Final answer:
BUN stands for blood urea nitrogen, a waste product synthesized by the liver and excreted by the kidneys in urine.
Step-by-step explanation:
BUN, which stands for blood urea nitrogen, is an estimate of urea in the blood and serves as an indicator of kidney function. Urea is synthesized in the liver from ammonia, which is produced when amino acids are broken down. The urea is then transported to the kidneys, where it is excreted in the urine.
During states such as fasting, starvation, or low carbohydrate diets, the liver also performs a process called gluconeogenesis, where it synthesizes new glucose molecules from non-carbohydrate sources to maintain blood glucose levels.
This additional role showcases the liver's function in both digestion and excretion, including the breakdown of waste products like ammonia into urea. The kidneys then filter this urea out of the blood and form urine.
Therefore, BUN is a byproduct synthesized by the liver as it converts toxic ammonia into less harmful urea, which the kidneys then excrete.