The Nephron Quiz

See Proximal Convoluted Tubule. Option E is correct. The proximal convoluted tubule (PCT) is the first part of the tubule after the glomerulus, and it does the bulk of the reabsorbing: it takes back the majority of the 180 litres of filtrate made each day, reclaiming around 130 litres, which is about 70%. It reabsorbs about 65% of the filtered water, sodium, potassium and chloride, all of the glucose and amino acids under normal conditions, and 85 to 90% of the bicarbonate. It can do this because its cells have a dense brush border (a fringe of tiny finger-like microvilli on the apical surface, which greatly increases the surface area) and many transporters. The driving force is the movement of sodium into the cells, which provides the energy for taking up glucose, amino acids and other solutes by secondary active transport (transport that borrows energy from the sodium gradient). Water follows the solutes by osmosis, so the fluid stays at about the same concentration as plasma. The aim is to avoid losing valuable nutrients and large volumes of water in the urine. Later segments then make smaller, regulated adjustments.

• Site: The proximal convoluted tubule is the first part of the tubule after the glomerulus.
• Water and ions: Reabsorbs about 65% of the filtered water, sodium, potassium and chloride.
• Nutrients: Reabsorbs all of the glucose and amino acids under normal conditions.
• Brush border: The dense microvilli greatly increase the surface area for reabsorption.