physiology Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical ...

, transport maximum (alternatively Tm or T_max) 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 The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...

. In

renal physiology Renal physiology (Latin ''rēnēs'', "kidneys") is the study of the physiology of the kidney. This encompasses all functions of the kidney, including maintenance of acid-base balance; regulation of fluid balance; regulation of sodium, potassium, ...

, the concept of transport maximum is often discussed in the context of

glucose Glucose is a simple sugar with the molecular formula . Glucose is overall the most abundant monosaccharide, a subcategory of carbohydrates. Glucose is mainly made by plants and most algae during photosynthesis from water and carbon dioxide, u ...

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 transport. * 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 maximum 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 Glycosuria is the excretion of glucose into the urine. Ordinarily, urine contains no glucose because the kidneys are able to reabsorb all of the filtered glucose from the tubular fluid back into the bloodstream. Glycosuria is nearly always caused ...

. * 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 In the renal system, peritubular capillaries are tiny blood vessels, supplied by the efferent arteriole, that travel alongside nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron. Peritubular capillaries s ...

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 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".Tubular Transport
/ref>

References

Circulatory system