In physiology, transport maximum (alternatively Tm or Tmax) refers to the point at which increases in concentration of a substance do not result in an increase in movement of a substance across a cell membrane.
In renal physiology, the concept of transport maximum is often discussed in the context of glucose and PAH.
For both substances (as with all substances), the quantity excreted can be determined with the following equation:
The proximal convoluted tubule of the nephron has protein channels that reabsorb glucose, and others that secrete para-aminohippuric acid (PAH). However, its ability to do so is proportionate to the channel proteins available for the transport.
In practice, the transport maximum is not all-or-nothing. As the concentration approaches the transport maximum, some of the channels are overwhelmed before others are. For example, with glucose, some sugar appears in the urine at levels much lower than 300 mg/dL. The point at which the effects start to appear is called "threshold", and the difference between threshold and transport maximum is called "splay".