How Is Breathing Rate Controlled?
Mechanics of Breathing
The mechanics of human and mammal breathing are different from that of amphibians, reptiles, and insects. We breathe by a process known as negative pressure breathing. It starts with the thoracic diaphragm---the muscle that separates the abdominal cavity from the upper torso---contracting. This contraction pulls the diaphragm away from the lungs, giving the lungs room to expand. This expansion, coupled with the timed dilation of the trachea, bronchioles, sinuses, mouth, and alveoli in the lungs, causes us to breathe in. The exact reverse happens when we breathe out. All of this is controlled by the ventral and dorsal respiratory groups in the medulla oblongata of the brain.
Conscious Control
The rate of breathing can be consciously increased in order to overoxygenate the blood and likewise consciously slowed in order to help calm the metabolic rate by the autonomic nervous system. This is a large system of nerves and electrochemical receptors that principally reside in the spinal cord. Almost like a second brain, the autonomic nervous system can regulate or alter the function of many systems within the body in regard to the "fight or flight" reflex and even at the host's will. When a person is consciously attempting to breathe, the system stimulates the beta 2 adrenergic receptor, which has the effect of forcing the bronchioles---the large airways directly leading to the lungs---to relax. Likewise when a person wants to let out a breath, the alpha 2 adrenergic receptor is stimulated, causing the bronchioles to constrict. In order to ensure that all the muscles involved in breathing are working in concert, the medulla oblongata syncs the constriction and relaxation of all the other muscles with the bronchioles. This is what allows a person to consciously control her rate of breathing.
Unconscious Control
As already stated, the medulla oblongata keeps both people and animals breathing even if they're not thinking about it. It also has two methods by which it ensures that it's keeping the optimum rate of respiration at all times: pH and carbon dioxide levels. The amount of carbon dioxide in the bloodstream increases with exertion. Likewise, lactic acid, which causes muscles to burn from exercise or work, lowers the pH of the blood. A chemoreceptor rests in the dorsal respiratory group of the medulla oblongata that measures pH levels. The glossopharyngeal nerve in the carotid veins and the vagus nerve in the aortic veins measure carbon dioxide levels. The medulla oblongata increases or decreases the rate of breathing based on this information.
Tags: medulla oblongata, , autonomic nervous system, carbon dioxide, nervous system, rate breathing