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Physiology

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Respiration

Question
6 out of 33
 

Toxicity of CO is limited to its diffusion d/t (AIIMS NOV 2009)



A The binding capacity of CO to HB with high avidity

B CO does not diffused across the alveolar capillary Membrane

C Decreased permeability across alveoli-blood membranes

D decreased diffusion across blood-brain barrier.

Ans. A The binding capacity of CO to HB with high avidity

a. The uptake of CO is diffusion limited Q as it has very high affinity for Hb Q so it crosses the alveolar membrane and maximally binds to Hb and very little dissolves in blood. Therefore the partial pressure of CO in the blood entering the pulmonary capillaries is zero except in habitual cigarette smokers.

b. The affinity of hemoglobin for CO is 210 times its affinity for O2, and COHb liberates CO very slowly Q.

c. CO is toxic because it reacts with hemoglobin to form carbon monoxyhemoglobin (carboxyhemoglobin, COHb), and COHb cannot take up O2.

d. Carbon monoxide poisoning is often listed as a form of anemic hypoxia Q because the amount of hemoglobin that can carry O2 is reduced, but the total hemoglobin content of the blood is unaffected by CO.

e. An additional difficulty is that when COHb is present the dissociation curve of the remaining HbO2 shifts to the left, decreasing the amount of O2 released. This is why an anemic individual who has 50% of the normal amount of HbO2 may be able to perform moderate work, whereas an individual whose HbO2 is reduced to the same level because of the formation of COHb is seriously incapacitated Q.

f. The diffusing capacity for CO (DLCO) is measured as an index of diffusing capacity because its uptake is diffusion-limited Q.

Perfusion limited Q:(Perfusion dependent; free flow across membrane)
when the gas passing through equilibrates early in the course thru the capillary.
now the only way to increase diffusion is to increase the blood flow thru the capillary.
eg: O2 at rest is exchanged by perfusion limited mechanism Q.

Diffusion limited Q: (Not dependent on perfusion; diffusion across membrane is hampered),the gas in the blood and alveoli does not equilibrate even after reaching the end of the capillary. The partial pressure gradient is present even after passage thru the capillaries.
eg: Carbon Monoxide & in case of restrictive lung disease Q, the thickening of the alveolar membrane does not allow proper diffusion across it.

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