Control of respiration
Encyclopedia
Control of ventilation refers to the physiological
mechanisms involved in the control of physiologic ventilation
. Gas exchange primarily controls the rate of respiration.
The most important function of breathing is gas exchange (of oxygen
and carbon dioxide
). Thus the control of respiration is centered primarily on how well this is achieved by the lungs.
There are four main centers in the brain to regulate the respiration:
1. Inspiratory center
2. Expiratory center
3. Pneumotaxic center
4. Apneustic center
The first two centers are present on the medulla oblongata whereas the last two centers on the pons region of brain.
The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centers in the medulla
and pons
. The nuclei known to be involved are divided into regions known as the following:
There is further integration in the anterior horn
cells of the spinal cord
.
, with only limited voluntary override. An exception to this is Ondine's curse
, where autonomic control is lost.
as determined by metabolic rate. Blood levels of oxygen
become important in hypoxia
. These levels are sensed by chemoreceptors in the medulla oblongata
for pH, and the carotid
and aortic
bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the glossopharyngeal nerve
(CN IX) and the vagus nerve
(CN X), respectively.
Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.
During moderate exercise, ventilation increases in proportion to metabolic
production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Lactate
produced during anaerobic metabolism lowers pH
and thus increases breathing. In aerobic metabolism, one molecule of acid (CO2) is produced in order to produce 6 molecules of the energy carrier ATP
, whereas in anaerobic metabolism, 6 molecules of lactate are produced to provide the same amount of energy.
Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindle
s via a reflex arc involving the spinal cord.
Drugs can greatly influence the control of respiration. Opioids and anaesthetic drugs tend to depress ventilation, especially with regards to Carbon Dioxide
response. Stimulants such as Amphetamines can cause hyperventilation
.
Pregnancy
tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased progesterone
levels and results in enhanced gas exchange in the placenta
.
Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, straining, burping, coughing and vomiting.
s play important roles in the regulation of respiration; central and peripheral chemoreceptors, and mechanoreceptor
s.
In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the limbic system
, or temperature
, via the hypothalamus
. Voluntary control of respiration is provided via the cerebral cortex
, although chemoreceptor reflex is capable of overriding conscious control.
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
mechanisms involved in the control of physiologic ventilation
Ventilation (physiology)
In respiratory physiology, ventilation is the rate at which gas enters or leaves the lung. It is categorized under the following definitions:-Sample values:...
. Gas exchange primarily controls the rate of respiration.
The most important function of breathing is gas exchange (of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
). Thus the control of respiration is centered primarily on how well this is achieved by the lungs.
There are four main centers in the brain to regulate the respiration:
1. Inspiratory center
2. Expiratory center
3. Pneumotaxic center
4. Apneustic center
The first two centers are present on the medulla oblongata whereas the last two centers on the pons region of brain.
Ventilatory Pattern
The pattern of motor stimuli during breathing can be divided into inspiratory and expiratory phases. Inspiration shows a sudden, ramped increase in motor discharge to the inspiratory muscles (including pharyngeal dilator muscles). Before the end of inspiration, there is a decline in motor discharge. Exhalation is usually silent, except at high minute ventilation rates.The mechanism of generation of the ventilatory pattern is not completely understood, but involves the integration of neural signals by respiratory control centers in the medulla
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
and pons
Pons
The pons is a structure located on the brain stem, named after the Latin word for "bridge" or the 16th-century Italian anatomist and surgeon Costanzo Varolio . It is superior to the medulla oblongata, inferior to the midbrain, and ventral to the cerebellum. In humans and other bipeds this means it...
. The nuclei known to be involved are divided into regions known as the following:
- medulla (reticular formationReticular formationThe reticular formation is a part of the brain that is involved in actions such as awaking/sleeping cycle, and filtering incoming stimuli to discriminate irrelevant background stimuli...
)- ventral respiratory groupVentral respiratory groupThe ventral respiratory group is a column of neurons located in the ventrolateral region of the medulla, extending from the caudal facial nucleus to -400μm obex...
(nucleus retroambigualis, nucleus ambigus, nucleus parambigualis and the pre-Botzinger complexPre-Botzinger complexThe Pre-Bötzinger Complex is a cluster of interneurons in the ventrolateral medulla of the brainstem, which is essential to the generation of respiratory rhythm in mammals...
). The ventral respiratory group controls voluntary forced exhalation and acts to increase the force of inspiration. - dorsal respiratory groupDorsal respiratory groupThe dorsal respiratory group is located in the dorsomedial region of the medulla, and is composed of cells in the solitary tract nucleus. The DRG is one of two known respiratory neuron localizations, with the other being the ventral respiratory group. The DRG is found in many types of fish and...
(nucleus tractus solitarius). The dorsal respiratory group controls mostly inspiratory movements and their timing.
- ventral respiratory group
- pons
- pneumotaxic centerPneumotaxic centerThe pneumotaxic center , also known as the pontine respiratory group , is a network of neurons in the rostral dorsal lateral pons. It consists of the Kölliker-Fuse nucleus and the medial parabrachial nucleus....
.- Coordinates transition between inhalation and exhalation
- Sends inhibitory impulses to the inspiratory area
- The pneumotaxic center is involved in fine tuning of respiration rate.
- apneustic centerApneustic centerThe apneustic center of the lower pons appears to promote inspiration by stimulation of the I neurons in the medulla oblongata providing a constant stimulus....
- Coordinates transition between inhalation and exhalation
- Sends stimulatory impulses to the inspiratory area – activates and prolongs inhalation (long deep breaths)
- overridden by pneumotaxic control from the apneustic area to end inspiration
- pneumotaxic center
There is further integration in the anterior horn
Anterior horn
The term anterior horn may refer to either of two separate anatomical structures within the central nervous system:...
cells of the spinal cord
Spinal cord
The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
.
Control of ventilatory pattern
Ventilation is normally controlled by the autonomic nervous systemAutonomic nervous system
The autonomic nervous system is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions. The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils,...
, with only limited voluntary override. An exception to this is Ondine's curse
Ondine's curse
Ondine's Curse, also called congenital central hypoventilation syndrome or primary alveolar hypoventilation, is a respiratory disorder that is fatal if untreated...
, where autonomic control is lost.
Determinants of ventilatory rate
Ventilatory rate (minute volume) is tightly controlled and determined primarily by blood levels of carbon dioxideCarbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
as determined by metabolic rate. Blood levels of oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
become important in hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
. These levels are sensed by chemoreceptors in the medulla oblongata
Medulla oblongata
The medulla oblongata is the lower half of the brainstem. In discussions of neurology and similar contexts where no ambiguity will result, it is often referred to as simply the medulla...
for pH, and the carotid
Carotid body
The carotid body is a small cluster of chemoreceptors and supporting cells located near the fork of the carotid artery ....
and aortic
Aortic body
In the human heart, the aortic body is one of several small clusters of chemoreceptors, baroreceptors, and supporting cells located along the aortic arch.-Function:...
bodies for oxygen and carbon dioxide. Afferent neurons from the carotid bodies and aortic bodies are via the glossopharyngeal nerve
Glossopharyngeal nerve
The glossopharyngeal nerve is the ninth of twelve pairs of cranial nerves . It exits the brainstem out from the sides of the upper medulla, just rostral to the vagus nerve...
(CN IX) and the vagus nerve
Vagus nerve
The vagus nerve , also called pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves...
(CN X), respectively.
Levels of CO2 rise in the blood when the metabolic use of O2 is increased beyond the capacity of the lungs to expel CO2. CO2 is stored largely in the blood as bicarbonate (HCO3-) ions, by conversion first to carbonic acid (H2CO3), by the enzyme carbonic anhydrase, and then by disassociation of this acid to H+ and HCO3-. Build-up of CO2 therefore causes an equivalent build-up of the disassociated hydrogen ion, which, by definition, decreases the pH of the blood.
During moderate exercise, ventilation increases in proportion to metabolic
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
production of carbon dioxide. During strenuous exercise, ventilation increases more than needed to compensate for carbon dioxide production. Lactate
Lactate
Lactate may refer to:*The act of lactation*The conjugate base of lactic acid...
produced during anaerobic metabolism lowers pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
and thus increases breathing. In aerobic metabolism, one molecule of acid (CO2) is produced in order to produce 6 molecules of the energy carrier ATP
Adenosine triphosphate
Adenosine-5'-triphosphate is a multifunctional nucleoside triphosphate used in cells as a coenzyme. It is often called the "molecular unit of currency" of intracellular energy transfer. ATP transports chemical energy within cells for metabolism...
, whereas in anaerobic metabolism, 6 molecules of lactate are produced to provide the same amount of energy.
Mechanical stimulation of the lungs can trigger certain reflexes as discovered in animal studies. In humans, these seem to be more important in neonates and ventilated patients, but of little relevance in health. The tone of respiratory muscle is believed to be modulated by muscle spindle
Muscle spindle
Muscle spindles are sensory receptors within the belly of a muscle, which primarily detect changes in the length of this muscle. They convey length information to the central nervous system via sensory neurons. This information can be processed by the brain to determine the position of body parts...
s via a reflex arc involving the spinal cord.
Drugs can greatly influence the control of respiration. Opioids and anaesthetic drugs tend to depress ventilation, especially with regards to Carbon Dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
response. Stimulants such as Amphetamines can cause hyperventilation
Hyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
.
Pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
tends to increase ventilation (lowering plasma carbon dioxide tension below normal values). This is due to increased progesterone
Progesterone
Progesterone also known as P4 is a C-21 steroid hormone involved in the female menstrual cycle, pregnancy and embryogenesis of humans and other species...
levels and results in enhanced gas exchange in the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
.
Ventilation is temporarily modified by voluntary acts and complex reflexes such as sneezing, straining, burping, coughing and vomiting.
Feedback control
ReceptorSensory receptor
In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
s play important roles in the regulation of respiration; central and peripheral chemoreceptors, and mechanoreceptor
Mechanoreceptor
A mechanoreceptor is a sensory receptor that responds to mechanical pressure or distortion. There are four main types in the glabrous skin of humans: Pacinian corpuscles, Meissner's corpuscles, Merkel's discs, and Ruffini corpuscles...
s.
- Central chemoreceptorsCentral chemoreceptorsCentral chemoreceptors of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the pH of their environment....
of the central nervous system, located on the ventrolateral medullary surface, are sensitive to the pHPHIn chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
of their environment.
- Peripheral chemoreceptorsPeripheral chemoreceptorsPeripheral chemoreceptors act principally to detect variation of the oxygen concentration in the arterial blood, whilst also monitoring arterial carbon dioxide and pH. They are located in the aortic body and carotid body, on the arch of the aorta and on the common carotid artery, respectively. The...
act most importantly to detect variation of the oxygenOxygenOxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
in the arterial bloodArterial bloodArterial blood is the oxygenated blood in the circulatory system found in the lungs, the left chambers of the heart, and in the arteries. It is bright red in color, while venous blood is dark red in color...
, in addition to detecting arterial carbon dioxide and pH.
- Mechanoreceptors are located in the airwayAirwayThe pulmonary airway comprises those parts of the respiratory system through which air flows, conceptually beginning at the nose and mouth, and terminating in the alveoli...
s and parenchymaParenchymaParenchyma is a term used to describe a bulk of a substance. It is used in different ways in animals and in plants.The term is New Latin, f. Greek παρέγχυμα - parenkhuma, "visceral flesh", f. παρεγχεῖν - parenkhein, "to pour in" f. para-, "beside" + en-, "in" + khein, "to pour"...
, and are responsible for a variety of reflex responses. These include:- The Hering-Breuer reflexHering-Breuer reflexThe Hering–Breuer inflation reflex, named for Josef Breuer and Ewald Hering, is a reflex triggered to prevent over-inflation of the lungs. Pulmonary stretch receptors present in the smooth muscle of the airways respond to excessive stretching of the lung during large inspirations.Once activated,...
that terminates inspiration to prevent over inflation of the lungs, and the reflex responses of coughing, airway constriction, and hyperventilationHyperventilationHyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
. - The upper airway receptors are responsible for reflex responses such as, sneezing, coughing, closure of glottisGlottisThe glottis is defined as the combination of the vocal folds and the space in between the folds .-Function:...
, and hiccups. - The spinal cordSpinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
reflex responses include the activation of additional respiratory muscles as compensation, gasping response, hypoventilation, and an increase in breathing frequency and volume. - The nasopulmonary and nasothoracic reflexReflexA reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es regulate the mechanism of breathing through deepening the inhale. Triggered by the flow of the air, the pressure of the air in the noseNoseAnatomically, a nose is a protuberance in vertebrates that houses the nostrils, or nares, which admit and expel air for respiration in conjunction with the mouth. Behind the nose are the olfactory mucosa and the sinuses. Behind the nasal cavity, air next passes through the pharynx, shared with the...
, and the quality of the air, impulses from the nasal mucosa are transmitted by the trigeminal nerve to the breathing centres in the brainstem, and the generated response is transmitted to the bronchiBronchusA bronchus is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles....
, the intercostal muscles and the diaphragmDiaphragm-Optics and photography:* Diaphragm , a stop in the light path of a lens, having an aperture that regulates the amount of light that passes* Diaphragm shutter, a type of leaf shutter consisting of a number of thin blades in a camera-Acoustics:...
.
- The Hering-Breuer reflex
In addition to involuntary control of respiration by the respiratory center, respiration can be affected by conditions such as emotional state, via input from the limbic system
Limbic system
The limbic system is a set of brain structures including the hippocampus, amygdala, anterior thalamic nuclei, septum, limbic cortex and fornix, which seemingly support a variety of functions including emotion, behavior, long term memory, and olfaction. The term "limbic" comes from the Latin...
, or temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, via the hypothalamus
Hypothalamus
The Hypothalamus is a portion of the brain that contains a number of small nuclei with a variety of functions...
. Voluntary control of respiration is provided via the cerebral cortex
Cerebral cortex
The cerebral cortex is a sheet of neural tissue that is outermost to the cerebrum of the mammalian brain. It plays a key role in memory, attention, perceptual awareness, thought, language, and consciousness. It is constituted of up to six horizontal layers, each of which has a different...
, although chemoreceptor reflex is capable of overriding conscious control.
External links
- Rabbany, Sina Y., "Breathing Coordination", Hofstra University http://people.hofstra.edu/faculty/sina_y_rabbany/engg81/breathingcoordination.html
- Webber, Charles L., Jr., Ph.D, Pulmonary Curriculum Function:"Neural Control of Breathing", Stritch School of Medicine, Loyola University-Chicago http://www.meddean.luc.edu/lumen/MedEd/medicine/pulmonar/physio/pf11.htm