Lecture 13: Pulmonary Shunts, Physiological Dead Space, LaPlace's Law (Exam III) Flashcards
What are the values of all four Starling pulmonary capillary forces?
PCAP: 7 mmHg
πCAP: 28 mmHg
PISF: -8 mmHg
πISF: 14 mmHg
What contributes to the negative interstitial capillary hydrostatic pressure?
The -8 mmHg comes from the intrathoracic pressure of -4 mmHg along with the actions of the lymphatic system acting as a small vacuum pump.
In Alveolus B there is adequate perfusion, but no ventilation. This mimics a ____________ shut.
Right-to-left shunt
A circulatory shunt is going from the right side of the heart to the left side of the heart. And in the process, the blood is not becoming oxygenated. So that would basically mimic a right-to-left shunt.
In Alveolus C, we have fresh air coming in, but it’s not participating in gas exchange. This will be known as ___________.
Alveolar Dead Space / Physiological Dead Space
What is the formula for Total Dead Space?
VTotalDS = VAnatomicDS + VAlveolarDS
What would be the alveolar dead space volume (VADS) for a healthy 20-year-old?
0 mmHg
Alveolar Deadspace Volume will increase after 20.
Alveolar and anatomical dead space share some similarities in that expired air coming from either of these places should have a similar composition as to __________.
What would expired PAO2 and PCO2 from alveolar dead space be equal to?
inspired air
Expired PAO2 = 150 mmHg
Expired PA CO2 = 0 mmHg
During the expiration portion of a normal tidal volume what components of the VT is expired first?
150 mL anatomical DS air
During the inspiration portion of a normal tidal volume the first amount of air inspired will go to the _______.
Alveoli
As expiration moves forward after the first 150 mL, the next 350 mL of air will contain ________(more/less) CO2 and (more/less) O2.
More CO2
Less O2
Last 350 mL will come from area in the lungs involved with gas exchange.
What happens if there is simultaneous emptying of both good alveoli and alveoli with dead space?
The gas coming from the alveolar dead space is going to typically dilute out the CO2 that’s going to be in the healthy parts of the lung as well as increased the O2 that’s coming out of the lung.
Good Expired Alveolar Air:
PAO2 =
PACO2 =
Good Expired Alveolar Air:
PAO2 = 100 mmHg
PACO2= 40 mmHg
Last 350 mL of expired breath.
What does it mean if our EtCO₂ is less than 40 mmHg?
The patient is either getting too much free air or there may be some alveolar dead space.
What is a good rule of thumb for just estimating somebody’s anatomical dead space?
When will this rule not work so well?
- 1mL of anatomical dead space for 1 pound bodyweight.
- If someone is incredibly overweight, it will be a gross overestimation of anatomical dead space.
Are pulmonary shunts or dead space easier to fix?
Alveolar dead space is easier to fix. Just put more fresh air into the lungs.
How will the body naturally compensate for a pulmonary shunt (Alveolus B)?
Hypoxic Pulmonary Vasoconstriction
If P1 and P2 had the same amount of surfactant, which alveolus will be more compliant and have the lower surface tension? (Ignore the blue arrow)
P1 will be more compliant and have less surface tension due to a higher concentration of surfactant which breaks more surface tension in the smaller alveolus.
Surfactant guards against uneven ventilation.
Demonstrating Laplace’s Law:
Which sphere will have a higher internal pressure?
P1
What is the relationship between wall tension and radius?
It is an inversely proportional relationship. Wall tension will decrease as the radius increases.
Laplace’s law states that the pressure inside an inflated elastic container with a curved surface (alveoli) is _________ proportional to the radius as long as the surface tension is constant and containers are filled to a reasonable size.
inversely
