Regular controlling mechanisms are needed for our bodily activities to function properly. This duty has been given to the nervous system. The autonomic nervous system (ANS), which is in charge of controlling the vital functions of the body, is designed to function in an involuntary, reflexive manner. The operating systems of several machines that make our life easier are developed by being modeled on the ANS. Take an air-conditioned car with a thermostat for example. When the air is cold, a heat sensitive mechanism automatically starts and it provides the engine with more gas and it produces more energy. And when it is warm enough inside the car, this time the thermometer urges the system to reduce the gas-flow back to normal. Likewise, sympathetic and parasympathetic nerves placed in the autonomic system are given the duty of a regulator that restores the altered functioning of organs back to normal so that they do not upset the balance of our body.
If the light coming to our eyes is too bright, vision is blurred. When the retina is exposed to excessive stimuli this causes the parasympathetic nerves to send signals to the eyes to contract the pupils so that the sensitive layers of the eyes are protected and the vision is cleared. In darkness or under dim light, the sympathetic system is called to duty again and this time the pupils are enlarged. The sympathetic-parasympathetic (autonomic) nervous systems granted to human beings play a role in optimizing eyesight under differing intensities of light.
Parasympathetic nerves are created in a way to stimulate the saliva and tear glands, as well as the glands in organs like the nose, stomach, intestines, pancreas, etc. When the secretion in these glands is surplus to our requirements, the canals in connection with them are shrunk and the secretion is lessened. Without such a system, germs would boom, morsels would not soften in our mouth, food intake would not decompose in our stomachs, the gastric mucus which protects the inner stomach from acid would not be secreted, and the final stage of digestion, absorption of digested nutrition, would not happen. Likewise, if our tear glands did not function, sores would emerge on our eyes; if there were no nasal mucus, dust and germs suspending in the air would easily reach our lungs.
The physiological functioning of the lungs and their protection are also maintained through the sympathetic and parasympathetic systems. When our tissues need more oxygen, the sympathetic system is activated. The air sacs are enlarged and more air is let in. If toxic gases, dust, cigarette smoke or other harmful elements enter the respiratory tract or the lungs are exposed to any destructive matter, the air sacs are narrowed by the immediate intervention of the parasympathetic system. In this way, the secretion in the air sacs increases and the harmful substance is prevented from going deeper into the lungs. Then the harmful substance is thrown out through secretion and the reflex of coughing.
When the blood pressure drops below 50mm Hg for any reason (due to hemorrhage, medication, body position, etc), the sympathetic system immediately works to send blood to the brain and the heart. As these are the most vital organs, they are given priority at receiving blood. Our blood circulation is carried out within a closed-circuit system and there is a constant amount of blood. Therefore, sending an organ more blood means lessening the blood sent to other organs. To maintain this, the sympathetic system again works to cause narrowing. When food intake reaches the stomach, the parasympathetic system is stimulated to enlarge the relative veins. More blood is pumped to the stomach.
Everything in both systems is designed to protect the organs, tissues, and systems; in other words, the entire body. When a person’s blood pressure goes up, the baro-receptors, which help regulate the pressure in the veins, are stimulated in order to ward off the danger and the narrowing effect of the sympathetic system on the veins is taken under control. In this way, the pressure applied by the blood to the walls of the veins is eliminated. During physical exercise or in a state of stress, anxiety, or worry, the tissues use more oxygen and the sympathetic alarm is switched on.
Blood is pumped faster to meet the need of the tissues. During sleep, the body needs less energy and the metabolism is slow. Therefore, a slower heartbeat is required. During a time of distress or fear, the sympathetic stimulators are under pressure due to hyperventilation. Then the parasympathetic system is put into service and the heartbeat and the blood flow to tissues slow down.
Sphincters are ring shaped muscles that maintain the constriction of a body passage or orifice. With sympathetic signals they constrict and block the passage, and the parasympathetic signals ease them to open the way. If it weren’t for the sympathetic system, the urine produced in the kidneys would not be under control and we would wet our trousers. However, what happens in practice is that when the kidneys produce a certain amount of urine, two sphincters controlled by the sympathetic system contract and they prevent an untimely emptying of the bladder.
Similarly, there are sphincters in the gastro-intestinal tract. If the sympathetic system had not been given the duty of controlling them, the food we eat would not stay with us until it was absorbed and it would be disposed of immediately. On the other hand, the malfunctioning of the parasympathetic system would cause obstructions and we would suffer greatly. Take the parasympathetic system working in our urinary tract for instance. It works without our control and if it did not work, the urine collected in the bladder would press back on the kidneys and cripple them.
In some functions, like the breaking down of fats, ejaculation, increase in brain activity, or the contraction of skeletal muscles, the parasympathetic system is not involved. Since its involvement might harm the body, it is not given a duty here, and the sympathetic system on its own suffices.
There are several other functions carried out by the autonomous nervous system. It works without our will or conscious control. As humans we tend to claim: “I did this, I did that.” When you eat something, your conscious control is limited to chewing the food and swallowing it. We cannot tell our stomach to digest or not to digest the food. In a healthy body, all of the involuntarily functioning mechanisms seem to know what to do, at the right time and in the right amount. But are these systems really “autonomous,” doing what they will? Can it be coincidence that each time the perfect choice out of thousands of possibilities is made? Is it at all possible for these fascinating systems to be a just a work of random causes?