Gas Exchange in the Lungs Flashcards
where does gas exchange take place?
within the alveoli forming air-blood barrier
specialisation of gas exchange system
large surface area
thin outer structure
richly innervated with capillaries
what is the pathway of oxygen into blood?
atmosphere –> alveolar airspace –> alveolar lining fluid –> through epithelium, BM, endothelial cells –> blood plasma –> Hb
how long for RBC to pass through pulmonary capillary
- 75 seconds
0. 25 in exercise
what determines rate of diffusion?
1- partial pressure gradient between two areas
2- size of diffusion distance
3- SA
factors for max diffusion?
- high pp gradient
- high SA
- low distance
what decreases SA
emphysema
what increases diffusion distance
fibrosis increases thickness of membrane and pulmonary oedema increases thickness of fluid
what decreases PA
hypoventilation (type II resp failure)
what decreases PC?
hypoperfusion (type I respiratory failure)
what is hypoventilation?
insufficient levels of breathing = excessive CO2 (pa < 4.9 kPa) and reduced O2
what is hyperventilation?
excessive levels of breathing = reduced CO2 (pa < 6.0 pKa) and increased O2
what does increasing rate of ventilation do?
increases alveolar oxygen partial pressure and decreases alveolar carbon dioxide pp
What affect does hypoventilation and hyperventilation have on PaCO2?
- Ventilation increase will decrease PACO2
- So hyperventilation will decrease it because constantly exchanging it with fresh atmospheric so less CO2 left in alveoli
- Hypoventilation will increase it
what is perfusion?
supply of blood
what should the ventilation/perfusion ratio be?
close to 1
what happens if V/Q > 1 (perfusion reduced relative to ventilation)?
inspired oxygen is wasted and not participating in gas exchange as blood supply to lungs reduced
what is physiologic dead space?
parts of lungs not participating in gas exchange despite presence of O2
what is a pulmonary embolism?
block of artery in lungs
what happens in pulmonary embolism?
overall perfusion of lungs may not decrease as blood diverted to other pulmonary arteries and capillaries
how is hypoxaemia and hypercapnia avoided?
ventilation of perfused alveoli increases
What is the V/Q ratio?
Describes the relationship between perfusion (Q) and alveolar ventilation (V) which both needs to match to enable efficient gas exchange –> as there’s a max amount of 02 each unit of blood can carry
V/Q less than 1
hypoperfusion (dead space effect)
V/Q more than 1
hypoventilation (shunt)
How is V/Q coupling maintained?
where it’s well ventilated there will be more perfusion/blood flow, if not then it will be diverted away –>this is to match ventilation and perfusion (so V/Q is close to 1)
more depth on V/Q maintenance
- By hypoxic vasoconstriction of capillaries (this is a homeostatic mechanism) –> this diverts blood flow from poor to well ventilated alveoli
- If ventilation of an alveoli decreases –> PACO2 will rise and PAO2 will fall –> this will reduce level of oxygen in pulmonary capillary as pressure gradient less steep–> starts to cause vasoconstriction because smooth muscle and tissues sensitive to lack of oxygen –>constriction increases resistance –>divert blood (because one capillary will have more
what is pulmonary shunt?
reduced ventilation of alveoli so decreased V/Q. Deoxygenated blood returns to left side of heart from right without gas exchange happening
what conditions are associated with pulmonary shunt?
cardiac shunts, pneumonia, acute lung injury, RDS and atelectasis
how does V/Q change with the shunt effect?
- V/Q ratio will decrease because Q remains same and V decreases –> response to oxygen therapy is poor because you can’t compensate for decrease of oxygen within regions affected by shunt by increasing ventilation to the regions that weren’t –> as blood can only carry a certain amount of oxygen (the extra oxygen you will add to regions not affected by shunt will have blood with slightly more oxygen but when mixed with the blood of the shunted under perfused alveoli blood –> it is not enough to compensate
what is hypoxic vasoconstriction?
ventilation to alveoli reduced, decreased ventilation results in rising CO2 and falling O2 so vascular smooth muscle contracts
what does the constriction of the blood vessels do?
diverts blood to other capillaries that innervate better ventilated alveoli
what does hypoxic vasoconstriction do?
protective mechanism to ensure efficient V-Q coupling
When does hypoxic vasoconstriction become pathological?
COPD, prolonged widespread pulmonary vasoconstriction. Increases resistance in pulmonary vasculature leading to pulmonary hypertension, right heart hypertrophy and right heart failure