In physiology, splay is the difference between urine threshold (the amount of a substance required in the

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...

s before it appears in the

urine Urine is a liquid by-product of metabolism in humans and in many other animals. Urine flows from the kidneys through the ureters to the urinary bladder. Urination results in urine being excreted from the body through the urethra. Cellular ...

) and saturation, or T_M, where saturation is the exhausted supply of

renal The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; bloo ...

reabsorption carriers. In simpler terms, splay is the concentration difference between a substance's maximum renal reabsorption vs. appearance in the urine. Splay is usually used in reference to glucose; other substances, such as

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

, have virtually no splay at all. Splay appears to occur because kidney nephrons do not have the same tubular maximum for glucose (TmG) therefore some nephrons may excrete before others and also because "the maximum reabsorption rate (or Tm) cannot be achieved until the amount/min of glucose being presented to the renal tubules is great enough to fully saturate the receptor sites". John Field of the

American Physiological Society The American Physiological Society is a non-profit professional society for physiologists. It has nearly 10,000 members, most of whom hold doctoral degrees in medicine, physiology or other health professions. Its mission is to support research an ...

said "Since the splay may occur when the residual nephrons are said to be free of anatomic abnormalities, the possibility exists that changes in the kinetics of glucose reabsorption may have been induced". One study found that glucose reabsorption exhibited low splay and another also found that the

titration curve Titrations are often recorded on graphs called titration curves, which generally contain the volume of the titrant as the independent variable and the pH of the solution as the dependent variable (because it changes depending on the composition o ...

s for glycine showed a large amount of splay whereas those for lysine showed none and the kinetics of carrier-mediated glucose transport possibly explains the level of splay in renal titration curves. As splay can be clinically important, patients with proximal tubule disease, mainly caused by hereditary nature and often in children, have a lower threshold but a normal Tm. Therefore, splay is suggested, probably because "some individual cotransporters have a low glucose affinity but maximal transport rate (renal glycosuria). Studies also show that if

sulfate The sulfate or sulphate ion is a polyatomic ion, polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salt (chemistry), ...

is reabsorbed by a Tm-limited process, it will have low splay and, in animals, the limits of

citrate Citric acid is an organic compound with the chemical formula HOC(CO2H)(CH2CO2H)2. It is a colorless weak organic acid. It occurs naturally in citrus fruits. In biochemistry, it is an intermediate in the citric acid cycle, which occurs in t ...

concentration normal in the body, citrate titration curves show a large amount of splay therefore a Tm for citrate reabsorption may actually happen. Also, tubular transport is Tm-limited and the reabsorption mechanism being saturated at a plasma concentration more than 20 times than usual shows a low level of splay. Renal abnormalities of glucose excretion, causing

glycosuria 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 caus ...

, may happen as either a result of reduced Tm for glucose or because of an abnormally wide range of nephron heterogeneity so splay of the glucose excretion curve is increased. Two causes are also listed for splay: "heteroginicity in glomerular size, proximal tubular length and number of carrier proteins for glucose reabsorption" and variability of TmG nephrons. Splay also occurs between 180 and 350 mg/dL %.

References

{{reflist Urology