Blood is filtered by millions of
nephrons, the functional units that comprise the kidneys. In each nephron, blood flows from the
arteriole into the
glomerulus, a tuft of leaky
capillaries. The
Bowman's capsule surrounds each glomerulus, and collects the filtrate that the glomerulus forms. The filtrate contains waste products (e.g.
urea),
electrolytes (e.g.
sodium,
potassium,
chloride),
amino acids, and glucose. The filtrate passes into the
renal tubules of the kidney. In the first part of the renal tubule, the
proximal tubule, glucose is reabsorbed from the filtrate, across the tubular epithelium and into the bloodstream. The proximal tubule can only reabsorb a limited amount of glucose (~375 mg/min), known as the
transport maximum. When the blood glucose level exceeds about 160–180 mg/dL (8.9-10 mmol/L), the proximal tubule becomes overwhelmed and begins to excrete glucose in the urine. This point is called the
renal threshold for glucose (RTG). Some people, especially children and pregnant women, may have a low RTG (less than ~7 mmol/L glucose in blood to have glucosuria). If the RTG is so low that even normal blood glucose levels produce the condition, it is referred to as
renal glycosuria. Glucose in urine can be identified by
Benedict's qualitative test. If yeast is present in the bladder, the sugar in the urine may begin to ferment, producing a rare condition known as urinary
auto-brewery syndrome. ==References==