Filtration, secretion, reuptake

Membrane ermeant dmgs are thus not efficiently eliminated in the urine, even if they do get filtrated in the glomemli. On the other hand, membrane-impermeant dmgs get eliminated in proportion to the extent of glomem-lar filtration. Glomemlar filtration therefore is an important parameter in the elimination of dmgs. It may vary considerably between different patients (example A patient who has donated one kidney. Not the most common case of reduced kidney function but a straightforward one). With 

Inulin is freely filtrated in the glomemli, so that the concentration in the filtrate equals that in the plasma  

Inulin is quantitatively retained in the urine, so that the number of molecules is the same in the filtrate and the final urine  

In the second step, p-AH is secreted from the tubule cell into the tubule lumen. This involves exchange with monovalent anions from the filtrate, driven not by charge but by concentration gradients. 

Since p-AH is nearly quantitatively extracted from all blood plasma that reaches the kidney (the commonly reported fraction is 92%), its clearance can actually be used to determine the renal flow of blood plasma, without any serious invasive action. Here is the rationale  

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Mechanistically, most small solutes - glucose, salts, amino acids - are taken up again by specific active transporters. Active secretion likewise works by way of active transport. Typically, one transporter will pick up the substrate in question from the interstitial space and move it to the cytosol, from where a second transporter located in the apical membrane expels it into the nascent urine (see Figure 2.19). Water is recovered by the ensuing osmotic effect. Some solutes are partially or totally excluded from reuptake. Note that the final urine volume is about 100 times smaller than the primary filtrate. This means that the bulk of the fluid, salt and metabolites are actually reabsorbed. Some dmgs are subject to reuptake to a similar extent, too. 

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