46 Tubular Reabsorption

Reabsorption starts as soon as filtrate enters the proximal convoluted tubules. Before the reabsorbed substances can reenter the blood, they must pass through three barriers: the luminal and basolateral membranes of cells in the tubule, and the endothelium of the peritubular capillaries.

Reabsorption returns about 99 percent of filtered water and many filtered solutes to the bloodstream. Although cells in the proximal convoluted tubule perform most reabsorption, epithelial cells all along the length of the renal tubule and the collecting duct also contribute to this process. To maintain normal plasma levels, almost every organic nutrient (e.g., glucose, amino acids) is completely reabsorbed. The amount of water and number of ions reabsorbed is determined by hormonal signals and is based on homeostatic needs. The scope of reabsorption can be appreciated by looking at water. Each day, about 180 liters (190 quarts) of water are filtered in the kidneys. Only 1–2 liters (1–2 quarts) are excreted in urine; the rest is reabsorbed in the kidneys.

(Note: It is not necessary to memorize the table, just wanting to give you an idea of what is reabsorbed vs. what is excreted.)

Reabsorbed substances can be returned to the bloodstream to maintain homeostasis by going between tubule cells (paracellular reabsorption) or by going through individual cells (transcellular reabsorption) on their way to entering a peritubular capillary. Cells in the renal tubule are joined together at tight junctions. Although these junctions are tight, they are just leaky enough to let some solutes pass through during paracellular reabsorption. Substances reabsorbed through the transcellular route move through the tubule cell’s apical membrane, then through the intracellular fluid, and finally through the basolateral membrane to reach the interstitial fluid. The apical membrane is in contact with the tubular fluid, and the basolateral membrane is in contact with interstitial fluid as well as the bottom and sides of cells.

The apical and basolateral membranes of renal cells contain a variety of transport proteins. The mechanism of transport depends on the substance being reabsorbed. In passive reabsorption, no ATP is required. In active reabsorption, the energy derived from the hydrolysis of ATP helps “pump” the substance across a membrane—either directly or indirectly—during at least one step in the reabsorption process.

Because all water is reabsorbed via osmosis, the reabsorption of water is coupled with the reabsorption of solutes. About 90 percent of water reabsorbed in the kidneys is accompanied by the reabsorption of sodium ions, chloride ions, glucose, and other solutes. Because the water is “obliged” to tag along with the reabsorbed solutes, this process is called obligatory water reabsorption. It occurs in the water-permeable proximal convoluted tubule and descending limb of the nephron loop. The remaining 10 percent of water is reabsorbed through facultative water reabsorption, which is controlled by antidiuretic hormone and takes place primarily in the collecting ducts.

When filtrate enters the proximal convoluted tubule, it is referred to as tubular fluid or tubular filtrate. The composition of the tubular fluid changes as it travels through the tubule and collecting duct, as each segment reabsorbs different substances.