Describe the three main processes of urine formation and explain how water intake affects urine concentration.

Urine formation involves three main processes: glomerular filtration, where blood is filtered in the glomerulus; selective reabsorption, where useful substances like water and salts are returned to the blood from the renal tubules; and tubular secretion, where additional wastes are added to the filtrate.

When Mary drinks a lot of water, less water is selectively reabsorbed from the filtrate due to lower levels of antidiuretic hormone (ADH), leading to a large volume of dilute urine. Conversely, with salty food and little water, more ADH is released, increasing water reabsorption to conserve body water, resulting in a small volume of dark, concentrated urine.