Respiratory System: Gas Exchange Flashcards
Describe and explain this diagram, label it as well
The rate of diffusion is determined by…
partial pressure gradient, the size of the diffusion distance, surface area.
Describe the role of ventilation in determining gas exchange
Changing metabolic demands (exercise, injury, infection) change levels of gas exchange.
This is done by changing rate of alveolar ventilation (volume of fresh air reaching alveoli per time), in order to modulate partial pa gradients between the alveoli and blood.
Why do ventilation and perfusion need to be closley matched? What is V/Q mismatch?
During gas exchange, u need sufficient Hb binding sites to absorb 02 arriving at the alveoli. Therefore ventilation (02 supply) and perfusion (blood supply) need to be closely matched, w a ratio close to 1.
V/Q Mismatch reduces gas exchange and decreases oxygenation of blood, leading to hypoxaemia.
What is the dead space effect?
If perfusion is reduced relative to ventilation, V/Q ratio increases >1.
Affected lung regions=’physiologic dead-space’ as they don’t participate in gas exchange, despite 02 presence
Causes: reduced blood supply to the lung (e.g. pulmonary embolism, damage/blockade of blood vessels).
Sometimes a pulmonary embolism may not decrease overall lung perfusion. Why is this?
In pulmonary embolism (a blocked lung artery), overall lung perfusion won’t decrease if blood diverts to other pulmonary arteries/capillaries.
Increased ventilation of these compensate for gas exchange reduction in other lung areas. Otherwise, hypoxaemia + hypercapnia (increased PaCO2) occur.
What is pulmonary shunt?
Pulmonary shunt is when V/Q ratio decreases due to hypoventilation of alveoli (eg from airway obstruction).
Deoxygenated blood returns to the heart from the lungs sin gas exchange, so blood is ‘shunted’.
Cardiac shunts, pneumonia, acute lung injury, RDS and atelectasis have severe pulmonary shunt.
Describe hypoxaemia caused by shunt
In hypoxaemia caused by shunt: even oxygenated blood from well-ventilated alveoli will eventually mix w deoxygenated blood returning from shunted areas, reducing overall PaO2.
Administering supplemental 02 cannot increase 02 sat in well-ventilated lung regions enough to compensate for eventual mixing w deoxygenated blood.
What is hypoxic vasoconstriction?
Occurs when ventilation to an alveoli is reduced; this causes rising CO2 and falling O2 levels.
This contracts the vsm within nearby capillaries. This blood vessel constriction diverts blood to other capillaries that innervate better-ventilated alveoli.
What is the disadvantage of hypoxic vasoconstriction?
In certain lung disease it can become pathological. Eg, in COPD, the chronic hypoventilation leads to prolonged and widespread pulmonary vasoconstriction.
This increases resistance within pulmonary vasculature, causing pulmonary hypertension. This can lead to right heart hypertrophy and eventually right heart failure.
Here are a list of factors that affect gas exchange. For each, give pathological states affecting each factor
How can you measure airway function by spirometry?
Patients produce a max forced expiration into a spirometer, which measures the volume of air passing through over time. This is then plotted on a graph.
FEV1 (forced expiratory volume in 1s): max expired volume in the 1st second of a max forced expiration. Shows how quickly air can pass through the airways.
FVC (forced vital capacity): the maximum volume an individual can exhale in one breath.
How do spirometry values change depending on restrictive or obstructive disease?
In obstructive airway diseases eg asthma + chronic bronchitis, FEV1/FVC ratio reduces <70%. FVC= unchanged as lung function is unaffected.
In restrictive lung diseases, eg pulmonary fibrosis, FEV1 and FVC reduces (<80%). FEV1/FVC ratio is normal >70%.
This is bc the decrease in FEV1 reflects an overall decrease in lung volume rather than airway obstruction.
COPD may be obstructive and restrictive.
