Question

Explain renal threshold and how diabetes mellitus causes polyuria. In a clinical setting, what is a

renal threshold test used for and what is a renal clearance test used?

Answer 1

Renal threshold:

In physiology, the renal threshold is the concentration of a substance dissolved in the blood above which the kidneys begin to remove it into the urine. When the renal threshold of a substance is exceeded, reabsorption of the substance by the proximal convoluted tubule is incomplete; consequently, part of the substance remains in the urine. Renal thresholds vary by substance – the low potency poison urea, for instance, is removed at much lower concentrations than glucose. Indeed, the most common reason for the glucose renal threshold ever being exceeded is diabetes, which is called glycosuria.

Renal thresholds vary by species and by physiological condition; thus an animal may have different renal thresholds while hibernating, Renal thresholds can also be altered by many drugs, and may change in characteristic ways during certain illnesses.

Taken together, the collection of a kidney's renal thresholds essentially define much of its function in renal physiology. Many tests of kidney function amount to measures of renal thresholds for various substances.

How DM causes polyurea:

The mechanism applies to the osmotic diuresis produced by infusions of mannitol, which cannot be reabsorbed from the nephron and hence traps water osmotically within the tubular lumen, resulting in a high urine flow rate. Osmotic diuresis can also occur during disease states, notably in uncontrolled diabetes mellitus. In this case, increased plasma glucose concentrations result in the filtration of a glucose load greater than that which can be reabsorbed by the proximal tubule glucose reabsorption mechanism (saturation of the sodium–glucose cotransport carrier), leading to glycosuria accompanied by increased water flow because of the osmotic effect of the glucose trapped in the lumen. This mechanism accounts for the polyuria and dehydration encountered in newly presenting or uncontrolled insulin-dependent diabetes. A broadly similar mechanism is occasionally seen during the development of chronic kidney disease, where high levels of urea have a diuretic effect.

Renal threshold test:

In physiology, the renal threshold is the concentration of a substance dissolved in the blood above which the kidneys begin to remove it into the urine.

Renal clearance test:

Creatinine clearance test: A test that helps determine whether the kidneys are functioning normally. ... The rate of creatinine clearance is measured by first noting the volume of urine excreted in a given time period, such as 24 hours.