Transport maximum

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In physiology, transport maximum (or Tm) refers to the point at which increases in concentration do not result in an increase in movement of a substance across a 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:

excretion = (filtration + secretion) - reabsorption

The body "wants" to retain glucose, and eliminate PAH. However, its ability to do so is proportionate to the channel proteins available for the transmission.

Glucose is not secreted, so excretion = filtration - reabsorption. Both filtration and reabsorption are directly proportional to the concentration of glucose in the plasma. However, reabsorption has a transport maximum of about 300 mg/dL in healthy nephrons, while filtration has effectively no limit (within reasonable physiological ranges.) So, if the concentration rises above 300 mg/dL, the body cannot retain all the glucose, leading to glucosuria.

PAH is not reabsorbed, so excretion = filtration + secretion. As with glucose, the transfer is at the proximal tubule, but in the opposite direction: from the peritubular capillaries to the lumen. At low levels, all the PAH is transferred, but at high levels, the transport maximum is reached, and the PAH takes longer to clear.

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 loewr than 300 mg/dL.^[1] The point at which the effects start to appear is called "threshold", and the difference between threshold and transport maximum is called "splay".^[2]