Final answer:
For passive reabsorption in renal tubules, the solute must permeate tubule epithelial cells' membranes and be in higher concentration in the tubule fluid than plasma, moving down its concentration gradient.
Step-by-step explanation:
For a solute to be passively reabsorbed from the renal tubules, the following condition must exist: The solute must be able to permeate the membrane of the tubule epithelial cells, and the solute must be in greater concentration in the tubule fluid than plasma. This creates a concentration gradient that facilitates diffusion across the membrane without the expenditure of energy.
Passive reabsorption in the kidneys relies on the principle that substances move from an area of higher concentration to an area of lower concentration—down their concentration gradient. This allows the kidneys to reclaim water, electrolytes, and other substances essential for homeostasis. In the context of the renal tubules, especially the proximal convoluted tubule (PCT), sodium ions and water are significant examples of solutes that are reabsorbed passively due to gradient differences. Water can independently move through aquaporins due to its concentration gradient while sodium is actively transported. Moreover, passive reabsorption is not just a function of concentration gradients but also involves the permeability of the membranes, supported by transport proteins like channels and transporters.