Breathing happens involuntarily because certain respiratory control centers in the brain monitor oxygen and carbon dioxide levels in the body and trigger a series of events if these levels are moving the body away from homeostasis. The main structures that control respiration are the medulla oblongata and the pons. The medulla sets the rhythm of breathing using two main networks of neurons. The first, called the ventral respiratory group (VRG), is the network of nerves that extends from the spinal cord to the site where the pons and the medulla oblongata meet and creates the rhythm of breathing. The ventral respiratory group creates the rhythm that causes the neurons to fire during inspiration (breathing in) and expiration (breathing out). During inspiration, nerve impulses travel along the intercostal nerves, those found in the ribcage. This excites the diaphragm and causes the thorax to expand, allowing air to rush into the lungs. When the neurons fire to stimulate expiration, the input stops and the diaphragm relaxes, forcing out carbon dioxide.
This process repeats over and over again, producing a rate of about 14 breaths per minute. Breathing in takes less time than breathing out. Normal, unlabored breathing (also known as quiet breathing) is called eupnea. The ventral respiratory group plays a major role during periods of hypoxia, which is a drop in normal oxygen level in the blood that induces the ventral respiratory group to increase breathing to restore blood oxygen level back to normal. The ventral respiratory group activity can be completely turned off if the body is exposed to certain chemicals, such as those in alcohol.
The other network of nerves that assists with respiration is called the dorsal respiratory group (DRG). The dorsal respiratory group is a series of nerves that collects information from the peripheral nervous system and from chemoreceptors and sends it to the ventral respiratory group.Another series of nerves, called the pontine respiratory group located in the pons, can influence and modify the activity of the medulla oblongata. The pontine respiratory group (PRG) consists of the pneumotaxic and apneuistic centers. These areas are believed to be responsible for controlling the rate of involuntary respiration. Breathing rhythms are fine-tuned during certain activities, such as sleeping, exercise, and speaking. This center receives input from the sensory organs of the peripheral nervous system and from the higher brain centers.
Sites of Respiratory Centers in the Brain Stem
The chemical that has the greatest impact on breathing rate is the concentration of carbon dioxide in the arteries. The partial pressure of carbon dioxide in the arteries is approximately 40 mm Hg (38 mm Hg to 42 mm Hg). As carbon dioxide levels rise, a condition called hypercapnia, or increased carbon dioxide levels in the brain, occurs. This causes the formation of carbonic acid and a drop in pH. This excites various chemoreceptors, which trigger the VRG. This increases breathing rate, bringing more oxygen into the body. It only takes an increase of 5 mm Hg of carbon dioxide to stimulate this response. When a person hyperventilates, or breathes heavily and rapidly during a panic attack, the opposite condition can arise. Carbon dioxide is rapidly expelled from the lungs and blood levels of carbon dioxide plummet. This is called hypocapnia, or reduced carbon dioxide levels in the body, and causes a respiratory alkalosis, which is an increase in blood pH level.