Respiration II Flashcards
What is the total ventilation?
The volume of air moved in and out of the lungs PER UNIT OF TIME
What is the total ventilation at normal resting conditions?
Why?
6 litres/min
Total ventilation = tidal volume x breathing frequency
At resting conditions:
- Tidal volume = 0.5
- Breathing frequency = 12
What is anatomical dead space?
What part of the respiratory system is this?
The space NOT INVOLVED in gas exchange
Conducting zone
How much of the total volume is in the conducting zone?
30%
~0.15litres
How big is the average breath?
How much of the breath is from the conducting zone?
500ml
150ml
Why does the conducting air zone impact on gas exchange?
- Not the same composition as ‘fresh’ air
- Impact on the composition of the air in the alveoli
What is ‘alveolar ventilation’?
How is it calculated?
The volume of ‘fresh’ air reaching the respiratory zone
Total ventilation - dead space ventilation
What is the alveolar ventilation?
4.3litres/min
6litres/min - (0.15x12)
What does ventilation rate impact on?
Alveolar and arterial gas COMPOSITION
How much CO2 does the body make per minute?
200ml
At steady breathing rate, what is the volume of the alveoli?
What does this cause the partial pressure in the alveolar to be?
Volume - 4.2L
Partial pressure 40mmHg
What happens if double ventilation rate?
- Body still makes 200ml of CO2 but in a 8.4L
- Partial pressure of the alveoli goes DOWN
- CO2 content of the blood DECREASES
- Alkalosis
What happens if half ventilation rate?
- Body still makes 200ml of CO2 but in a 2.1L
- Partial pressure of the alveoli goes UP
- CO2 content of the blood goes UP
- CO2 content of the blood INCREASES
- Acidosis
How does the lung ventilation vary?
Why does it vary?
Varies according to the position in the lung:
At the base = higher ventilation
Due to:
- Posture
- Gravity
- Starting volume of the alveoli
What are the changes in the intrapleural pressure in the lungs?
Why?
Apex:
- More NEGATIVE
- Lungs PULLING down
Base:
- More POSITIVE
- Lungs PUSHING down
Describe the properties of the apex of the lung
- HIGH transpulmonary pressure
- Alveoli have a HIGH starting volume (large alveoli)
- DILUTED surfactant
- LOW compliance
- HARDER to expand alveoli
- Lower ventilation
- Lower perfusion
- Higher v/p ratio
Describe the properties of the base of the lung
- Alveoli have a SMALL starting volume (small alveoli)
- HIGH LEVELS of surfactant
- HIGH compliance
- EASIER to expand alveoli
- Higher ventilation
- Higher perfusion
- Lower v/p ratio
What does LOW compliance mean?
SMALL changes in pressure - SMALL changes in volume
What does HIGH compliance mean?
SMALL changes in pressure - LARGE changes in volume
What is perfusion?
BLOOD FLOW to the lung
Does the pulmonary circulation operate at higher or lower pressures than than the systemic circulation?
LOWER
What are alveolar vessels?
Capillaries and slightly larger vessels that are SURROUNDED on all sides by the alveoli (travel THROUGH the alveoli)
What is the resistance in the alveolar vessels linked to?
TransMURAL pressure of the vessels and LUNG VOLUME
What is transmural pressure?
Difference in pressure between 2 sides of the wall or equivalent boundary
What does the resistance of the alveolar vessels increase?
How?
When INSPIRE:
1) Alveoli EXPAND more
- Slightly COLLAPSES the blood vessels and reduces the DIAMETER of the vessels
2) Also STRETCHES the vessels, reducing the diameter
What are the extra-alveolar vessels?
Vessels that are NOT surrounded by the alveoli
What is the resistance in the extra-alveloar vessels linked to?
IntraPLEURAL pressure
What happens to the resistance in the extra-alveolar vessels when breath in?
Intrapleural pressure becomes MORE NEGATIVE
- Transmural pressure of the vessels INCREASE
- Vessels EXPAND
What happens to the resistance in the extra-alveolar vessels when breath out?
Intrapleural pressure becomes MORE POSITIVE
- Transmural pressure of the vessels DECREASE
- Vessels are RESTRICTED
What is the total pulmonary resistance the sum of?
Alveolar and extra-alveolar resistances
When is the total pulmonary resistance at the lowest?
When the lungs are at functional residual capacity:
- Alveoli aren’t expanded (pressure of 0)
- Pressure in the intrapleural space is subatmospheric (Expansion of the chest wall balances the collapse of the lung)
What does a positive transmural pressure cause?
The airways to expand - inspire
What does a negative transmural pressure cause?
The airways to constrict - expire
How can the pulmonary circulation accommodate increases in pulmonary pressure and flow rate?
What does this do?
By RECRUITMENT of more and more capillaries:
Reduces system resistance
What does the recruitment of capillaries in the pulmonary system depend upon?
The pressure head forcing the blood through the vessels
What is the reference point for pulmonary circulatory pressures?
Outside of the heart, level with the left atrium
What is Ppa?
What is the mean value?
Pressure in the pulmonary ARTERIOLES
15mmHg (20 cmH2O)
What is Pva?
What is the mean value?
Pressure in the pulmonary VENULES
8mmHg (10 cmH2O)
What is Pva or Ppa affected by?
How?
Height above or below the lungs:
- FALL by 1cm H2O for every 1cm ABOVE the left atrium
- INCREASE by 1cm for every 1cm BELOW the left atrium
What is Pa?
Alveolar pressure (relative to the atmosphere)
When is Pa 0? (relative to the atmosphere)
When there is no difference between atmospheric an alveolar pressure
(No movement of air)
What happens to the flow rate through the vessels as drop down through the lung?
Why?
Increases as get to the level of the heart
- Increase in transmural pressure - eventually becomes positive
- Expansion of the blood vessels
- Higher perfusion rate
What does a positive transmural pressure mean?
The pressure in the alveoli is GREATER than that in the intapleural space
What happens to the resistance in flow in the lungs below the heart level? (zone 4)
Why?
Increased resistance
Even though the transmural pressure expands the capillaries:
- Interplural pressure causes contraction of the alveolar vessels
What is intraplural pressure normally?
Less than atmospheric
What happens if the intrapleural pressure becomes positive?
Lung collapse will occur
What 3 things alters perfusion?
- Alveolar CO2 level
- Alveolar O2 level
- Interstitial pH
What is hypoxia?
What does it cause?
Decreased O2 in the alveoli or increases CO2 in a region
Causes:
- Constriction of the blood vessels (opposite to the systemic circuit)
- Acidification (increased CO2 - fall in pH)
What is the ventilation/perfusion ratio?
Ratio of the RATE of alveolar ventilation and pulmonary blood flow
Ratio = ventilation/perfusion
What does the ventilation/perfusion ratio influence?
Gas composition
What happens if the alveoli are not ventilated?
How does this affect gas composition in the alveoli?
- Ventilation is 0
- Ratio is 0
Gas composition will reach EQUILIBRIUM with MIXED VENOUS BLOOD
- 40 mmHg O2
- 46 mmHg CO2
What happens if the alveoli are not perfused?
How does this affect gas composition in the alveoli?
- Perfusion is 0
- Ratio is infinite
Gas composition will reach EQUILIBRIUM with INSPIRED HUMIDIFIED AIR
- 149 mmHg O2
- OmmHg Co2
What is the composition of the air at the apex of the lungs?
Closer to the composition of the atmospheric air
What is the composition of the air at the base of the lungs?
Closer to the composition of the mixed venous blood
What makes the final composition of the blood that goes into the systemic circulation?
The blood from both the APEX and the BASE
What should the anatomical and physiological dead space be?
The SAME
What happens if there is an increase in the physiological dead space?
What can cause this?
- Part of the lungs aren’t being perfused properly
- Ratio to infinity
- Gas composition becomes the same as inspired air
Caused by pulmonary embolism
What is the compensation mechanism for a reduction in perfusion due to a pulmonary embolism?
- Blood REDIRECTED to other areas
- Bronchiolar CONSTRICTION (redivert air)
Reduction is SURFACTANT production:
- Reduction in COMPLIANCE
- Harder to inflate alveoli
Why is surfactant production decreased when the lungs are poorly perfused?
Minerals are needed to make surfactant
These minerals are carried in the blood
What is a pulmonary shunt?
What can cause this?
- Local reduction in VENTILATION (but the perfusion remains the same)
- Composition of gas becomes the same as mixed venous blood
- Ratio becomes 0
Caused by foregin body in the airways
What is the compensation mechanism for pulmonary shunt?
- Air directed to other parts of the lungs
- Reduced O2 in TRAPPED air
- Pulmonary VASOCONSTRICTION
