Week 1 Ventilation and Perfusion Flashcards
Give 4 key functions of the respiratory system
1) Gas exchange (O2 in CO2 out)
2) Reservoir of blood and O2 ( pulmonary circulation stores 7-10% blood and 2.5 L O2 after expiration)
3) Metabolism of circulating compounds- e.g ACE/bradykinin/ prostaglandins/ serotonin
4) Filter blood - microthrombi removed
Describe the general divisions and structure of the respiratory system
23 divisions in the airway:
General structure: trachea splits into: R and L primary bronchus, leads into secondary and tertiary bronchi. Up to 10 divisions before bronchi become bronchioles. After 10th division leads into bronchioles that become terminal bronchioles, respiratory bronchioles, alveolar ducts, alveolar sacs.
- 1-16 are the conducting airways - up to division 16 there are no alveoli and form the anatomical dead space.
Air movement is via bulk flow down a pressure gradient
-17- 23 are the respiratory airways - terminal bronchioles change to respiratory bronchioles with alveolar ducts leading into alveolar sacs. Air movement is dependent on diffusion which relies on partial pressure gradients of O2/CO2.
What histological changes occur as you descend the respiratory tree?
How does the internal structure of the bronchi and bronchioles differ?
- Generally as you descend the respiratory tract, the amount of cilia, mucus producing cells and cartilage all decrease.
- Up to the 10th division i.e within Bronchi C-shaped rings of hyaline cartilage keep airways open providing structural support.
- After 10th division in Bronchioles, there is no cartilage and bronchioles rely on negative intrapleural pressure to remain open. Smooth muscle forms part of the bronchial wall, which can contract or relax to constrict or dilate the airway. Contraction reduces lumen, increases resistance and reduces flow. Dilation increases lumen diameter, reduces resistance, increases flow.
Define ventilation and perfusion
Ventilation is the amount of air reaching the alveoli and describes the passage of air in and out of the respiratory tract.
Perfusion is the amount of blood reaching the alveoli via the pulmonary capillaries.
Alveolar gas composition relies on both ventilation and perfusion.
Describe the structure and properties of the capillaries within the respiratory system
What reflex may occur when there is low O2 within an alveolus?
- Dense network of pulmonary capillaries that surrounds each alveolus.
- Forms a thin blood- gas barrier that facilitates gas exchange
- Pulmonary circulation and alveolar capillary network are distensible- they can alter their vessel diameter to increase flow
- Links to the Hypoxia - induced vasoconstriction reflex:
- low O2 detected by alveolus
- Leads to contraction of vascular smooth muscle in capillaries surrounding hypoxic alveolus
- Vasoconstriction of the capillary network which redirects blood to better ventilated alveoli.
- Can be used during normal physiological response to exercise, some vessels are closed off at rest then recruited during exercise - more alveoli are recruited, blood is directed towards increasingly ventilated alveoli.
Define diffusion
Net movement of molecules from an area of high concentration to area of low concentration.
what effect would increased ventilation have on alveolar partial pressures?
What effect would increased perfusion have on alveolar partial pressures?
What are the normal partial pressures within alveolus/ pulmonary capillary network?
Increased ventilation leads to an increase in pO2 and decrease in pCO2 (more oxygen delivered, more CO2 removed.
Increased perfusion leads to a decrease in pO2 as more O2 is removed from the alveolus into the circulation, and increased pCO2 as more is delivered to the alveolus.
Alveolus: pO2 - 13.3 kPa pCO2 - 5.3 kPa
Pulmonary capillary - pO2- 6.0 kPa pCO2- 6.5 kPa
Describe the layers of the diffusion barrier
- oxygen needs to diffuse through the alveolus
- alveolar epithelium
- epithelial BM
- Interstitial space - tissue fluid and connective tissue
- through endothelial cell BM
- Through endothelial cell of pulmonary capillary
- Through blood plasma
- RBC membrane and cytoplasm
what is the equation for diffusion rate?
Fick’s law:
Diffusion rate = A x △P x d / T
Where A = surface area available
△P = partial pressure gradient
d= diffusion constant which = Solubility / square root of molecular weight
T= thickness of diffusion barrier
Describe gas transfer in the lungs and what it is limited by under normal circumstances?
Why is this beneficial in exercise?
What becomes the limiting factor in disease states?
Why would this lead to desaturation of haemoglobin on exercise?
- Gas transfer relies on both ventilation and perfusion of the lungs
- Under normal circumstances (with 13.3kPa O2) haemoglobin is saturated at 25% of the way along the capillary bed. Gas transfer is perfusion limited in the healthy individual.
- This leaves a large reserve which is beneficial during exercise as blood is moved through the pulmonary circulation faster, giving haemoglobin less time to saturate. In a healthy individual reserve means haemoglobin still reaches full saturation during exercise.
- In disease states where the diffusion barrier may have becomed thickened (fibrosis) haemoglobin saturation becomes Diffusion limited due to increased thickness and reduced diffusion rate. It now takes the whole length of the capillary to fully saturate haemoglobin, lost reserve.
- This means diffusion becomes limiting factor rather than perfusion.
- Leads to desaturation of haemoglobin on exercise as blood is moved through pulm circulation faster, less time to saturate and lost reserve as increased diffusion barrier thickness.
Define each of the volumes shown
- Tidal volume= volume of air moved in and out the lungs during normal quiet respiration
- Inspiratory reserve volume = the added volume of air that can be inhaled over normal tidal volume with maximal inspiratory effort
- Expiratory reserve volume= the added volume of air that can be exhaled over normal tidal volume with maximal expiratory effort
- Residual volume= the volume of air that remains in the lungs after maximal expiration.
What is a lung capacity measurement? How does it differ from lung volume measurements?
Define the following lung capacities:
- A lung capacity measurement incorporates two or more lung volumes and is measured from fixed points in the respiratory cycle.
- Unlike lung volumes which change with respiratory pattern, lung capacities are fixed.
- Total lung capacity = total volume of air within the lungs after maximal inspiration (RV + ERV + TV + IRV).
- Vital capacity = total volume of air that can be inspired and expired from the lungs with maximal respiratory effort (ERV + TV + IRV)
- Inspiratory capacity= The total volume of air that can be inhaled with maximum inspiration. (IRV+ TV)
- Functional residual capacity (FRC) = the volume of air remaining within the lungs after normal expiration. (RV + ERV).
What is total minute ventilation?
How would you calculate it?
What is a typical minute ventilation?
What can it increase to during exercise?
Total minute ventilation= the volume of air entering and leaving the lungs within 1 minute
MV = TV x RR (Tidal volume x respiratory rate)
Typical minute ventilation = 6-8 L
Can increase to 70 L during exercise
What is the alveolar ventilation rate?
What does it allow for and how would you calculate it?
The alveolar ventilation rate is the total amount of air that reaches the alveoli / minute.
It allows for dead space and is calculated by:
Alveolar ventilaton rate= (TV- Vds) x RR
Where TV = tidal volume Vds= dead space volume RR = respiratory rate.
What types of dead space are there?
What is the normal volume of dead space within the lungs?
- Two types of dead space:
- Anatomical/ serial dead space formed by the conducting airways ( no gas exchange occuring in conducting airway)
- Distributive dead space formed by the alveoli unable to take part in gas exchange either due to disease/ lack of perfusion.
- Total of anatomical and distributive dead space = physiological dead space
- Normally not a lot of physiological dead space ~ 0.15L
Describe the effects of different breathing patterns on the alveolar ventilation rate:
- Alveolar ventilation rate= (TV - Dead space volume) x RR
- Normal lung TV = 0.5L and normal Dead space volume = 0.15L. For normal Respiratory rate of 12 breaths per minute this gives:
- Alveolar ventilation rate= 0.35 x 12 = 4.2 L/ min
- When breathing becomes quick and shallow (hyperventilation) this exaggerates the effects of physiological dead space:
- TV becomes 0.25 L, RR= 24
- Alveolar ventilation rate = 2.4 L/ min
- Deep, slow breathing reduces the effect of dead space volume:
- TV = 1L and RR = 6
- Alveolar ventilation rate= 5.1 L/ min
Describe the effects of increased alveolar ventilation rate on diffusion rate of carbon dioxide
- Increased AVR will lead to increased removal of CO2 from alveoli leading to an increase in the steepness of the concentration gradient for CO2 from venous blood to alveolus.
What would be the effect of no ventilation on alveolar partial pressures and therefore V/Q ratio?
What would be the effect on alveolar partial pressures and V/Q ratio when there is no perfusion?
- No ventilation: No delivery of Oxygen to alveolus and no removal of carbon dioxide would lead to drop in pO2 and increase in pCO2 within the alveolus- dissipating the partial pressure gradient. Venous blood carrying pO2 of 6.0kPa would begin to equilbrate with alveolar O2 until the PO2 concentration becomes 6.0 kPa in the alveolus. Venous blood carrying pCO2 of 6.5kPa would also equilbrate.
- No perfusion: No removal of oxygen or deliver of CO2 to the alveolus. Alveolus continues to be ventilated, pO2 within rises to match atmospheric concentrations due to no removal by perfusion, pC02 decreases to 0 as it continues to be expired with no delivery via venous blood.
Describe the effect of gravity on perfusion
- Perfusion increases from the apex to the base of the lung largely due to the effect of gravity pooling blood in bases where there are more dependent areas of the lung.
- At the apex of the lung above the level of the heart perfusion is close to zero
- In the middle of the lung at the level of the heart perfusion is sporadic, occuring with the contraction of the heart/ systole.
- At the base of the lung (Below level of the heart), perfusion is constant and at its highest
- Sitting or standing will affect perfusion in an upright patient.
Describe the effect of gravity on ventilation and how this relates to ventilation perfusion ratio.
At rest:
- At the apex the lungs are stretched as gravity pulls lung tissue down, meaning in this region all alveoli are open and well ventilated. Intrapleural pressure is more negative here also due to lungs being pulled down from the apex.
- The bases of the lungs tend to be compressed as they contact the diaphragm and intrapleural pressure is not as negative at the base as the apex
However this means during ventilation:
- The apex is already hyperinflated due to effects of gravity and more negative IP pressure, meaning ventilation cannot increase much here.
- The bases of the lungs which are relatively compressed at rest have a much higher potential for expansion, therefore ventilation increases much more at the base compared to the apex of the lung.
- However this increase is not as large as that of perfusion in the base of the lung therefore V/Q ratio decreases as you descend from apex to base.
What is the effect of a significant V/Q mismatch?
Why can the effects of a high V/Q in one area and low V/Q in another area not cancel each other out?
- The lung is unable to transfer gases efficiently which can lead to arterial hypoxaemia.
- High V/Q in one area cannot cancel out low V/Q in another area as :
- High V/Q area has high ventilation and low perfusion and therfore any blood entering capillaries is well saturated but there is a lower volume. (100% saturation)
- Low V/Q area has low ventilation and higher perfusion, but blood in these regions will not be fully saturated. (58% saturation)
- Blood from high V/Q area and blood from low V/Q area will be carried back to the Left side of the heart and mixed, leads to overall reduced saturation.
- Termed shunting: where blood enters the Left side of the heart without properly taking part in gas exchange.
