Control of Ventilation Flashcards
What is the equation for minute ventilation?
V(E) = f x V(T)
minute ventilation = breathing frequency x tidal volume
What is average minute ventilation and why?
5 L/min
Around 10 breaths are taken each minute (f) and each one is around 500 ml (VT)
Why doesn’t the total volume of each breath reach the lungs?
Due to the dead space which does not partake in gas exchange
How are physiological and anatomic dead space related in healthy individuals?
Anatomic and physiological dead space are roughly the same in healthy individuals
How are physiological and anatomic dead space related in diseased individuals?
The physiological dead space is greater than the anatomic dead space
Less area is available for gas exchange
What is the normal total volume for dead space?
150 ml
What is average minute ventilation at rest, taking into account the affect of dead space?
around 3.5 L/min
What 3 factors influence why breathing may need to vary?
- activity vs. at rest
- altitude
- disease
How does blood flow through the alveoli change during exercise?
Increased cardiac output means that blood flow through the alveoli increases
How does the amount of oxygen consumed vary at rest and during exercise?
During exercise, oxygen consumption can increase by more than 10 times
What is the exercise tolerance test used for?
To determine a patient’s anaerobic threshold
This will determine whether they are fit for surgery
As altitude increases, how does the amount of oxygen available change?
The amount of oxygen in the air decreases meaning the lungs need to work harder to maintain the oxygen supply
What is the result of the change in partial pressure of oxygen as altitude increases?
Partial pressure of oxygen decreases
As saturation of Hb is dependent on pO2, there is less oxygen binding to Hb
How does cardiac output change as altitude increases?
Cardiac output increases to deliver more oxygen to the tissues per minute
Why does disease lead to an altered breathing frequency?
Many disease can compromise gas exchange or delivery of oxygen
How are the activities of the cardiac and respiratory systems altered in sepsis?
Septic tissues have to work harder to produce energy
The cardiac and respiratory systems must work harder to deliver adequate oxygen to the tissues
How does sleep apnoea and opioids affect breathing?
Sleep apnoea leads to periodic cessation during sleep
Opioids lead to depression of breathing
How is chronic hypercapnia related to hypoxic drive?
Chronic hypercapnia is where patients require oxygen sensing
They have become dependent on oxygen concentrations to drive respiratory functions - this is hypoxic drive
How are chemoreceptors involved in developing hypoxic drive?
Chemoreceptors become tolerant to high levels of CO2
High levels of CO2 are no longer the patient’s drive to breathe
The hypoxic drive now drives the patient to breathe
What component of hypoxic drive actually drives someone to breathe?
Low levels of oxygen in the blood
How is hypercapnia defined?
A blood gas CO2 level over 45 mmHg
What condition can result from hypercapnia?
Elevated blood gas CO2 levels drive serum pH down
This leads to respiratory acidosis
What are the sensors, input and output in the control of ventilation pathway?
- Sensors in the periphery, brain and muscles/joints
- Sensors provide an input to the respiratory centre in the medulla
- The respiratory centre sends an output to thoracic and abdominal muscles involved in control of ventilation
Where is the respiratory centre and what does it detect?
Medulla oblongata
It detects the levels of O2 and CO2 in the blood
Where does the respiratory centre send signals to?
Muscles in the heart, lungs and diaphragm
This signals to increase or decrease ventilation
How is the pons involved in ventilation control?
The pons processes inputs that affect respiratory control on a metabolic level
It controls the respiratory rate depending on the body’s needs as it influences the speed of inspiration or expiration
How are the higher sensors in the brain involved in control of ventilation?
They process inputs that affect respiratory control on an emotional level, or due to voluntary effect
What are the three major nuclei that make up the respiratory centre and where are they found?
Ventral respiratory group and Dorsal respiratory group in the medulla
Pontine respiratory group in the pons
What are the 2 areas of the pontine respiratory group?
The pneumotaxic centre and the apneustic centre
What is the role of the dorsal respiratory group (DRG)?
What does it control?
It is responsible for core control of ventilation and initiates inspiration
It controls muscle contraction of inspiratory muscles
Under what circumstances would it be possible to survive with only the DRG?
If only passive expiration was used
When is the expiratory drive required?
Why?
It is only required when exercising
This is because elastic recoil of the lungs during expiration is a passive process
What kind of neurones are found in the DRG?
Neurones which fire during inspiration ONLY
What kind of neurones are found in the ventral respiratory group (VRG)?
Mixed neurones
Some will fire during inspiration and some will fire during expiration
What kind of control on ventilation does the VRG have?
It has some inspiratory and some expiratory control
At rest, what nucleus is in control of respiration? Why?
DRG as there is no expiratory nerve activity
How does inspiratory nerve activity change at rest?
It increases during inspiration and decreases during expiration
During exercise, why does reciprocal inhibition occur?
The inspiratory and expiratory muscles cannot be active at the same time
The DRG and VRG work against each other
What is reciprocal inhibition?
The way in which the stretch of one muscle inhibits the activity of the antagonistic muscle
How does the apneustic centre influence the DRG?
It has a positive role on the DRG as it stimulates inspiratory neurones
How does the pneumotaxic centre influence the DRG?
It has a negative role on the DRG as it inhibits inspiratory neurones
What are the 3 main factors that depress inspiratory activity?
- hypoxia
- therapeutic drugs
- inhibition of blood supply
What are the main therapeutic drugs which depress inspiratory activity?
- opiates
- barbiturates
- anaesthetic agents
What are the two divisions of the higher brain centre influences on ventilation?
Cortical centre and hypothalamic centre
Where is the cortical centre located and what is it involved with?
In the cerebral cortex
It is involved with voluntary control of breathing
What does voluntary hyperventilation lead to?
Hypocapnia and alkalosis
What does voluntary breath-holding lead to?
Why is this unsustainable?
Hypoxia
The higher sensor will inhibit the medulla and pons
The pons senses it is becoming acidotic and so you cannot continue to hold your breath
What is the hypothalamic centre involved in?
Visceral and emotional responses
What do emotions such as anger and anxiety lead to?
Hyperventilation
What do sensory reflexes, such as pain and cold, lead to?
Gasping and hyperventilation
What are pulmonary stretch receptors?
Afferent fibres from the smooth muscle of the trachea and bronchi
How do pulmonary stretch receptors signal to the respiratory centre?
They run in the vagus nerve to the respiratory centre in the medulla
How do the pulmonary stretch receptors influence the brain?
They give sensations to the medulla and the pons
This allows them to gage how much you are breathing, how fast, etc.
How do the lungs feedback to the brain in a negative feedback loop?
Inspiration - the thoracic cage expands and the stretch receptors are activated
Stretch receptors inhibit breathing to prevent over-inflation of the lungs
Expiration - stretch receptors relax to allow inspiration
What is the role of the pulmonary stretch receptors?
How do their impulses change as inspiration progresses?
As inspiration progresses, the impulses from the stretch receptors increases
They inhibit breathing to prevent over-inflation of the lungs
What is the Hering-Breuer lung inflation reflex?
The reflex stimulated to prevent over-inflation of the lung
In the Hering-Breuer reflex, how are the pulmonary stretch receptors activated?
They respond to excessive stretching of the lungs during large inspirations
They are activated and send action potentials through the vagus nerve
In the Hering-Breuer reflex, where do the action potentials from the stretch receptors feed into?
The inspiratory area in the medulla and the apneustic centre in the pons
How are the inspiratory area and apneustic centre inhibited?
The inspiratory area in the medulla is inhibited directly
The apneustic centre in the pons is inhibited from activating the inspiratory area
what does inhibition of inspiration allow for?
It allows expiration to occur
What are irritant/cough receptors?
Receptors throughout the airways that initiate an explosive expiration (cough) when activated
Where do the afferent fibres from cough receptors run to the respiratory centre?
Vagus nerve
Where are irritant receptors found?
In the upper airways and the nose
What does stimulation of the irritant receptors lead to?
Hyperpnoea (deep inhalation) and airway constriction
This leads to coughing and contribute to sneezing
What is the role of the irritant receptors?
They are designed to prevent aspiration and things entering the lungs that shouldn’t
What procedures suppress the actions of the irritant receptors?
Local anaesthetics that are used when passing endotracheal or nasogastric tubes into patients
It is increasingly easier for patients to aspirate when they are suppressed
Where are muscle spindles found?
They are present in large numbers in the intercostal muscles
There are a few in the diaphragm
How are muscle spindles activated?
They are activated by a stretch associated with the contraction of breathing
How can muscle spindles in exercising muscles stimulate breathing?
Muscles produce lactic acid which increases ventilation through chemoreceptors
What is the role of proprioceptors in ventilation?
Where are they found?
They are found in joints
They relay information about activity induced motion, which can influence ventilation
Where are baroreceptors found?
In the cross-arch and aortic arch
What do baroreceptors sense?
How do they influence ventilation?
They sense blood pressure
Blood pressure increases in exercise as cardiac output increases
This leads to an increase in ventilation
What are J-receptors?
Where are they found?
Juxtacapillary receptors
They lie close to capillaries around the alveolar walls
How are J-receptors activated?
Through trauma
What type of traumas activate J-receptors?
What does activation of J-receptors lead to?
Pulmonary oedema, inflammatory agents and pneumonia
Activation of J-receptors leads to increased ventilation
What are central chemoreceptors?
Specialised regions that are close to medulla respiratory centres and close to a rich blood supply
What are central chemoreceptors sensitive to?
CO2 and H+
When H+ and CO2 reach the blood-brain barrier, what happens?
H+ ions cannot cross the blood-brain barrier
CO2 will penetrate the barrier easily
What happens once CO2 has penetrated the blood-brain barrier?
It diffuses into the CSF to inform the chemoreceptors about the H+ levels
What will a rise in CO2 (rise of H+) do to the central chemoreceptors?
It stimulates the central chemoreceptors to increase ventilation
What happens in the CSF when H+ concentration increases?
Acidosis will occur in the CSF as there is no buffering action
What do peripheral chemoreceptors detect?
CO2, O2 and H+
What are the main peripheral chemoreceptors?
The carotid and aortic bodies
What are the peripheral chemoreceptors sensitive to?
- hypoxia - low O2
- hypercapnia - high CO2
- acidosis - high H+
What do the peripheral chemoreceptors feed into?
What does this allow?
They feed along the vagus nerve into the medulla and pons
This allows them to change the ventilatory rate according to what is required
What type of cell is the primary sensor of hypoxia?
The glomus cell
What happens when the glomus cell detects hypoxia?
It feeds into the afferent nerve
This leads to an action potential reaching the hindbrain
What does hypoxia trigger in the glomus cell?
Ca2+ influx into the glomus cell, which leads to depolarisation
What does the Ca2+ influx into the glomus cell trigger?
It triggers the release of transmitters which initiate action potentials in the afferent nerve
What is the neurotransmitter that is released from glomus cells?
Dopamine is an excitatory neurotransmitter that is released
The excitation is from ATP and ACh
