The Science of Gas Exchange and the Control of Breathing Flashcards
What is local matching of ventilation and perfusion important for
To optimise gas exchange in the lungs
Where is there better blood flow when upright and healthy
At the base of the lungs
What is each alveolus surrounded by
A dense capilliary network
What are the values of PO2 and PCO2 in normal lungs
PO2= 100 mmHg/ 1.3. kPa PCO2= 40 mmHg/ 5.3 kPa
What is the most efficient V:Q ratio
1
What does not participate in gas exchange
The anatomical dead space
What does the fact that Va:Q= infinity mean
The alveoli are ventilated but not perfused meaning there is an increase in physiological dead so no blood flow therefore gas in the lungs remains the same as atmospheric air
What does a clot in a capillary result in
An increase in physiological dead space so alveoli are ventilated but not perfusedso no blood flow and gas in the lungs remains the same as atmospheric air
What does poor perfusion in one lung result in
Overcompensation of perfusion in the other lung therefore little gas exchange in the lung with poor perfusion even though it is ventilated
Why are the lungs susceptible to emboli
When a blood clot in the leg breaks away it passes through widening vessels until it reaches the right atrium and then enters the pulmonary circulatory system. The pulmonary circulatory system has narrower vessels meaning that clots become trapped (therefore pulmonary circulatory system is most susceptible to emboli)
How is the pulmonary circulatory system arranged
It runs in series
How is the systemic circulatory system arranged
It runs in parallel
What does Va:Q=0 mean
There is a right-to-left shunting of blood so blood passes through the lung without coming into contact with air. There is perfusion but no ventilation. There is no airflow if the airways are completely blocked
Describe a physiological shunt that occurs in health
Bronchial blood supply (bronchial blood supply is a very minor portion of cardiac output, less than 2%)
Describe pathophysiological shunt
Due to fluid filled alveoli. Alveoli can be fileld with fluid due to infection, inflamamtion, pulmonary hypertension or trauma to the lungs. Fluid in the alveoli impairs gas exchange as the volume of alveoli is decreased and so diffusion is reduced.
What are mechanisms to defend Va:Q matching
Principally achieved by modulation of blood flow rather than ventilation. Vasoconstriction but low PO2 results in blood being directed away from poorly ventilated areas, the response is very non-linear
What is global hypoxia due to
Issues such as altitude and results in global vasoconstriction
What happens at low alveolar PO2
There is a significant drop in the amount of blood in a blood vessel due to localised hypoxia which causes vasoconstriction. There is higher blood in the lungs without an embolism
What happens in high alveolar PO2
Little effect on dilation
What happens in the systemic system in low PO2
There is an increase in blood supply (in pulmonary circulation a decrease in PO2 results in a decrease in blood flow)
What controls the rhythm and pattern of breathing
The medulla (brainstem)
What are the respiratory muscles
Diaphragm and intercostals
What do changes in the effectors do
Wither stimulate sensors or remove the stimulus
What plays a role in the fine tuning of breathing
The pons
What does the contraction of the diaphragm depend on
Neural input from the spinal cord to the respiratory muscles (C3, C4, C5 keeps the phrenic diaphragm alive)
What does the fact that the medulla is the respiratory centre mean
It is the rhythm and pattern generator
What does the medulla generate
Discrete bursting patterns resulting in a cycle of inflation and deflation
What is breathing modified by
The pons and other sensors
What is inspiration controlled by
The Pre-Botzinger complex which fires before inspiration
What is pre-inspiration/ expiration controlled by
The reterotrapezoid nucleus/ parafacial respiratory group which fires at the end of inspiration
What does the Pre-Botzinger complex consist of
Specialised nerve endings located in the ventrolateral surface of the medulla which fire spontaneously resulting in a burst pattern causing contraction of the diaphragm
What does the spontaneous pacemaker control
The diaphragm
What does the pons consist of
The pneumotaxic centre and apneustic centre which is responsible for fine tuning of breathing
What feed into the pons
Higher regions of the brain
What does voluntary control mean
You are able to override your medulla and therefore can hold your breath, however you can’t hold your breath indefinitely
Where are lung receptors
In the respiratory system itself and the peripheral receptors are in the body and the CNS
What is the role of stretch receptors
Stimulate lung inflation, terminate inflation
What is the role of juxta-pulmonary “J” receptors
Changes in pulmonary circulation halt inflation in the lungs
What is the role of irritant receptors
Prevent damage to the lungs when you inhale an irritant resulting in a large expulsion of air, therefore these receptors provide a protective function
What is the role of proprioceptors (position/ length sensors)
Sensitive to changes in stretch, they are golgi tendon organs in the ribcage which respond to changes in load. These receptors are also found in limbs
Describe chemoreceptors
They also modify breathing and are the most important and major class of receptors
What is the importance of the fact that ventilation must me matched to metabolism
The respiratory system should not be the limiting factor as there is lots of reserve in health
What is CO2 production estimated from
PCO2
What is O2 production estimated from
PO2
What is H+ production estimated from
pH
What are the 3 indices of metabolism in the body
CO2, O2 and H+
What are changes in blood chemistry detected vy
Chemoreceptors which detect changes in indices
What are the two types of chemoreceptors
Central and peripheral
Where are central chemoreceptors located
In the brainstem near the ventrolateral surface of the medulla
What do central chemoreceptors consist of
Chemosensitive neurones which detect changes in the cerebrospinal fluid
What are central chemoreceptors sensitive to
pH of CSF (index of PCO2)
Why does the pH in the capillary blood stay relatively constant
Due to protein buffers
What moves freely across the blood brain barrier
Carbon dioxide
What does an increase in carbon dioxide in the capillaries result in
An increase in carbon dioxide in the CSF
What does an increase in CO2 in the CSF cause
There are no protein buffers in the CSF therefore pH changes regarding an increase in the concentration of H+ stimulate chemoreceptors
What does acidification of the CSF result in
An increase in firing and then an increase in minute ventilation
Describe central chemoreceptors
located near the ventrolateral surface of the medulla; sensitive to pH of CSF (index of PCO2); relatively slow response time, relatively insensitive to changes in PO2. Central chemoreceptors are not stimulated by hypoxia, therefore carbon dioxide is more important
Where are peripheral chemoreceptors located
Neat the carotid and aortic arteries
Why are the peripheral chemoreceptors located in the aortic and carotid arteries
They are areas of high blood flow, nerve endings project directly into the blood
Why peripheral chemoreceptors are more significant in adults
Carotid bodies
What do peripheral chemoreceptors respond to
Hypoxia, hypercapnia and acidification very rapidly resulting in an increase in firing and an increase in ventilation
What is the peripheral chemoreceptor firing pattern due to
Rapid on and off responses from the carotid sinus nerve
What stimulates peripheral chemoreceptors
Hypoxia and acidification of the blood
Describe peripheral chemoreceptors
Located in carotid and aortic bodies (high blood flow); decrease in PO2 (hypoxia) resulting in an increase in firing; increase in the concentration of H+ or PCO2 results in an increase in firing; respond rapidly (detect oscillations between each breath)
How is blood chemistry constrained to a very small range in health
Peripheral and central chemoreceptors act together to rapidly modify respiration
What is normal arterial PCO2
5.3 kPa
What is normal ventilation
6-7L/min
What does the fact that there is a flat line between 4-5 arterial PCO2 show
Breathing doesn’t cease altogether so it is not completely dependent on chemoreceptors
What controls basal tone
Chemoreceptors
What is the ventilatory response to an increase in PCO2 (hypercapnia)
The ventilatory responds to increased CO2: breath air results in an increase in PCO2 resulting in an increase in tidal volume. 5 minutes later tidal volume is 1.5L (starts at 0.5L), there is also an increase in breathing frequency.
What does hypoxia result in
(Decreased oxygen in inspired air) causes only a small increase in tidal volume, therefore our bodies are not very sensitive to hypoxia
How do hypoxia and hypercapnia work together
There is a synergistic effect resulting in a more rapid increase in tidal volume (means that they both happen and the response is greater than the sum of both individually)
Are you more sensitive to hypoxia or hypercapnia
Hypercapnia
What is the main drive to breathe
A rise in PCO2
