Ventilation and Perfusion Flashcards
What are the roles of the lungs?
- Gas exchange
- Reservoir for blood and oxygen
- Metabolism of some circulating compounds
- Filter blood
How is the respiratory system organised?
Conducting airways:
- Anatomical dead space
- Bulk flow
- Trachea
- Bronchi
- Bronchioles
- Terminal bronchioles
Respiratory airways:
- Gas exchange
- Gases move down their individual pressure gradients
- Respiratory bronchioles
- Alveolar ducts
- Alveolar sacs
What are the different airways made up of?
Bronchi: cartilage
Bronchioles: smooth muscle and
What type of blood does the pulmonary artery carry?
Deoxygenated
What type of blood does the pulmonary vein carry?
Oxygenated
What does the distensibility of the pulmonary capillaries allow?
Allows for low resistance throughout the pulmonary circulation allowing blood flow to the lungs to increase without resistance and pressure increasing.
How does the respiratory system regulate blood flow?
Hypoxic vasoconstriction:
- Shunting blood to areas of the lung with high partial pressures of oxygen and away from alveoli with low partial pressures of oxygen.
Define diffusion
The movement of gases across the alveolar membrane
If perfusion is increased? What will happen to the partial pressure of O2 in the alveoli?
Decrease
If ventilation is increased, what will happen to the partial pressure of O2 in the alveoli?
Increase
What is the diffusion barrier made up of?
- Epithelial cell of alveolus
- Tissue fluid and connective tissue
- Endothelial cell of capillary
- Plasma
- Red blood cell membrane
- Red blood cell cytoplasm
Under normal circumstances, what is Hb saturation limited by?
At what point along the pulmonary capillaries are Hb completely saturated with oxygen?
Perfusion limited- perfusion limitation occurs with both oxygen and carbon dioxide under normal circumstances.
25%
In disease states, how may Hb saturation be limited?
Diffusion limited- full equilibration may not occur until the end of capillaries or not at all
What is the effect of disease processes on the oxygen reserve?
Hb may not be able to fully saturated (reach full equilibration) therefore there is just enough to provide tissues with oxygen but not enough to cover increased demand (e.g. exercise)
Define ventilation
Movement of gas in and out of the lungs
What are the 4 lung volumes?
Tidal volume: Volume of air moved in and out of lungs in normal (quiet) breathing
Inspiratory reserve volume: Extra voolume that can be breathed in over that at rest
Expiratory reserve volume: Extra volume that can be breathed out over that at rest
Residual volume: Volume of air left in the lungs after maximum expiration
Lung volumes change with breathing patterns
What are the 4 lung capacities?
Total lung capacity: Inspiratory reserve volume + expiratory reserve volume + tidal volume + residual volume
Inspiratory capacity: Tidal volume + inspiratory reserve volume
Expiratory capacity: Tidal volume + expiratory reserve capacity
Functional residual capacity: Reserve of air left in lungs at the end of passive exhalation (resting expiratory level)
Lung capacities do not change with breathing patterns
Define dead space airways
Areas of the airways not involved in gas exchange
Air here last in and first out: enters and leaves via same airways, does not enter alveoli.
2 types:
- Serial: conducting airways (anatomical dead space)
- Distributive: damaged alveoli or those with poor perfusion
Define total/minute ventilation
What is a normal minute ventilation at rest and in exercise?
The amount of air being moved in and out of the lungs per minute
Normal = 6-8L/min at rest, can reach 80L/min during exercise
MV = tidal volume x respiratory rate
Define the alveolar ventilation rate
How is it calculated?
The amount of air that reaches the alveoli per minute
Need to account for ‘dead space’ (air that sits in conducting airways and does not reach alveoli). 2 types:
- Serial: volume of conducting airways (anatomical dead space)
- Distributive: parts of the lungs that are not conducting airways but cannot take part in gas exchange (damaged alveoli or those with poor perfusion)
Serial + distributive = physiological dead space
AVR = (Vt - Vds) x RR
What effect does shallow breathing have on dead space?
Amplifies the effect of dead space- less air reaches alveoli
What effect does alveolar ventilation rate have on diffusion?
Increasing AVR will remove CO2 from the alveoli, increasing the partial pressure gradient of CO2 between pulmonary capillaries and alveoli, thus increasing its diffusion from the blood into the lungs and its removal from the body.
What is the ideal V/Q (ventilation/perfusion) ratio?
1 (equal)
What does a V/Q ratio less than 1 mean?
What does a V/Q ratio more than 1 mean?
Less than 1 = perfusion > ventilation
More than 1 = ventilation > perfusion
What is the main reason for defective gas exchange in respiratory diseases?
Exacerbated V/Q mismatches:
- Increased V/Q ratio= blood saturated, little O2 added by ventilation
- Decreased V/Q ratio= O2 reduction due to low ventilation compared to perfusion.
How do ventilation and perfusion change from the apex to the base of the lung?
Both increase from the apex to the base of the lung
- Perfusion increases more rapidly therefore V/Q ratio decreases towards the base of the lung
- Increase in blood flow at the base of the lung due to gravity.
- Ventilation increases as the base of the lung is more compressed therefore more room for expansion
What happens to the V/Q ratio in:
Pneumonia
P.E
P.E: Increased V/Q ratio (decreased perfusion)
Pneumonia: Decreased V/Q ratio (decreased ventilation)
Why is there increased ventilation towards the base of the lung?
The base of the lung is more compressed due to gravity therefore has more room for expansion
What happens to apical blood vessels during exercise?
Blood vessels at the apex of the lung which are normally closed open during exercise to accommodate increased cardiac output.
This increases oxygen perfusion at the apex of the lung.
Why can high and low V/Q mismatch areas not cancel eachother out?
Areas with high V/Q ratios cannot add anymore oxygen to blood as it is already saturated with oxygen.
Areas with low V/Q will therefore result in a significant oxygen decrement in mixed capillary blood.
How do V/Q ratios explain the oxygen dissociation curve?
At the top of the curve where blood is fully saturated, there is a high V/Q ratio.
At the bottom of the curve, where blood is poorly saturated, there is a low V/Q ratio.
How can pneumonia produce low oxygen saturations
Well saturated blood from high V/Q ratio areas of the lung mixes with poorly saturated blood from low V/Q ratio areas of the lung to produce overall lower than normal oxygen saturation
Blood from low V/Q area is effectively shunted as it does not pick up oxygen as it passes through the lung.
