Lecture 26 - Control of Ventilation Flashcards
Partial pressures of O2 and CO2 in the blood are determined by what two factors?
alveolar ventilation and blood flow to the alveoli
What is the ventilation/Perfusion (V?Q) ratio?
it is the relationship between blood flow and alveolar ventilation. The nature of the ratio will determine how much gas exchange is taking place
What does a V/Q = 1 suggest?
this implies that the ventilation and perfusion are equal, and that there is a balance such that optimal gas exchange is occurring
What is a “V/Q mismatch” or “V/Q inequality”?
any deviation from V/Q = 1
What does V/Q = zero mean?
there is no ventilation, even though perfusion continues. therefore, there can be no gas exchange, and blood perfusing that part of the lung will return to the heart without releasing CO2 or picking up O2 (aka shunting)
what are three possible reasons for a V/Q = zero?
small airway collapse
hypoventilation
absorption of O2 (atelectasis)
alveolar hypoxia (PaO2 <70 mmHg) induces ______ constriction to the arterioles that supply the hypoxic alveoli
constriction. The net effect of this is to divert the blood away from poorly ventilated areas of the lung, minimizing the shunt effect and maximizing PaO2
what does a V/Q = infinity mean?
there is no perfusion, while ventilation continues. Therefore there is no gas exchange, and air that is being moved in and out of the lung will not pick up CO2, and the blood will not have its O2 absorbed.
What is another term for when V/Q= infinity?
This is referred to as “dead space ventilation” since gas moves in and out of the body without any gas exchange occurring
what are two things that can cause V/Q= infinity?
blood clot, profound hypotension
What are the different mechanisms by which CO2 is transported in the body?
- Dissolved in plasma
- bound to Hb
- bicarbonate ions in the plasma
what are the two methods of O2 transport in the body?
1.Dissolved in plasma
2. bound to Hb
what are the three forms of Co2 in the plasma?
- dissolved in the plasma
- combined with plasma proteins
- combined with water to form H2Co3
overall, Co2 transport in the plasma only accounts for __% of the total CO2 transport in the body
10%
Out of all the methods of CO2 transport, what are the most important?
the reactions that occur in the red blood cells.
Explain the three mechanisms of C02 transport in red blood cells
Co2 readily diffuses into RBCs
Terminal proteins on RBCs can buffer Co2 levels by forming carbamino compounds
Inside RBCs, carbonic anhydrase can convert CO2 + H2o ->H2Co3 -> HCO3- + H+
what is the name of the enzyme that facilitates the conversion of CO2 into carbonic acid in the RBCs?
carbonic anhydrase
write out the chemical equation for the hydration reaction of water with CO2 that occurs in RBCs.
CO2 + H2O <-> H2CO3 <-> HCO3- + H+
what happens as more and more HCO3- is formed from the hydration of CO2 within RBCs?
the HCO3- will diffuse out of the RBC along its gradient into the plasma. This will cause a charge imbalance where the inside of the RBC will lose negative charge. To compensate this, Cl- diffuses into the RBCs, in a mechanism known as a “chloride shift”
explain a chloride shift and why it happens
a chloride shift describes how Cl- will diffuse into an RBC after the RBC has lost it’s HCO3- to the plasma. This happens to maintain the charge balance within the RBC and the plasma
why are RBC’s in venous blood slightly larger than those found in arteriole blood?
water diffuses into the RBC’s during venous circulation
When venous blood reaches the capillaries in the lungs, CO2 that was dissolved in the plasma will:
diffuse out of the blood along its gradient into the alveoli. (ventilation of alveoli keeps the CO2 levels in the lungs low so that the gradient of CO2 diffusion always favours exchange in the lungs)
What does loss of CO2 from the plasma trigger in the lungs?
Co2 then travels down its diffusion gradient, leaving its highly concentrated area in the RBCs to the lower concentrated area of the plasma.
Additionally, the reactions in the RBCs are reversed so that CO2 comes off of the proteins and H2Co3 is dehydrated to form Co2 and water
what helps reverse the reactions to enhance the release of CO2 into the alveoli for elimination from the body via ventilation?
Oxygenation of hemoglobin
what is the normal level for arterial Co2?
around 40 mmHg
what are two methods of measuring CO2 levels?
- Co2 in the blood (artery, vein) with blood gas analyzer
- expired air (end-tidal) with a capnograph
what are the three components of respiratory control system?
- sensors (peripheral and central) gather info about Co2, O2, ph, movement, etc
- central controller in the brain coordinates info and determines what actions to take
- effectors (muscles) induce a response and ventilate the animal
What is the overall goal of the respiratory control system?
maintain normal levels of O2 (100 mmHg) and CO2 (40 mmHg) in arterial blood
where are the central chemoreceptors located?
near the ventral surface of the medulla
what do central chemoreceptors respond to?
changes in H+ levels in the interstitial fluid of the brain
CENTRAL CHEMORECEPTORS: ventilation is stimulated by increases in ____
PaCo2
What do central chemoreceptors NOT respond to?
O2
If central chemoreceptors only respond to changes in H+, why do increases in CO2 in the brain lead to stimulation of the DRG (and therefore stimulate ventilation)?
in the CSF, Co2 can undergo the hydration reaction we previously saw inside the RBCs, leading to indirect increases of H+ from the production of HCO3-
What is DRG?
is is the inspiratory centre in the brain that can cause increases in tidal volume and resp rate
where are the peripheral chemoreceptors located?
in areas that are highly perfused with arterial blood
(aortic bodies along arch of aorta)
(carotid bodies at bifurcation of carotid arteries)
what do peripheral chemoreceptors respond to?
changes in H+, PaCO2, and PaO2.
PERIPHERAL CHEMORECEPTORS:
ventilation is stimulated in response to:
low O2 or high Co2
there are two nerves that stimulate ventilation when excited by the peripheral chemoreceptors. What are these, are where are they first excited?
- Vagus nerve, which stimulates ventilation after being excited by the peripheral chemoreceptors of the aortic bodies
- The glossopharyngeal nerve, which stimulates ventilation after being excited by the peripheral chemoreceptors of the carotid bodies
peripheral chemoreceptors account for __% of the normal ventilatory drive in response to changes in CO2, while the central receptors account for __%
peripheral = 30%
Central = 70%
what is a key feature of peripheral responses to PaO2?
The nervous system doesn’t care too much about fluctuations in O2 until it crops below 60 mmHg. When this happens, the receptors will become very active and stimulate ventilation as a last ditch attempt to bring o2 into the body
What are the four main components of the respiratory center?
- dorsal Respiratory group (DRG) in the medulla
- Ventral respiratory group (VRG) in the medulla
- pneumotaxic centre in the rostral pons
- Apneustic centre in the caudal pons
What is the main function of the DRG?
generate the basic rhythm of breathing.
What are the main inputs to the DRG?
the vagus and glossopharyngeal nerves, which are stimulated by mechanoreceptors in the pleura and the peripheral chemoreceptors
what is the output of the DRG?
the phrenic nerve stimulates contraction of the diaphragm to initiate inspiration
what is the main function of the VRG?
active expiration
True or False: The DRG and VRG are always active with normal respiration
False. The VRG is not usually active with normal respiration, as expiration is typically a PASSIVE process.
what is the main function of the pneumotaxic centre?
inhibits inspiration by regulating inspiratory volume and rate. Helps in fine-tuning of rhythm
True or False: normal respiratory rhythm can still exist in the absence of the pneumotaxic centre
True!
What is the main function of the apneustic centre?
not really known, possibly involved with deep breathing.
what are two situations where you may see apneustic breathing?
ketamine based anesthesia
brain injury or tumours
the main muscles of inspiration are:
diaphragm and external intercostals
the main muscles of expiration are:
internal intercostals
The muscles of inspiration include:
Diaphragm
Intercostals
Abdominal muscles
accessory muscles
