Lung Physiology 4 Flashcards
Respiration requirement is to
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
CO2 is inversely proportional to alveolar ventilation
Breathing is automatic
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
Basic breathing rhythm
-comes in summarise assessments-
Where are the major locations of the higher centre involvements in the brain?
In the Pons the centres are
-Pneumotaxic and Apneustic Centres
These are responsible for physical discharges
Medulla Oblongata
-Phasic discharge of action potentials
-Two main groups that set off these physicians rhythmic responses
Dorsal respiratory group (DRG)
Ventral respiratory group (VRG)
DRG; predominantly active during inspiration
VRG; active in both inspiration and expiration
Each are bilateral, and project into the bulbo-spinal motor neuron pools and interconnect
Central Pattern generator
Contains Neural network (interneurons)
Located within DRG/VRG
Precise functional locations not known
-Start, stop and resetting of an integrator of background ventilatory drive
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
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
Input signals
Central chemoreceptors
Chemoreceptors- sensitive to CO2
Central (60% influence from PaCO2) and peripheral (40% influence from PaCO2)
-Stimulated by [H+] concentration and gas partial pressures in arterial blood
-This stimulates Brainstem [primary influence is PaCO2]
Carotids and aorta [sensitive to 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 ONLY and respond to most CO2 changes
Blood brain barrier relatively impermeable to H+ and HCO3-
PaCO2 preferentially diffuses into CSF and so influences your ability to take a breath
CO2 crosses the blood brain barrier, CO2 then combines with water, which then pushes this direction along. This leads to H+ concentration increasing which then stimulates the medullary inspiratory neurones to increase ventilations (70%)
Ventilation response to PaCO2
As theres an increase in alveolar CO2, theres an increase in the ventilators response
If you fix this at a standard, but lower, Oxygen level, there is a greater response for a given CO2
So the more hypoxic you are, the more likely you are to want to breathe for a given CO2.
Peripheral chemoreceptors
Location
These are located in;
Carotid bodies
Bifurcation of the common carotid
(IX) cranial nerve afferents
Aortic bodies
Ascending aorta
Vagal (X) nerve afferents
These are in the ascending aorta and these are heading towards the brain
Peripheral chemoreceptors
Role
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
neurotransmitters increase the number of action potentials along the afferent nerve fibres stimulating the medullary inspiratory neurones, this increases inspiration.
The receptors also respond to an increase in the arterial [H+] increasing stimulation of the medulla inspiratory neurones increasing ventilation (30%)
Peripheral chemoreceptors
Response to CO2
Linear response to PaCO2
Interactions between responses
[Poison (e.g. cyanide) and blood pressure responsive]
Ventilatory response to PaO2
As the PaO2 goes down, people begin to breathe extremely rapidly particularly when their CO2 is fixed. This is so called eucapnic hypoxia.
There is a much more blunted response if you’re allowed to alter your CO2, theres a much more marked response if the CO2 is fixed
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, J and irritant
Stretch
Smooth muscle of conducting airways
Sense lung volume, slowly adapting
Irritant
Larger conducting airways
Rapidly adapting [cough, gasp]
J; juxtapulmonary capillary
Pulmonary and bronchial C fibres
Airway receptors (NOT necessary to learn)
Nose, nasopharynx and larynx
Chemo and mechano receptors
Some appear to sense and monitor flow
-Stimulation of these receptors appears to inhibit the central controller
Pharynx
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
An example; ascent
Ascending; PiO2 falls (FiO2 remains constant)
Decreased PAO2
Decreased PaO2
Peripheral chemoreceptors fire (e.g carotid)
Activates increased ventilation (VA)
Increased PAO2
Increased PaO2
