skeletal muscles is greatly enhanced which is required to cope with emergencies. It also increases the force of contraction of the heart by increasing the contractility of cardiac cells. The release of catecholamines by the sympathetic nervous system increases the heart rate. It does so by acting on the beta receptors of the heart. The sympathetic nervous system increases the heart rate in emergency situations. The main function of the sympathetic nervous system is the “fight and flight” response of the body in emergency situations. The sympathetic nervous system is antagonistic to the parasympathetic nervous system because the latter stimulates the body to “feed and breed”. The post-ganglionic fibers then extend and innervate almost every organ system of the body. The pre-ganglionic fibers travel to a ganglion, paravertebral or prevertebral, and the post-ganglionic fibers arise from that ganglion. These fibers are termed pre-ganglionic fibers. The sympathetic nerves arise from the thoracolumbar region (thoracic and lumbar vertebras) of the spinal cord. The sympathetic nervous system is also called the “fight or flight” system of the body. The sympathetic and the parasympathetic nervous system have opposite actions but this antagonistic role of theirs is what regulates some very important functions of the body. The first is the Sympathetic, the second is the Parasympathetic, and the third one is the Enteric branch. The autonomic nervous system has three divisions. It controls breathing, heart rate, peristalsis, vasomotor activity, and it is also involved in sexual activity. The autonomic nervous system regulates many body functions and acts almost unconsciously.

The peripheral nervous system is further divided into the Somatic and Autonomic Nervous System. The nervous system has two divisions, one is Central Nervous System (CNS), and the other is Peripheral Nervous System (PNS).

Autonomic nervous system lens