Advanced Dental Admission Test (ADAT)

Which part of the nephron is responsible for isosmotic reabsorption of solutes and water?

1. Distal convoluted tubule

2. Glomerulus

3. Early proximal tubule

4. Collecting duct

The correct answer is: Early proximal tubule

The early proximal tubule plays a crucial role in isosmotic reabsorption of solutes and water within the nephron. During this process, approximately 65-70% of the filtered water and various solutes, including sodium, bicarbonate, chloride, and glucose, are reabsorbed back into the bloodstream. This reabsorption occurs through both active and passive transport mechanisms that enable the movement of solutes and water together, maintaining the osmotic balance. As water is reabsorbed alongside solutes, the osmolarity of the tubular fluid remains relatively unchanged—hence the term "isosmotic." This is significant because it allows for efficient reabsorption without concentrating the tubular fluid unduly at this segment of the nephron, which is critical for protecting metabolic functions and maintaining homeostasis. In contrast, other components of the nephron, such as the distal convoluted tubule and collecting duct, have different roles regarding solute and water reabsorption, often involving selective transport processes and contributing to the concentration of urine. The glomerulus is primarily involved in filtration, not reabsorption, meaning that it does not play a role in isosmotic reabsorption.