Key Takeaways

Blood in the upper gastrointestinal (GI) tract is degraded to its constituent parts, including hemoglobin-derived amino acids. These amino acids are absorbed by the intestinal epithelial cells and transported to the liver.

In the liver amino acids may be used to synthesize new protein (anabolism) or they may undergo catabolism/degradation to generate energy and/or to synthesize glucose and fatty acids.

Amino acid catabolism/degradation results in the release of ammonia (NH3), which is then converted to urea by the urea cycle (the latter being confined to the liver).

The urea is released into the circulation and filtered by the kidney.

Blood urea concentration is dependent on renal function and on non-renal factors including the amount of protein ingested. Upper GI bleeding is akin to a blood meal, presenting a bolus of amino acids to the liver.

By contrast, creatinine concentration, another marker of renal function, reflects muscle mass and is not influenced by dietary protein intake (or, by extension, blood in the gastrointestinal tract).

As a result, the BUN to creatinine ratio is increased in upper GI bleeding.

Blood has low biological value (poor nutritional availability for use by the body).

Proteins of low biological value give rise to higher ammonia and urea production compared with balanced proteins. As a result, blood in the stomach (e.g., from a bleeding peptic ulcer) will cause a greater increase in the BUN compared with an isonitrogenous protein meal. 

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