What substance does the body primarily excrete nitrogen through in critically ill patients?

The body primarily excretes nitrogen through urine, especially in critically ill patients. This is due to the breakdown of proteins and amino acids, which releases nitrogen as waste in the form of urea. When the body metabolizes protein, the amino groups (−NH2) are converted into ammonia, which is then converted into urea in the liver. Urea is soluble in water and is excreted in the urine by the kidneys.

In critically ill patients, metabolic processes may be altered due to increased protein catabolism to meet energy demands or to repair tissues, making the excretion of nitrogen and urea through urine more pronounced. Other methods of excretion, such as through sweat or saliva, play a negligible role in the removal of nitrogen when compared to urine. The significant volume of urine produced helps the body maintain homeostasis while effectively eliminating nitrogen waste.