Control Of Respiration Flashcards
Carbonic acid equilibrium
CO2 + H2O <—> H2CO3 <—> H+ + (HCO3)-
Carbonic acid equilibrium enzyme
Carbonic anhydrase
Requirement of respiration
- Ensure haemoglobin is as close to full saturation with oxygen as possible
- Efficient use of energy resource
-Regulate PaCO2 carefully
variations in CO2 and small variations in pH can alter physiological function quite widely
Breathing is autonomic
No conscious effort for the basic rhythm
Rate and depth under additional influences
Depends on cyclical excitation and control of many muscles
-Upper airway, lower airway, diaphragm, chest wall
-Near linear activity
-Increase thoracic volume
Input signals to respiratory control centres
Central chemoreceptor a
Voluntary control (cerebrum)
Lung receptors; stretch, J receptors, irritant
Peripheral chemoreceptors: carotid, aortic
Muscle proprioceptors
Respiratory control centres- basic breathing rhythm
Medulla and pons
3 types of Lung receptors
Stretch
J receptors
Irritant
Pons
Pneumotaxic and apneustic centres
Medulla oblongata
Phasic discharge of action potentials
2 main groups:
1. Dorsal respiratory group (DRG)
2. Ventral respiratory group (VRG)
Each are bilateral, and project into the bulbo-spinal motor neuron pools and interconnect
When is DRG active
predominantly active during inspiration
When is VRG active
active in both inspiration and expiration
Central pattern generator
Neural network (interneurons)
Located within DRG/VRG
-Precise functional locations not known
-Start, stop and resetting of an integrator of background ventilatory drive
What does desire to take a breath come from
PaCO2
Inspiration
Progressive increase in inspiratory muscle activation
-Lungs fill at a constant rate until tidal volume achieved
-End of inspiration, rapid decrease in excitation of the respiratory muscles
DRG and VRG prevent over inflation of the lungs
Expiration
Largely passive due to elastic recoil of thoracic wall
-First part of expiration; active slowing with some inspiratory muscle activity
-With increased demands, further muscle activity recruited
-Expiration can be become active also; with additional abdominal wall muscle activity
DRG and VRG prevent over deflation of the lungs
What % influence from PaCO2 on central chemoreceptors
60%
What % influence from PaCO2 on peripheral chemoreceptors
40%
Chemoreceptors
Stimulated by [H+] concentration and gas partial pressures in arterial blood
Brainstem [primary influence is PaCO2]
Peripheral chemoreceptors
Carotids and aorta [PaCO2, PaO2 and pH]
-Significant interaction
Central chemoreceptors
Located in brainstem
Pontomedullary junction
Not within the DRG/VRG complex
Sensitive to PaCO2 of blood perfusing brain (also influenced by PaO2)
Blood brain barrier relatively impermeable to H+ and HCO3-
PaCO2 preferentially diffuses into CSF
Carotid bodies
Bifurcation of the common carotid
Glossopharyngeal (IX) cranial nerve afferents
Aortic bodies
Ascending aorta
Vagal (X) nerve afferents
Peripheral chemoreceptors
Responsible for [all] ventilatory response to hypoxia (reduced PaO2)
Generally not sensitive across normal PaO2 ranges
When exposed to hypoxia, type I cells release stored neurotransmitters that stimulate the cuplike endings of the carotid sinus nerve
Linear response to PaCO2
Interactions between responses
[Poison (e.g. cyanide) and blood pressure responsive]
What are central chemoreceptors sensitive to
PaCO2
What are peripheral chemoreceptors sensitive to
PaO2, PaCO2 and pH
What mediates the response to CO2 by central chemoreceptors
H+ produced during the carbonic acid equilibrium bind to receptors and increase ventilation
Lung receptors
Stretch, J and irritant
Afferents; vagus (X)
Combination of slow and fast adapting receptors
Assist with lung volumes and responses to noxious inhaled agents
Stretch lung receptors
Smooth muscle of conducting airways
Sense lung volume, slowly adapting
Irritant lung receptors
Larger conducting airways
Rapidly adapting [cough, gasp]
J (juxtapulmonary capillary) lung receptors
Pulmonary and bronchial C fibres
Nose, nasopharynx and larynx airway receptors
Chemo and mechano receptors
Some appear to sense and monitor flow
Stimulation of these receptors appears to inhibit the central controller
Pharynx airway receptors
Receptors that appear to be activated by swallowing
- respiratory activity stops during swallowing protecting against the risk of aspiration of food or liquid
Muscle proprioceptors
Joint, tendon and muscle spindle receptors
Intercostal muscles > > diaphragm
Important roles in perception of breathing effort
If ascending up a hill….
Ascending; PiO2 falls (FiO2 remains constant)
Decreased PAO2
Decreased PaO2
Peripheral chemoreceptors fire (e.g carotid)
Activates increased ventilation (VA)
Increased PAO2
Increased PaO2
PaCO2 and pH
The pH of the CSF is established by the ratio of pCO2 : [HCO3–].
The HCO3– levels remain relatively constant. CO2 freely diffuses across the blood brain barrier, from the arterial blood supply into the CSF. CO2 reacts with H2O, producing carbonic acid, which lowers the pH. This means that the pH of the CSF is inversely proportional to the arterial pCO2.
A small decrease in pCO2 leads to an increase in the pH of the CSF, which stimulates the respiratory centres to decrease ventilation.
A small increase in pCO2 leads to a decease in the pH of the CSF, which stimulates the respiratory centres to increase ventilation.
Ventral respiration group
the VRG sends inhibitory impulses to the apneustic centre, decreasing the duration of inspiration, leaving more time for longer expiration during forced expirations
Dorsal respiratory group
initiates respiration and determines the basic rhythm of breathing by adjusting the frequency of inspiration. When it receives information regarding an increase in PaCO2, the DRG causes the diaphragm to contract (via the phrenic nerve) to increase the vertical length of the thoracic cavity, and through the intercostal nerves to the external intercostal muscles, which contract and cause the ribs to move up and out, increasing the lateral size of the thoracic cavity. This causes airflow into the lungs
Apneustic centre
When O2 requirement is higher (eg during exercise) the apneustic centre of the pons is activated, stimulation excites the DRG in the medulla, prolonging the period of action potentials in the phrenic nerve, prolonging contraction of the diaphragm and so preolonging inspiration
Pneumotaxic centre
To prevent over inflation of the lungs, the pneumotaxic centre is activated, which limits the burst of action potentials in the phrenic nerve, making the diaphragm contract less- stopping inspiration and allowing for expiration to happen
When low PaO2 detected
afferent impulses travel via the Glossopharyngeal and vagus nerves to the medulla oblongata and pons in the brain stem. In order to restore PaO2, respiratory rate and tidal volume are increased to allow more oxygen to enter the lungs and diffuse into the blood. Also, blood flow is directed towards the kidneys and brain (as these organs are most sensitive to hypoxia), and cardiac output is increased to maintain blood flow
Hypercapnia
Hypercapnia, also known as hypercarbia, is a condition that occurs when a person has too much carbon dioxide (CO2) in their bloodstream. It can cause confusion as increased CO2 levels cause the brain to reduce metabolism and spontaneous neural activity and enter a lower arousal state.
In response to what stimuli cause chemoreceptors in the medulla to increase respiratory rate
Increase in carbon dioxide of cerebral spinal fluid
Normal respiratory rate
10-12 breaths per minute
Acid-base balance
CO2 +H20 <-> H2CO3 <-> (HCO3)- + H+
Carbonic anhydride enzyme
Respiratory control of acid-base balance is
Rapid
Renal control of acid base balance is
Slow
Opioids
Depress control of respiration
Amphetamines
Stimulate respiration
Large respiratory reserve
Minute ventilation : 7.5 L/min
Can increase to 30L/min
Flow of air
(Alveolar pressure - atmospheric pressure) / resistance
Pneumotaxic centre
Inhibits inspiration- enables expiration
Apneustic centre
Increases inspiration
Pontine respiratory group
Pneumotaxic centre
Apneustic centre
Dorsal respiratory group
Inspiration: contracts
Diaphragm
External intercostals
Pre-Bötzinger complex
Rhythm generator of breathing (pacemaker cells)
Ventral respiratory group
Inspiration and forced expiration
Contracts:
Internal intercostal muscles
Accessory muscles
Central chemoreceptors
detect changes in PaCO2. The pH of the CSF is established by the ratio of pCO2 : [HCO3–].
• The HCO3– levels remain relatively constant.CO2 freely diffuses across the blood brain barrier, from the arterial blood supply into the CSF. CO2 reacts with H2O, producing carbonic acid, which lowers the pH. This means that the pH of the CSF is inversely proportional to the arterial pCO2.
• A small decrease in pCO2 leads to an increase in the pH of the CSF, which stimulates the respiratory centres to decrease ventilation.
• A small increase in pCO2 leads to a decease in the pH of the CSF, which stimulates the respiratory centres to increase ventilation.
• The DRG (dorsal respiratory group) initiates respiration and
What is main driver for respiration
CO2
chemoreceptors respond to small CO2 changes but large O2 changes
Prolonged hypoxia
Type II sustenacular cells (supporting) ——> type I glomus cells (O2 sensing)
Carotid sinus is innervated by
Glossopharyngeal nerve
Aortic arch is innervated by
Vagus nerve
Which nerve do pulmonary stretch and irritant receptors inhibit the respiratory centre by
Vagus nerve
Irritant receptors
Larger conducting airways
Fast adapting- cough
Stretch receptors
Lung inflation in smooth muscle
Slowly adapting
Inhibit inspiration (inflation)
J receptors
Alveoli
Unmyelinated C fibres
Irritants, noxious agents, volume
Bronchoconstriction and shallow breathing
Rapidly adapting stretch receptors
Airway epithelial cells that respond to rate of change of volume and irritants
Can cause bronchoconstriction and long deep breathing
The central chemoreceptors, located in the ventral medulla help regulate the internal environment.
What do they respond to?
CSF pH
Chemoreceptors regulate the internal environment by monitoring changes to various substances.
Changes in which of these substances stimulate the carotid chemoreceptors?
Oxygen, carbon dioxide and H+ ions
The body uses a variety of chemoreceptors to regulate the internal environment.
Where are the main peripheral chemoreceptors located?
Carotid arteries and aortic arch
