individuals with a low GFR. It is estimated by measuring the creatinine molecule, which is a protein from muscle cell metabolism that is minimally secreted by the nephron.
SECRETION AND REABSORPTION About 180 liters of filtrate get through the glomerulus per day—much of it needing reabsorption back into the bloodstream. As you have learned, this occurs in the PCT, the loop of Henle, DCT, and collecting ducts. Different parts of that system reabsorb and secrete things in various ways. Much of this is passive reabsorption along concentration gradients; however, the reabsorption of water is highly regulated by the kidneys (and the body as a whole). Aldosterone, antidiuretic hormone, and renin (indirectly) will affect the water recovery. Most of water recovery happens in the nephron with only ten percent reaching the collecting ducts. This is where ADH kicks in to recover all, some, or none of the water, depending on how much water is needed by the body. Solutes like glucose, amino acids, oligopeptides, vitamins, and lactate get reabsorbed in the PCT for the most part so they aren’t lost in the urine. Creatinine is secreted in the PCT, while urea is secreted and partially reabsorbed in the PCT (it is later secreted and reabsorbed in other parts of the kidney). Sodium and chloride are mostly reabsorbed throughout the nephron. Bicarbonate is reabsorbed at about 80-90 percent in the PCT, while hydrogen ions are secreted. Potassium can be reabsorbed in the PCT and loop of Henle but is secreted under the regulation of aldosterone in the collecting ducts. Calcium, magnesium, and phosphate are all reabsorbed throughout the nephron. The mechanisms necessary for the reabsorption or secretion of these solutes include diffusion, active transport, facilitated diffusion, osmosis, and secondary active transport. These have already been discussed in previous chapters. Some of these mechanisms need ATP energy, while others are completely passively transported across the cell membranes. The initial filtrate is similar to blood except for the lack of cells and large proteins in the filtrate. There is continual modification after that to make the final urine end product. This modification starts in the PCT. Some substances are secreted in the PCT, while others are reabsorbed. Ultimately these are returned to the circulation by the peritubular and vasa recta capillaries.
356
