RBCs are permeable to urea, but does this have any physiological significance? Ideas are explored.
The first significant point to realise is the cell membrane is selectively permeable. In other words some molecules can move freely through it, whilst others can hardly move through it at all.
The red blood cell membrane is pretty much freely permeable to urea, so any urea in the surrounding solution will move into a red blood cell by diffusion.
Imagine some red blood cells in a solution ...
This job focuses on red blood cell membranes.