Secretion from endocrine glands adds hormones. Metabolism reactions contribute waste products. Various mineral ions are lost in sweat. Ammonia is formed when amino groups are removed from amino acids. Urea is formed by reactions in the liver that unite two ammonia molecules with carbon dioxide.
Uric acid is formed in reactions that degrade nucleic acids.
Mammals A. A pair of kidneys, located in the lower abdomen, filter a variety of substances from the blood. The kidneys regulate the volume and solute concentrations of extracellular fluid. B. Kidney Structure 1. The outer coat of connective tissue is the renal capsule. Nephrons filter and retain water and solutes, leaving a concentrated urine to be collected in the central renal pelvis.