Renal Excretory Function

The rate at which substances are filtered out of the glomerulus

90 mL/min/1.73 m² or above is generally considered normal in adults

Glomerular basement membrane; capillary endothelium; podocyte filtration slits

The size of the molecule; the polarity of the molecule - positively charged molecules are filtered more readily; net filtration pressure; podocyte slit pores

Ultrafiltration - free movement of smaller molecules but restricted movement of larger molecules

Water; electrolytes - Na, K, Cl and phosphate; glucose; urea; amino acids

Large proteins; red blood cells; white blood cells

The total amount of fluid filtered through the glomeruli per unit of time

Creatinine because it is readily filtered by the glomerulus

Efferent arteriole

Creatinine clearance = (urine concentration × urine volume) / plasma concentration

Measure the concentration of creatinine in the plasma; collect urine for a fixed period to calculate urine flow; measure the concentration of creatinine in the collected urine

Nuclear medicine methods using an exogenous filtration marker; estimated GFR using serum creatinine or cystatin C

An increased creatinine in the blood indicates a lower GFR

Approximately 60–110 micromol/L in adult men and 45–90 micromol/L in adult women

Creatinine is produced by the muscles, so someone with more muscle bulk would produce more creatinine; malnourished individuals have low serum creatinine, so eGFR may be overestimated; some drugs inhibit tubular secretion of creatinine, leading to raised plasma creatinine even though GFR may be unchanged

Solutes are transported through passive diffusion and active transport, which can be primary or secondary.

Passive diffusion; active transport - primary and secondary

Cystinuria; renal glycosuria; proximal renal tubular acidosis

Bartter syndrome type 1

Gitelman syndrome

Abnormality in the function of the PCT, causing impaired reabsorption of multiple substances

Bulk reabsorption of solutes and water, including amino acids and glucose

The concentration of a substance in the blood above which it begins to appear in the urine

Reabsorption of that substance by the PCT is incomplete and some of the substance remains in the urine

Yes - the renal threshold varies between substances, with glucose normally having a higher threshold than urea

Glucose transporters are saturated, so not all glucose is reabsorbed in the PCT and some is excreted in the urine

The maximum rate at which glucose can be reabsorbed by the renal tubules

The descending limb is permeable to water, so water leaves the tubule. The ascending limb is impermeable to water and reabsorbs NaCl, creating a medullary osmotic gradient that allows the kidneys to concentrate urine.

Sodium-potassium-chloride cotransporter (NKCC2)

Potassium excretion; regulation of sodium delivery to the collecting ducts; urine acidification; urinary concentration

NaCl cotransporter (NCC)

Alpha intercalated cells

Reabsorption of water in the presence of ADH; final regulation of urine concentration before urine passes to the renal pelvis, ureter and bladder

Located in the collecting duct; allows water to be reabsorbed in the collecting duct in response to ADH