The body cannot store excess amino acids. When there are more amino acids than are needed to build proteins, the liver breaks them down in a process called deamination. The part of the amino acid containing nitrogen is removed and converted into urea. The rest of the molecule can be used in respiration to release energy.
Urea is a waste product and is toxic if it builds up. It is released from the liver into the blood, carried to the kidneys, filtered out in the nephrons, and lost from the body in urine. Urea is produced all the time, so the kidneys must work constantly.
Questions on this topic often use rates and graphs. For example, if a person produces 90 cm3 of urine in 60 minutes, the rate is 90 ÷ 60 = 1.5 cm3 per minute. On a graph of the mass of urea excreted per hour, the line shows how the amount of urea lost in urine changes as protein intake changes. A person who eats more protein makes more urea, so the amount excreted rises.