In physiology,<unk>  (alternatively<unk>  or<unk> ax) refers to the point at which increase in concentration of a substance does not result in an increase in movement of a substance across a cell membrane. In renal physiology, the concept of<unk>  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 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<unk> . * Glucose is not secreted, so excretion = filtration - reabsorption. Both filtration and reabsorption are directly proportional to the concentration of glucose in the plasma. However, while the average<unk>  reabsorption is about 375 mg/min in healthy individuals, filtration has effectively no limit (within reasonable physiological ranges.) Therefore, if the concentration rises above 375 mg/min, the body cannot retain all the glucose, leading to glucosuria. * PAH is not reabsorbed and is secreted, 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<unk>  is reached, and the PAH takes longer to clear. In practice, the<unk>  is not all-or-nothing. As the concentration approaches the<unk> , 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<unk>  is called "splay".
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Free association 
Banana equivalent dose