<Ul> <Li> The co-transport sodium - potassium pump actively transports sodium out of the PCT (proximal convoluted tubule) wall (using energy from converting ATP to ADP + Pi) to maintain a low Na concentration gradient in the wall . </Li> <Li> This low concentration gradient means that Na ions from the glomerulus filtrate can easily passively diffuse into the wall of the PCT . </Li> <Li> 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 . </Li> <Li> 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 . </Li> <Li> 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 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 reabsorbed into the blood in the loop of Henle and the collecting duct system . </Li> <Li> 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 . </Li> </Ul> <Li> The co-transport sodium - potassium pump actively transports sodium out of the PCT (proximal convoluted tubule) wall (using energy from converting ATP to ADP + Pi) to maintain a low Na concentration gradient in the wall . </Li> <Li> This low concentration gradient means that Na ions from the glomerulus filtrate can easily passively diffuse into the wall of the PCT . </Li> <Li> 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 . </Li>

Where does selective reabsorption occur in the nephron