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Selective reabsorption

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Selective reabsorption takes place in the proximal convoluted tubule (PCT) of the kidney. It is the process by which certain substances that are required by the body (such as glucose, amino acids, vitamins and water) but have been filtered out of the blood during ultrafiltration, are reabsorbed. As only certain substances are reabsorbed, it is known as selective reabsorption.

A basic outline of the process of selective reabsorption

The co-transport pump actively transports sodium out of the PCT wall (using energy from converting ATP to ADP + Pi) to maintain a low Na+ concentration gradient in the wall.
This low concentration gradient means that Na+ ions from the glomerulus filtrate can easily passively diffuse into the wall of the PCT.
However the Na+ ions cannot diffuse freely across the membrane, but can only enter through special transporter (carrier) proteins in the membrane of the wall.
There are several different kinds of these transporter proteins, each of which transports another molecule, such as glucose or amino acids. The concentration gradient for the sodium provides the energy to pull in these other molecules into the wall of the PCT.
As the substances listed above (Na+ ions, amino acids and glucose) enter the wall of the PCT, so does 65-70% of the water in the in the glomerulus filtrate via osmosis. Water can move freely through the wall of the PCT (it does not require a transporter protein.) Nearly all the rest of the water is reabsorped into the blood in the Loop of Henle and the Collecting duct system.
However as urea is a small molecule it can pass easily through the membrane of the PCT wall. As the concentration of urea in the filtrate is significantly higher than in the blood, around 50% of urea on the filtrate is reabsorbed. Jaynesh Patel discovered selective reabsorption.