Consider a simplified metabolic model to describe nitrogen metabolism in the bod

Question

Consider a simplified metabolic model to describe nitrogen metabolism in the body. Nitrogen is ingested in the food (F), a fraction of which is actually absorbed (A), the remainder being lost in the feces (W). The absorbed nitrogen is taken up by the muscle to make protein. At small fraction of the stored muscle protein undergoes degradation and the liberated nitrogen (L) is released from the muscle. Of the liberated nitrogen, a fraction is recycled back to the muscle (R), while the rest is lost to excretion (E) in the urine. Gut Muscle F W A E R a) If the nitrogen ingested in food is 100 g/day, but only 80% of the food is actually absorbed, while the rest goes to waste, what is the amount of nitrogen lost by excretion? Assume steady-state. b) Of the nitrogen liberated from muscle, 90% is excreted, while the rest is recycled. What fraction of the nitrogen entering the muscle is from the absorbed food? (In other words, what is the ratio A/(A+R)). Again, assume steady-state

Explanation / Answer

80% of 100gm of nitrogen is absorbed = 80g

So, 20% is waste =20gm

Waste is in the form of ammonia (max) = NH4

So fraction of N in ammonia = 14/18

And hence the amount of N wated = 14/18*20 = 15.5gm

b. Of the nitrogen liberated from muscle, 90% excreted. while rest is recycled

So 90% of 80gm (Absorbed nitrogen) = 72gm

10% is recycled, so the recycled N = 8 gm

So, the fraction of N entering the muscle from the absorbed food = A/(A+R)). = 8/80 = 1/10

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