| Reabsorption and Secretory Transport Maximums |
As already mentioned, glucose and amino acids are reabsorbed by means of carrier mediated transport. Carrier mediated transport shows the property of saturation. Saturation means that when the transported substance is present at a sufficiently high concentration then all the carrier positions are filled up and the transport reaches a "maximum". Thus an amount of substance remains unabsorbed and would be excreted in the urine. The concentration of the substance that is just enough to achieve the maximum transport rate is called the transport maximum (Tm).
Glycosuria
The presence of glucose in urine is called glycosuria. This occurs when the plasma glucose level reaches 180 to 200 mg per 100ml. This plasma level is called the renal plasma threshold for glucose.
Glycosuria occurs most frequently in two conditions:
| 1. | Diabetes
Mellitus. In diabetes mellitus, fasting hyprglycaemia occurs due to inadequate secretion or action of insulin. In this condition the plasma glucose level increases to a level above the renal plasma threshold. |
| 2. | Lowering of
renal plasma threshold. This is usually a non-pathological condition and occurs most frequently during pregnancy. In this case glycosuria occurs even though the normal renal plasma threshold is not surpassed. |
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