AEMCA Practice Exam

During reabsorption within the kidneys, which substance is primarily reabsorbed from the renal tubule?

1. Urea

2. Water

3. Cretinine

4. Hydrogen ions

The correct answer is: Water

In the kidneys, a significant process called reabsorption occurs in the renal tubules, where essential substances and water are taken back into the bloodstream after filtration. Water is predominantly reabsorbed throughout the nephron, particularly in the proximal convoluted tubule and, to a greater extent, in the loop of Henle and collecting ducts. The primary mechanism for water reabsorption is osmosis, driven by the concentration gradients established by solutes being reabsorbed, such as sodium. This process is crucial for maintaining the body’s fluid balance and blood pressure. The kidneys adjust the volume of water reabsorbed based on the body's hydration status and hormonal influences, particularly from antidiuretic hormone (ADH), which increases water reabsorption in response to dehydration. Other substances mentioned, such as urea, creatinine, and hydrogen ions, are not reabsorbed to the same extent. Urea undergoes some passive reabsorption, but it is not the primary substance reabsorbed. Creatinine is typically not reabsorbed and is instead excreted as a waste product. Hydrogen ions may be secreted depending on the body's acid-base balance but are not primarily reabsorbed. Thus, water stands out as the principal