Cl- is pumped into cells from the hindgut lumen; K+ follows through potassium channels along an electrochemical gradient and water follows via osmosis.
In the basolateral membrane, Na+/K+-ATPase sets up gradients that favor movement of Cl-, K+, and H2O into the hemolymph.
– Water is not pumped directly—it moves only by osmosis via
osmotic gradients set up by active transport of ions.
– The formation of the filtrate is not particularly selective.
– Unlike filtrate formation, reabsorption is highly selective for certain molecules and ions.
– Also in contrast to filtrate formation, reabsorption is also tightly regulated in response to osmotic stress.
– Water cannot be transported actively—it only moves by osmosis.
– To move water, cells in the kidney set up strong osmotic gradients.
– By regulating these gradients and specific channel proteins, kidney cells exert precise control over loss or retention of water and electrolytes.
1. Na+/K+-ATPase in the basolateral membranes removes intracellular Na+, creating a gradient for Na+ entry from the lumen.
2. In the apical membrane, Na+-dependent cotransporters use the gradient to remove valuable ions and nutrients selectively from the filtrate.
3. The solutes that move into the cell diffuse across the basolateral membrane into nearby blood vessels.
4. Water follows ions from the proximal tubule into the cell and then into the blood vessels.
1. ADH triggers the insertion of aquaporins into the apical membrane. As a result, cells become much more permeable to water and large amounts of water are reabsorbed.
2. ADH increases permeability to urea, which increases the osmolarity of the surrounding fluid and thus water loss from the filtrate.