Pulmonary Ventilation Part 1 Flashcards
What is the active part of breathing process?
Inspiration
What is inspiration initiated by?
The respiratory control centre in the medulla oblongata, part of the brainstem
What does activation of the medulla cause?
Contraction of the diaphragm and the external intercostal muscles
Leading to an expansion of the thoracic cavity and a decrease in the pleural space pressure
What part of breathing is passive?
Expiration due to elastic recoil of the lung?
What occurs during forced expiration?
Internal intercostal muscles and anterior abdominal muscles contract and accelerate expiration by raising pleural pressure
Why are there pressure differences between the 2 ends of the conducting zone?
Changing lung volumes
What are important physical properties of the lungs?
Compliance, elasticity and surface tension
What is intrapulmonary or intra alveolar pressure?
Pressure in the lungs
What is intrapleural pressure?
Pressure within the intrapleural space (between parietal and visceral pleura) - contains a thin layer of fluid to serve as a lubricant
What allows air flow into the lungs?
Intrapulmonary pressure is lower than atmospheric pressure
What allows air to flow out of the lungs?
Intrapulmonary pressure is greater than atmospheric pressure
What is transpulmonary pressure?
Difference between intrapulmonary and intrapleural pressure
Keeps the lungs against the thoracic wall and allows the lungs to expand as the thoracic wall expands
What is Boyle’s Law?
A.The pressure of a gas is inversely proportional to its volume
B. An increase in lung volume during inspiration decreases intrapulmonary pressure to sub atmospheric levels - air goes in
C. A decrease in lung volume during expiration increases intrapulmonary pressure above atmospheric levels - air goes out
What happens to the diaphragm in breathing?
Contracts in inspiration - flattens, increasing the volume of the thoracic cavity
Relaxes in expiration -raises, decreasing the thoracic cavity
What is the role of the external intercostal muscles?
Raise the rib cage during normal or quiet inspiration
What is the role of internal intercostal muscles?
Lower the rib cage during forced expiration
What muscles are used for forced expiration?
Internal intercostal muscles
Scalenes, pectoralis minor and sternocleidomastoid
Abdominal muscles
What are the muscles of inspiration?
Sternocleidomastoid
Scalenes
External intercostals
Parasternal intercostals
What is the mechanism of inspiration?
Volume of thoracic cavity increases vertically when diaphragm contracts and laterally when parasternal and external intercostals raise the ribs
Thoracic and lung volume increases -> intrapulmonary pressure decreases -> air in
What is the mechanism of expiration?
Volume of thoracic cavity decreases vertically when diaphragm relaxes (dome) and laterally when external and parasternal intercostals relax for quiet expiration or internal intercostals contract in forced expiration to lower the ribs
Thoracic and lung volume decreases -> intrapulmonary pressure increases -> air out
During a 10km run, what muscles are used during expiration?
Internal intercostals and eg. Abdominal recti
To generate the highest extra try flow, which muscle is most effective at producing a maximal effort?
Rectum abominis
What are the basic elements of the respiratory control system?
- A central controller
- Strategically placed sensors (mechanoreceptors and chemoreceptors)
- Respiratory muscles
What are the 3 important areas of control of breathing?
- The medullary respiratory centre comprising the dorsal medullary respiratory neurones and the ventral medullary neurones
- The apneustic centre
- The pneumotaxic centre
What is breathing triggered by?
Normal automatic and periodic nature of breathing is triggered and controlled by the respiratory centres located in the pons and the medulla
What are the dorsal medullary neurones or DRG associated with?
Inspiration
What does the DRG control?
The spontaneous intrinsic periodic firing of these neurones is responsible for the basic rhythm of breathing
As a result, these neurones exhibit a cycle of activity that arises spontaneously every few seconds and establish the basic rhythm of respiration
What happens when the DRG is active?
When the neurones are active their action potentials travel through RETICULOSPINAL TRACT in the spinal cord and PHRENIC and INTERCOSTAL NERVES and finally stimulate the respiratory muscles
What are the ventral medullary neurones / VRG associated with?
Expiration
What is the role of VRG in expiration?
Neurones are silent during quiet breathing because expiration is a passive event
They are activated during forced expiration when the rate and the depth of the respiration is increased
What does VRG do during heavy breathing?
Increased activity of the DRG neurones activates the expiratory system
In turn, the increased activity of the expiratory system inhibits the inspiratory centre DRG and stimulates muscles of expiration
How are the dorsal and ventral groups paired?
They are bilaterally pared and there is cross communication between them
As a consequence they behave in synchrony and the respiratory movements are symmetric
Where is the apneustic centre located?
The lower pons
What do lesions coverings the apneustic centre cause?
A pathological respiratory rhythm with increased apnoea frequency (missed breaths)
What o nerve impulses from the apneustic centre stimulate?
Stimulate the inspiratory centre and without constant influence of this centre, respiration becomes shallow and irregular
Where is the pneumotaxic centre located?
The upper pons
What effect do the group of neurones in the pneumotaxic centre have?
They have an inhibitory effect on both the inspiratory (DRG) and apneustic centres
It is probably responsible for the termination of inspiration by inhibiting the activity of the dorsal medullary neurones
What does the pneumotaxic centre regulate?
Primarily the volume and secondarily the rate of the respiration
What is responsible for the fine tuning of the respiratory rhythm?
Upper pons
Pneumotaxic centre located here
What does hypoactivation of the pneumotaxic centre cause?
Prolonged deep inspirations and brief limited expirations by allowing the inspiration centre to remain active longer than normal
What does hyper-activation of the pneumotaxic centre result in?
Shallow inspirations
How do they apneustic and pneumotaxic centres function in co ordination in order to provide a rhythmic respiratory cycle?
- Activation of the inspiratory centre stimulates the muscles of inspiration and also the pneumotaxic centre
- Then the pneumotaxic centre inhibits both the apneustic and the inspiratory centres resulting in
- Initiation of expiration
- Spontaneous activity of the neurones in the inspiratory centre starts another similar cycle again
Where are mechanoreceptors placed?
In the walls of bronchi and bronchioles of the lungs
What is the main function of mechanoreceptors?
Prevent the over inflation of the lungs
What does activation of stretch receipts cause?
Inhibits the neurones in the inspiratory centre via the vagus nerve
What happens to the stretch receptors when expiration starts?
Activation of the stretch receptors gradually ceases allowing. Neurones in the inspiratory neurones to become active again
Phenomenon is called Hering-Breuer reflex
When is the Hering-Breuer Reflex important?
In infants and in adults it is functional only during exercise when the tidal volume is larger than normal
What are chemoreceptors activated by?
Changes in O2 or CO2 levels in the blood and the brain tissue
They are involved in the regulation of respiration according to the changes in PO2 and pH
Where are O2 sensitive chemoreceptors (peripheral chemoreceptors) located?
At the bifurcation of the carotid artery in the neck and the aortic arch
What are peripheral chemoreceptors?
Small vascular sensory organs encapsulated with the connective tissue
What are the carotid body chemoreceptors connected to the respiratory centre in the medulla by?
Glossopharyngeal nerve
What are the aortic body chemoreceptors connected to the respiratory centre in the medulla by?
The vagus nerve
Where are central chemoreceptors located?
Bilaterally in the chemosensitive area of the medulla oblongata and exposed to the cerebrospinal fluid (CSF), local blood flow and local metabolism
What do central chemoreceptors respond to?
Changes in H+ concentration
When the blood partial PCO2 is increased, CO2 diffuses into the CSF from cerebral vessels and liberates H+
(When CO2 combines with water it forms carbonic acid and liberates H+ and HCO3-)
What does an increase in H+ stimulate?
Stimulates chemoreceptors resulting in hyperventilation which in turn reduces PCO2 in the blood and therefore the CSF
What always accompanies an increased PCO2 and enhances the diffusion of CO2 into the CSF?
Cerebral vasodilation
Why does CSF have a much lower buffering capacity than blood?
As it has less protein
What is the result of CSF having a much lower buffering capacity than the blood?
Changes in pH for a given change in PCO2 is always bigger than the change in blood
What is most important to maintain normal respiration?
CO2
What is an increase in CO2 called?
Hypercapnia
What do even very small changes in CO2 levels in the blood cause?
Large increases in the rate and depth of respiration
What does hypocapnia, lower than normal PCO2 result in?
Periods in which respiratory movements do not occur
What is a decrease in PO2 called?
Hypoxia
Only after 50% decrease in PO2 can produce significant changes in respiration
Why to produce a significant change in respiration does PO2 levels have to decrease by at least 50%?
Due to the nature of O2-Hb saturation that any PO2 level above 80 mm Hg Hb is saturated with o2
Consequently only big changes in PO2 produce symptoms otherwise it is compensated by O2, which is bound with Hb
What is the primary motor cortex?
The neural centre for voluntary respiratory control
How does the primary motor cortex work?
By sending signals to the spinal cord, which sends signals to the muscles it controls, such as the diaphragm
-> called the ascending respiratory pathway
What is the ascending respiratory pathway required for?
When we talk, cough and vomit
It is also possible voluntarily change the rate of breathing
What does hyperventilation cause?
Can decrease blood partial carbon dioxide pressure due to loss of CO2 resulting in peripheral vasodilation and decrease in blood pressure
What occurs when you stop breathing voluntarily?
This results in an increase in arterial partial oxygen pressure, which produces and urge to breathe
When eventually PCO2 reaches the high enough level it overrides the conscious influences from the cortex and stimulates the inspiratory system
What s the ventilation/perfusion (V/Q) ratio?
This describes the dynamic relationship between the amount of ventilation in the alveoli and the amount of perfusion through the alveolar capillaries
What does the V/Q ratio determine?
The quality fo gas exchange across the alveolar capillary membrane, which in turn determines the amount of oxygen entering the blood and CO2 off loading from the blood
What would occur in an ideal lung?
Each alveolus would receive an adequate amount of ventilation and a matching mount of blood flow through the surrounding capillary
If a V/Q ratio is 1…
Ventilation = perfusion
The ideal conditions never exist because of the effects of gravity on blood flow, the structure of the lungs and shunting of blood
What causes V/Q mismatching?
- shunt is the perfusion of poorly ventilated alveoli
- physiologic dead space is the ventilation of poorly perfumed alveoli
What is HPV?
Hypoxia pulmonary vasoconstriction
Physiological mechanism which is there to combat shunt
What is shunt?
The perfusion of poorly ventilated alveoli
In diseased lungs where there are a lot of under ventilated alveoli, what can this lead to?
Pulmonary hypertension
What is the molecular mechanism of HPV?
The classic explanation involves inhibition of hypoxia sensitive voltage gated potassium channels in pulmonary artery smooth muscle leading to depolarization
• This depolarization activates voltage dependent calcium channels which increases intracellular calcium and activates smooth muscle leading to vasoconstriction.
• Later studies show other ion channels and mechanisms play a role.
• Recently it was proposed that hypoxia is sensed at the alveolar/capillary level rather than in the pulmonary artery smooth muscle cell.
