Which of the following is not true about the anatomy of the urinary system?

A. Urine concentration and volume are determined by countercurrent mechanisms: The countercurrent multiplier (nephron loop) and countercurrent exchanger (vasa recta) help establish a medullary osmotic gradient that allows for urine concentration or dilution.
B. The kidneys produce a large volume of dilute urine when overhydrated: In overhydration, ADH secretion is suppressed, leading to reduced water reabsorption and a larger volume of dilute urine.
C. The concentration of urine is lower when urine volume is reduced: This is incorrect. When urine volume is reduced (e.g., dehydration), the urine is more concentrated, not less.
D. The kidneys produce a small volume of concentrated urine when dehydrated: In dehydration, ADH is secreted, causing increased water reabsorption, resulting in concentrated urine with low volume.

A. Potassium ion concentration in extracellular fluid: High levels of extracellular potassium stimulate aldosterone secretion and enhance potassium secretion into the renal tubules for excretion. This is the most direct and critical factor.
B. Intracellular sodium levels: While sodium levels affect membrane potentials, they do not directly regulate potassium secretion.
C. The pH of the intracellular fluid: pH changes can influence potassium distribution between compartments but are not the main driver of secretion.
D. The potassium ion content in the renal tubule cells: This reflects storage, not the signal for secretion. The extracellular fluid concentration is what regulates the process.