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Home , Central chemoreceptors

SUPPLEMENTSUPPLEMENT RESPIRATORY CARE KNOWLEDGE

Marion Richardson, BD, CertEd, RGN, RNT, DipN, is senior lecturer and programme leader, Physiology for practice: the emergency nursing, University of Hertfordshire, Hatfield mechanisms controlling

Marion Richardson explains , or respiration, is something we do not FIG 1. NERVES INVOLVED IN THE CONTROL how respiration is controlled have to think about – it just happens; indeed, if we OF RESPIRATION and regulated within do think about it, our breathing pattern changes. the body Nurses count and record patients’ respirations Cerebral cortex Respiratory centre in every day with perhaps little thought as to the KEY WORDS physiological mechanisms that maintain this vital Glossopharyngeal Medullary respiratory centres function, or to the wealth of information available nerve Respiration from careful assessment of a patient’s breathing. Control Knowledge of these processes is important to be able to understand the symptoms, treatment and management of .

The medullary respiratory centre Rhythmic breathing (normal, quiet breathing at rest or during sleep) is initiated by the respiratory centre in the medulla Carotid artery oblongata of the brainstem. This centre has two groups of neurones: a ventral group and a dorsal group. Spinal cord The dorsal group is sometimes referred to as the inspiratory centre because it acts as the respiratory ‘pacemaker’. The neurones may be self-excitatory (working automatically without the need for stimulus Intercostal Phrenic nerve from nerve pathways) in a similar way to the cardiac nerves to to diaphragm cells at the sinoatrial node, although this is not certain intercostal (Marieb, 2003). Without other influences, these muscles neurones switch on for approximately two seconds and Ribs off for three in a constant, rhythmic pattern. A simple calculation provides the number of breaths per minute which are generated by the inspiratory centre: One minute = 60 seconds One respiratory cycle = five seconds (two on, three off) per minute = 60/5 = 12 breaths per minute In practice, the normal respiratory rate is 12–18 breaths per minute in adults and 18–20 breaths per minute in children (Martini and Bartholomew, 2003). Intercostal Diaphragm muscles For breathing to happen, there must be a connection from the medullary inspiratory centre to the muscles of the . The active inspiratory centre neurones stimulate the nerves to the inspiratory muscles, the strength of the nerve stimulus to the muscles of phrenic nerve to the diaphragm and intercostal nerves respiration – the greater the stimulus, the greater the to the external intercostal muscles. These muscles depth of respiration. contract leading to expansion of the thorax so that air is The ventral group of respiratory neurones in the drawn into the lungs. When the inspiratory centre medulla become more active during forced respiration, neurones are not active, the stimulus to the muscles especially forced expiration (Marieb, 2003). Impulses stops, the muscles relax and expiration occurs from the ventral neurones travel via nerves to the passively. This rhythmic pattern continues unless some muscles involved in forced expiration, notably the other influence affects the inspiratory centre neurones internal intercostal muscles (supplied by intercostal and either increases or decreases breathing rate. nerves) and abdominal muscles (supplied by the vagus Respiratory rate is determined by the length of time nerve). This group of neurones appears to play a role in the inspiratory centre is active before it is switched off. inspiration as well as expiration, but the precise The depth of each inspiration is determined by the mechanism is unclear (Marieb, 2003) (Fig 1).

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REFERENCES Bourke, S.J. (2003) Lecture Notes on Respiratory Medicine. Oxford: Blackwell Publishing. Marieb, E.N. (2003) Human Anatomy and Physiology. New Jersey: Pearson Educational. Martini, F.H., Bartholomew, M.S. In the absence of other stimuli, respiration would include gasping with fear or cold, a rise in respiratory (2003) Essentials of Anatomy and continue in this same rhythmic pattern throughout life. rate when the body temperature is high, and breath- Physiology. New Jersey: Prentice This would not be a problem if people sat quietly every holding during times of anger. Hall. Richardson, M. (2002) day but respiratory rates alter with activities such as From the cerebral cortex we can also voluntarily Physiology for practice: delivering running, climbing and singing, laughing or crying. In change our respiratory pattern by sending signals oxygen to the cells. Nursing order to help us to tailor our breathing to meet the direct to the muscles of inspiration and bypassing the Times; 98: 40, 62–63. needs of everyday life, a number of physiological medullary centres (Marieb, 2003). The cortex is the area Saaresranta, T., Polo, O. (2002) mechanisms influence this basic breathing pattern. of the brain where we interpret and manipulate Hormones and breathing. Chest; 122: 6, 2165–2182. information and when we need, for example, to swim a Stocks, J. (1996) Respiration. In: Influence from the pons Centres in the pons length under water, sing or simply chat to friends, we Hinchliff, S.M. et al (1996) (close to the medulla in the brain stem) influence the can consciously control our breathing pattern. Many of Physiology for Nursing Practice. respiratory neurones in the medulla (Fig 1). The pontine us, in our younger days, tried to hold our breath until London: Baillière Tindall. respiratory group of neurones (once known as the we collapsed, but it is impossible to alter our breathing pneumotaxic centre) is responsible for the ‘fine-tuning’ beyond certain limits because the other respiratory of our breathing and for preventing overinflation of the control mechanisms ultimately override the influence of lungs. It achieves this by sending constant inhibitory the higher centres. impulses to the inspiratory centre in the medulla to limit the period of inspiration. Chemical influences Perhaps the most important influence on respiratory rate and depth are chemicals. Influence from the lungs The lungs contain stretch Specialised receptors () respond to receptors (or baroreceptors) which also appear to chemical changes in the blood and influence respiration. When the lungs expand during (CSF). Peripheral chemoreceptors in the aortic arch inspiration, stretch receptors in the walls are and the carotid bodies respond to changes in the activated and act via the vagus nerve to inhibit the oxygen (O2), (CO2) and acidity (pH) inspiratory centre in the medulla oblongata and allow levels in arterial blood. reflex expiration to occur (Bourke, 2003). These Central chemoreceptors in the medulla oblongata receptors are particularly important in animals and in respond to changes in arterial CO2 levels and in the pH young babies who have a poorly organised brainstem level of CSF. It is these chemoreceptors that are

(Stocks, 1996) but their role in adults remains uncertain, ultimately responsible for the of O2 and especially during quiet respiration. Marieb (2003) CO2 levels in the blood. They ensure that there is suggests that this mechanism is probably protective adequate oxygen circulating for the needs of cells rather than regulatory. throughout the body and that the waste products of

Other receptors in the lungs are sensitive to irritants cellular metabolism, carried as CO2, can be deposited such as gases, debris, inhaled foreign bodies and in the lungs. Arterial pressures of O2 and, especially, excess . When they are activated, these receptors CO2 are maintained within narrow limits despite large influence the respiratory centre via the vagus nerve so changes in consumption and production. that coughing can occur to clear the irritant. Normally it is a small rise in arterial CO2 that triggers these chemoreceptors and results in a negative Influence from the higher brain centres The feedback homoeostatic response to reduce these higher centres of the brain are the areas where we levels. The CO2 diffuses from the blood into the CSF understand and manipulate information and experience where there is very little protein to mop up or ‘buffer’ thoughts, feelings and emotions. These centres can the acid produced. As a result, the CSF rapidly also influence respiration. becomes more acidic and its pH falls. This increased Respiratory rate and depth alter when the centres of acidity stimulates the central chemoreceptors, which the limbic system involved with emotions such as pain, act directly upon the medullary and pontine centres to anger or excitement are activated, though this effect is increase both the rate and depth of respiration by involuntary and outside our control. Centres in the increasing the strength and duration of neuronal hypothalamus are activated and influence both the rate impulses from the inspiratory centre to the muscles of and the depth of respiration via the pons (Martini and inspiration. The result is that the excess CO2 is blown Bartholomew, 2003) and the medullary inspiration out of the lungs. When arterial CO2 and CSF pH levels centre. Respiration can be increased or decreased via return to normal, the response ceases (Fig 1) and this pathway. Examples of this mechanism in action rhythmic respiration resumes.

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FIG 2. CONTROL OF RESPIRATION Peripheral chemoreceptors, bathed as they are in newly oxygenated blood, are sensitive to arterial O2 levels. While they are involved in the response to Cerebral cortexcortex/ – increased acidity (rise in arterial partial pressure of CO , higherhigher brainbrain centrescentres 2 fall in pH) they also respond to falls in the arterial partial (limited(Limited control) control) ▼ StretchStretch receptorsreceptors inin pressure of oxygen (PO2). There are vast reserves of CentresCentres inin ponspons lungslungs (prevent(Prevent arterial oxygen bound to haemoglobin in the blood (‘fine(‘Fine tuning’) tuning’) overinflation)overinflation) (Richardson, 2002), and a large fall in PO2 is needed before these begin to be depleted and the peripheral chemoreceptors are stimulated. Neuronal messages via ▼ the glossopharyngeal nerves (from carotid receptors) HormonesHormones DrugsDrugs and & medicinesmedicines and the vagus nerve (from the aortic receptors),

(various(various influencesinfluences ▼ ▼ (variety(variety ofof actions)actions) stimulate the medullary inspiratory neurones. Rate and andand actions)actions) depth of respiration are increased and more O2 is RespiratoryRespiratory centrecentre inhaled and absorbed into the blood. Once arterial O2 ▼ inIn levels return to normal, the stimulus ceases.

medullaMedulla oblongataoblongata These peripheral receptors assume a vital importance

EmotionalEmotional stimulistimuli viavia ▼ CentralCentral in patients who retain CO2 due to pulmonary disease

hypothalamus such as emphysema or chronic bronchitis. In these

hypothalamus ▼ chemoreceptorschemoreceptors

patients, the central chemoreceptors become ▼ (involuntary(Involuntary ▼ inin brainbrain (maintain(Maintain influences) unresponsive to the constant stimulus of CO and the influences) arterial arterial PCO PCo22 2 andand CSFCSF pH)pH) peripheral chemoreceptors assume the function of driving respiration (the hypoxic drive). These patients PeripheralPeripheral chemoreceptorschemoreceptors IrritantIrritant receptorsreceptors inin will only breathe when arterial PO2 is low enough to inin aorticaortic archarch and& carotid carotid lungslungs (allow(Allow trigger the peripheral chemoreceptors. It is essential that bodybody (maintain(Maintain arterialarterial clearanceclearance ofof debrisdebris nurses understand this physiological alteration as giving OO22 levels)levels) andand fumes)fumes) high doses of oxygen therapy to these patients will stop them breathing because O2 levels do not fall low enough to stimulate respiration. MusclesMuscles ofof respirationrespiration Influence from hormones Hormones are not only involved in the transmission of nerve impulses within the , but recent work suggests that many

When arterial CO2 levels are abnormally low are involved in the control of respiration (Saaresranta (hypocapnia), respirations become shallow and slow and Polo, 2002). Progesterone and thyroxine, for (hypoventilation) and periods of apnoea may occur as example, are known to stimulate respiration, while the stimulus to breathe is absent. This reaction can somostatin and dopamine have a depressant effect. occur as a result of a panic attack and can usually be rectified by rebreathing into a paper bag. Rebreathing Influence from drugs and medications Many

expired CO2 leads to a rise in arterial CO2 levels which different drugs affect our respiratory rate. Barbiturates, triggers the response. alcohol, anaesthetics and opiates have a depressant A fall in the pH of CSF can be triggered not only by effect, while stimulants such as caffeine and respiratory changes but by metabolic causes. Typical amphetamines increase respiratory rate. A variety of causes of metabolic acidosis are poorly controlled mechanisms are involved and readers are referred to diabetes mellitus, which allows organic acids to build specialist respiratory pharmacology texts. up, or the increased production and accumulation of lactic acid during exercise. Conclusion The simple act of breathing in and out is Whether the cause of the reduction of pH in the CSF regulated by numerous physiological mechanisms is respiratory or metabolic, the central chemoreceptors (Fig 2). This complicated system enables us to adjust will be stimulated. The body will react in an attempt to our respirations with great precision, ensuring that every rid itself of excess acids and raise the pH by eliminating cell in the body receives a constant supply of oxygen

CO2 via the lungs. and has a means of ridding itself of waste products. ■

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