Final answer:
During diuresis, osmolarity decreases in the PT, then increases in the DL, decreases in the AL, and continues to decrease in both DT and CD. In antidiuresis, osmolarity increases in the PT, decreases in the DL and AL, remains the same in the DT, and increases in the CD.
Step-by-step explanation:
The question pertains to the changes in osmolarity in different parts of the nephron during diuresis and antidiuresis. Osmolarity is the measure of solute concentration per unit of solution, and in the context of the nephron, it describes the concentration of urine as it is formed. In diuresis (increased urine production), the osmolarity changes are as follows:
- In the proximal tubule (PT), osmolarity goes down because of the isotonic reabsorption of water and solutes.
- In the descending limb (DL) of the loop of Henle, osmolarity goes up due to water leaving the permeable descending limb, concentrating the filtrate.
- In the ascending limb (AL), osmolarity goes down as salts are actively transported out, but water cannot leave.
- In the distal tubule (DT) and collecting duct (CD), osmolarity goes further down if there is limited water reabsorption, as seen during diuresis.
During antidiuresis (decreased urine production), the osmolarity changes are somewhat opposite:
- In the proximal tubule, osmolarity goes up, as less water is reabsorbed relative to solutes.
- In the descending limb, osmolarity still goes down as water leaves the filtrate.
- In the ascending limb, osmolarity down because of active salt transport.
- In the distal tubule, osmolarity remains relatively the same, but water reabsorption increases in the collecting duct, which leads to an increase in osmolarity, creating concentrated urine.
Given this understanding, the series of osmolarity changes for diuresis is down, up, down, down, down, and for antidiuresis, it is up, down, down, down, up. Therefore, the correct answer is option b.