reabsorption

Meaning: Reabsorption is the movement of water and solutes from the filtrate in the nephron tubule, across tubule cells, and back into the blood of the peritubular capillaries. Most of it happens in the proximal tubule, which returns about two-thirds of filtered Na⁺ and water, plus nearly all glucose and amino acids by Na⁺ cotransport. These carriers saturate, so each has a transport maximum. In the loop of Henle, the descending limb is permeable to water, while the ascending limb is not but reabsorbs salt. In the distal tubule and collecting duct, aldosterone raises Na⁺ reabsorption and ADH raises water reabsorption by adding aquaporins.

Equation: Excretion rate = filtration rate − reabsorption rate + secretion rate, with each term in mol/s (or kg/s for mass).

On the MCAT: Passages give a table of filtrate concentrations along the nephron, or a hormone change, and ask which segment or direction is involved. Higher ADH means more water reabsorbed and a smaller volume of more concentrated urine. Glucose appears in urine when the filtered load exceeds its transport maximum.

Common confusion: Reabsorption is not secretion. Reabsorption moves substances from filtrate to blood; secretion moves them from blood into filtrate. It is also different from digestive absorption in the gut.