14.7 Why do diabetic patients lose glucose in the urine?

Glucose is precious, and in healthy individuals the concentration of glucose in the urine is neglible. Why does glucose appear in the urine in diabetic patients? To understand this, let's have a brief look at how the kidney produces urine. The kidney contains several thousand functional units, each of which is called a nephron. Within each nephron, urine production proceeds in two stages (Figure 14.7-1):

Ultrafiltration of the blood plasma. This occurs in the glomerulus; all small solutes found in the blood plasma (glucose, ions, amino acids and so on) will appear in the filtrate.
Selective, active reuptake of solutes and of water in the tubuli of the nephron.

The capacity of glucose reuptake is limited by the abundance of glucose transporter molecules in the tubuli. While quite a few processes in metabolism have a functional reserve, the transport of glucose is not one of them; its capacity saturates just slightly above the physiological range of the glucose concentration in the blood plasma and the primary urine filtrate.