Tubule The glomerular filtrate, which is isosmotic to plasma, enters the proximal convoluted tubule where its volume is reduced by about 80%. This reduction in fluid v6lume is 'accomplished by the active reabsorption of NaCl, glucose, amino acids, etc. from the tubular fluid. As these solutes are removed, the tubular fluid becomes hypoosmotic and the interstitial fluid surrounding the tubular wall becomes hyperosmotic. This causes passive diffusion of water out of the tubule to restore isosmotic condition between the tubular fluid and the surrounding cortical interstitium.
This reabsorption of water is secondary to solute transport and occurs inevitably irrespective of the water requirements of the body. Hence, it is called the obligatory reabsorption of water. Thus, in the proximal convoluted tubule, there is a drastic reduction in fluid volume without any change in its osmolarity. At the normal plasma glucose concentration of 100 mg per 100 ml, glucose is totally reabsorbed from the ultrafiltrate in the proximal convoluted tubule. If the plasma glucose concentration increases above the .normal level, its concentration in the ultrafiltrate also increases correspondingly. If it exceeds, the capacity of the transport mechanism to reabsorb, some glucose appears in the urine as happens in diabetic patients.