What part of the kidney adds the urea and nitrates from the food you ate?

The part of the kidney responsible for adding urea and nitrates is primarily the nephron, specifically during the process of filtration in the glomerulus and subsequent tubular secretion in the renal tubules. Urea is formed in the liver from amino acid metabolism and is transported to the kidneys for excretion. Nitrates, which can come from dietary sources, are also filtered through the glomerulus and can be secreted into the tubular fluid. The final composition of urine, including urea and nitrates, is then formed as the filtrate passes through the nephron's various segments.