Gas Exchange In The Lung Flashcards
How is partial pressure calculated?
Multiplying total pressure by mole fraction
What is concentration of a gas dissolved within a liquid determined by?
Partial pressure and solubility of a gas
What does partial pressure of a gas dissolved in a liquid mean?
The amount of gas that would dissolve at equilibrium if the liquid was placed in contact with a gas phase of equivalent partial pressure
What are the 5 steps in gas exchange?
1- o2 enters the alveolar airspace from the atmosphere
2- o2 dissolves in the alveolar lining fluid
3- o2 diffuses rough alveolar epithelium, basement membrane and capillary endothelial cells
4- o2 dissolves in blood plasma
5- o2 binds to Hb molecule
What is gas exchange?
Diffusion of gases between air and blood
What determines rate of diffusion in the lungs?
Alveolar surface area
Partial pressure gradient between alveolar air and capillary blood
Epithelial and and endothelial cell thickness and basement membrane thickness and fluid layer depth
What features do you want for maximum diffusion?
Inc partial pressure gradient
Inc surface area
Dec distance
What happens if you have a low partial pressure gradient in the lungs?
Hypoventilation (type 2 respiratory failure)
What happens if you have a decreased surface area in the alveoli?
Emphysema
What is an increased basement membrane thickness called?
Fibrosis
What is VQ coupling?
Matching of ventilation to perfusion
What is the ideal VQ ratio?
1
What happens if the VQ ratio is more than 1?
Hypoperfusion (dead space effect)
What happens if the VQ ratio is less than 1?
Hypoventilation (shunt)
What is VQ coupling maintained by?
Hypoxic vasoconstriction
What is hypoxic vasoconstriction?
Homeostatic mechanisms exist to reduce ventilation so there isn’t perfusion mismatching
Hypoxic vasoconstriction of capillary diverts blood flow from poor to well ventilated alveoli
What happens if the ventilation of specific alveoli decreases?
Partial pressure of CO2 will rise and O2 will fall
Leads to decreased oxygenation of blood flowing through innervating capillaries
What happens when ventilation and perfusion to alveolar units are not matched?
Gas exchange is reduced
What happens in most cases when CO2 partial pressure increases?
Reflex hyperventilation that clears the excess CO2 but doesnt increase o2 partial pressures
What happens when an embolism blocks the pulmonary artery supplying a region of the lung?
Perfusion to the other alveoli increases as cardiac output is diverted
Unless ventilation of alveoli is increased, hypoxia and hypercapnia will occur
What causes reduced perfusion to the lungs?
Heart failure
Blocked vessels
Loss/damage to capillaries
What is physiologic dead space?
Ventilates alveoli that aren’t perfused
Does oxygen help with the dead space effect?
Yes
What is a shunt in the lungs?
Perfusion without ventilation
What causes a shunt In the lungs?
Cardiac shunts
Pneumonia, acute lung injury, RDS, atelactasis
What does reduced ventilation do to the VQ ratio?
Decrease it
Does O2 therapy help when there’s a lung shunt?
Not really
