Proximal convoluted tubule

The glomerular filtrate passes first into the proximal convolute tubule. The reabsorption is essentially isotonic. Here energy dependent mechanisms reabsorb all of the following constituents:

• glucose - this mechanism is saturated when the blood glucose is more than 10 mmol/l. When this level of blood glucose is reached then glycosuria occurs.
• amino acids
• potassium
• sodium - about 80% reabsorbed
• bicarbonate - there is titratable acid and ammonium secretion
• active secretion also transports other compounds e.g. penicillins, probenecid

There is passive reabsorption of water and chloride.