Osmose

Osmosis is the movement of a solvent, such as water, from a lower to a higher solute concentration across a semipermeable membrane.

The number of solute particles per liter of a solution is called osmolarity. It determines the osmotic pressure— the minimum pressure required to prevent osmosis.

In a cell, if the osmolarity inside is higher than outside, water continues to move into the cell until both sides attain equal solute concentration and osmotic pressure becomes zero.

Usually, the polar water molecules diffuse slowly across the lipid bilayer. For rapid osmosis, water-specific channel proteins called aquaporins selectively transport water molecules according to the osmotic gradient.

In kidneys, the renal tubules reabsorb water and solutes from the plasma filtrate via osmosis and return them to the bloodstream.

The filtrate from the tubule lumen that enters tubule wall cells contain high ion concentrations.

As the ions are actively transported out of the tubule cells, their osmolarity within the cell becomes lower than in the surrounding capillaries. To balance, water molecules move through the aquaporins into the bloodstream.