The regulation of breathing is the result of a complex interaction involving a system of sensors, a respiratory control center, and an effector system to carry out its commands to the muscles and organs involved in breathing.
Description
In the body, cells obtain their energy by combining oxygen with various nutrients, producing carbon dioxide as a waste product. Thus, they need a constant supply of oxygen and also need to have the carbon dioxide removed. This is accomplished by breathing, also known as respiration or ventilation, terms used by physiologists to mean the inhaling and exhaling of air.
Breathing is spontaneously initiated in the central nervous system and is performed by the body's respiratory system. The overall purpose of this system is to allow the body to inhale the oxygen-containing air, and to exhale the harmful carbon dioxide produced by the metabolic reactions.
Air contains 21% oxygen as a component gas. Air is inhaled into the body through the mouth and nose. It then travels past the throat and voice box (larynx), through the windpipe (trachea), which divides into two main air passages leading to the right and left lungs. The two passages divide again in the lungs into smaller branches (bronchi) that separate into even smaller ones (bronchioles or alveolar ducts) much like a tree branches out. These air passages contain little air sacs at their ends called alveoli. There are approximately 150 million alveoli in the human lungs. They have very thin walls that release the oxygen into the blood, receiving in exchange its carbon dioxide, which is then exhaled out of the lungs through the same path leading back to the nose and mouth.
Breathing is an automatic process triggered in a complex area of the brain called the brain stem, a part of the brain that connects with the spinal cord and its nerves. The brain stem contains the involuntary respiratory control center. This means that breathing is more a reflex activity than an activity based on will, meaning that it happens without our having to think about it or decide that it should occur. Breathing is in fact such a strong reflex that it is very hard to willfully stop breathing for any length of time.
Function
The brain stem respiratory control center regulates breathing with the help of nerves, muscles and special sensors called chemoreceptors. Chemoreceptors are specialized cells that can detect chemical substances in the body and pass that information to nerves that are part of the central nervous system for relay to the respiratory control center. In this way, breathing is constantly monitored and adjusted to maintain appropriate pH and partial pressure of oxygen in the arterial blood, that is the blood coming from the heart and that contains the highest levels of oxygen for distribution in the body. The regulation of breathing is a homeostatic control mechanism, meaning that it seeks to maintain the stability of the body's internal environment via negative feedback mechanisms. For example, high levels of carbon dioxide in the body automatically trigger quicker and deeper breathing, which in turn decreases the level of carbon dioxide by increasing the intake of oxygen.
The respiratory control center is the central controlling area for breathing. It receives information from other parts of the body and produces an automatic coordinated response. The response is a reaction that triggers the various organs and muscles involved in breathing and located in different parts of the body to act together. It is located in the lower part of the brain stem, called the medulla oblongata, a scoop-shaped structure connected with the spinal cord. The medulla contains special breathing nerve cells (neurones). One type are the inspiratory neurones, which are active during inhaling and inactive during exhaling. The other type are the expiratory neurones, which are active during exhaling and inactive during inhaling. These two groups of neurones generate an automatic cycle of inhaling and exhaling. This automatic cycle can be modified or even temporarily stopped, depending on the information received by the respiratory control center from a variety of origins, such as reflexes from the lungs and airways, information from the chemoreceptors, and commands from other areas of the brain.
The major centers providing the information required by the respiratory control center to regulate breathing are the following:
Central chemoreceptors. The central chemoreceptors are located at the bottom of the fourth ventricle (region of the brain stem) and they respond to the acidity—or pH—of the cerebrospinal fluid (CSF), the fluid that bathes the brain and spinal cord. The acidity of any fluid is measured by the pH; this value is related to the number of hydrogen ions present in the fluid. The normal pH of the body is 7.4; values higher than this represent alkaline conditions in the body, meaning a lower amount of hydrogen ions, and values of pH less than7.4 represent acidic conditions, meaning a higher amount of hydrogen ions. Thus, when the acidity of the CSF changes, the messages sent by the central chemoreceptors to the respiratory control center then have an effect on breathing.
Peripheral arterial chemoreceptors. There are two types of peripheral chemoreceptors: the carotid bodies and the aortic bodies. They are small pieces of tissue containing chemoreceptors that respond to the amounts of oxygen and carbon dioxide in arterial blood. The carotid bodies are located where the common carotid artery divides into the two carotid arteries on both sides of the neck. The aortic bodies are located on the aorta, the body's largest blood vessel, which starts from the heart. The information from the carotid bodies is carried along the ninth cranial nerve and the information from the aortic body is passed along the tenth cranial nerve to the respiratory control center. The information from the carotid bodies is believed to affect the immediate regulation of breathing, breath by breath, by the respiratory control center.
Brain. Breathing can also be voluntary, that is influenced by other parts of the brain, especially the cerebral cortex, the part of the brain where thought processes reside and are responsible for willful action. We can all consciously breathe more deeply and more rapidly (hyperventilation), as for example, before starting heavy exercise. Strong emotions can also result in hyperventilation.
Lung. There are various receptors located in the lung that can also affect breathing. For example, a type of receptor in the bronchi respond to irritating inhaled substances and cause coughing, breath holding, and sneezing. Other receptors located in the flexible tissues of the lung and the chest wall are sensitive to stretch. The exact role played by these receptors in the regulation of breathing is not fully understood, but they are thought to be responsible for various reflexes that have been discovered in laboratory studies performed on animals. These are stretch reflexes that occur when the lung and chest wall are distended so as to prevent further inhaling. Also, when the air volume is low in the lungs, then there are opposite reflexes.
After receiving information, the respiratory control center needs a pathway to send its regulatory messages. This is done by special nerves, called efferent nerves. They leave the respiratory control center and pass down the spinal cord to the diaphragm, and to the muscles located between the ribs (intercostal muscles) and to other muscles located in the neck and used to breathe. The diaphragm is a thin, sheet-like muscle located at the bottom of the chest cavity below the lungs and heart, and it is the most important muscle involved in breathing. When a person inhales, the diaphragm contracts and moves downward, thus stretching the cavity that contains the lungs (thorax or thoracic cavity), while the intercostal muscles contract and widen the thoracic cavity, which results in air filling the lungs through suction. When a person exhales, the diaphragm and intercostal muscles relax, which decreases the size of the thoracic cavity and forces air out of the lungs. During normal breathing, inhaling is an active muscular process. Exhaling is passive and relies on the natural elasticity of the tissues to deflate the lung.
Role in human health
Breathing is an essential activity of the body, required to maintain life. If it is stopped, death quickly follows. Thus, the regulation of breathing is also essential, meaning that any disruption of this function will affect the respiratory ability of the body, with consequences depending on the extent of the impairment.
Common diseases and disorders
If the rate of oxygen intake and of carbon dioxide elimination is not matched by delivery of the first and removal of the second, an imbalance occurs that may result in respiratory abnormalities associated with serious diseases such as pneumonia (inflammation of the lungs), emphysema (excessive and abnormal accumulation of air in the lungs resulting from a reduction of the area of the lung membranes), heart failure, anemia (lack of red blood cells in the blood, resulting in insufficient oxygen), and asthma (narrowing of the bronchi). The following conditions are the result of such imbalances:
Hypoxia. Also known as anoxia. It means "without oxygen" and is used to describe a condition of insufficient oxygen being provided to the cells of the body.
KEY TERMS
Acidity—Refers to a compound that is acid or sour. When dissolved in water, acids yield hydrogen ions.
Brain stem—Lowest part of the brain that connects with the spinal cord. It is a complicated neural center with several neuronal pathways between the cerebrum, spinal cord, cerebellum, and motor and sensory functions of the head and neck. It consists of the medulla oblongata, the part responsible for cardiac and respiratory control, the midbrain, which is involved in basic, involuntary body functions, and the pons, where some cranial nerves originate.
Bronchi—Air passages in the lungs.
Central nervous system (CNS)—In humans, the CNS consists of the brain, the cranial nerves and the spinal cord.
Cerebrospinal fluid (CSF)—A clear fluid that contains small quantities of sugar (glucose) and protein. The CSF fills the brain and the central canal of the spinal cord and its normal pH is 7.4. Any change from this value is detected by chemoreceptors.
Chemoreceptors—Specialized cells that can detect chemical substances in the body and relay that information to the central nervous system. The substances detected may be external, such as when an individual smells or tastes something or they can be internal, such as the oxygen or carbon dioxide present in the blood.
Cranial nerve—In humans, there are 12 cranial nerves. They are connected to the brain stem and basically control the head as well as help regulate the organs of the thoracic and abdominal cavities.
Diaphragm—Thin, sheet-like muscle located below the lungs and heart at the bottom of the chest cavity and that separates it from the abdomen. The diaphragm is a breathing muscle, it allows air to get in and out of the lungs by moving up and down. When the diaphragm moves down, a person breathes in. When the diaphragm moves up, the person breathes out.
Homeostasis—Stability of the body's internal environment, achieved by a system of integrated control systems activated by feedback systems. For example, high levels of carbon dioxide in the CSF triggers stronger ventilation in the lungs which in turn decreases the level of carbon dioxide by increasing the intake of oxygen.
Involuntary activity—Activity that is not under the influence or control of the will.
Lungs—Large pair of respiratory—or breathing— organs located in the chest. The lungs bring the oxygen present in the inhaled air into the blood and remove toxic carbon dioxide from the blood.
Metabolism—The sum of all the physical and biochemical processes occurring in the body to produce what is required to maintain life. This includes the transformation of nutrients into energy and the use of energy by the body.
Peripheral chemoreceptors—Chemoreceptors not located in the brain stem. There are two types: the aortic bodies located in the aorta, the largest artery in the body and the carotid bodies located in the carotid arteries that pass on either side of the neck, carrying oxygenated blood from the aorta to the brain.
pH—A measure of the acidity of a solution, or of its hydrogen ion concentration. In the human body, the solution can be the blood or the CSF. The normal pH of body fluids is 7.4.
Reflex—Action or movement of the body that is the sum of involuntary activity.
Respiration—In humans, breathing, meaning the inhaling of oxygen-containing air and the exhaling of carbon dioxide-containing air. It can also refer to the exchange of oxygen and carbon dioxide in the blood. The term respiration has yet another meaning in biochemistry, where it refers to the complex chemical reactions occurring in the body, called oxidation reactions, by which chemical substances transfer electrons and convert energy into forms required to maintain life.
Respiratory system—The organs that are involved in breathing: the nose, the throat, the larynx, the trachea, the bronchi and the lungs.
Trachea—The windpipe, a tube made of fiber and cartilage that connects the voice box (larynx) to the bronchi.
Ventilation—Air entering an exiting the body. Pulmonary ventilation refers to the exchange of air between the lungs and the ambient air and alveoli ventilation refers to the exchange of oxygen and carbon dioxide with the blood.
Hypoxic hypoxia. This condition is characterized by an inadequate supply of oxygen to the arterial blood and abnormally low partial pressure of oxygen in the arteries. Cyanosis (blueness of the skin and membranes) is a major symptom, due to lower levels of hemoglobin, the protein that carries oxygen in the blood.
Stagnant hypoxia. This is failure to transport oxygen to the tissues and is due to a slow blood flow, as can occur with heart failure.
Anemic hypoxia. In anemic hypoxia, the partial pressure of oxygen in the arteries is normal, but the oxygen content of the arterial blood is lower than normal.
Hypercapnia. This means an excess of carbon dioxide.
Acidosis. Increase of the amount of hydrogen ion (acidity) in the blood and CSF, leading to respiratory acidosis. It occurs when the lungs are not ventilating properly and results in excessive amounts of carbon dioxide in the blood. Non-functional carotid bodies have been shown to cause acidosis.
Alkalosis. Loss of hydrogen ion in the blood and CSF, leading to respiratory alkalosis, meaning excessive loss of carbon dioxide from the body. An alkaline CSF inhibits the respiratory control center.
BOOKS
Adams, Francis V. The Breathing Disorders Sourcebook. Verulam Publishing Ltd, 1999.
Albert, R. K. et al, eds. Comprehensive Respiratory Medicine. St. Louis: Mosby, 1999.
Murray, J. F. Textbook of Respiratory Medicine. St. Louis: WB Saunders, Harcourt, 2000.
PERIODICALS
Zwillich, C.W. "The control of breathing in clinical practice." Chest 117 (January 2000):205-225.
OTHER
American Lung Association. 1740 Broadway New York, NY 10019. (212) 315-8700. <http://www.lungusa.org>.
