The kidneys reabsorb most of the fluid filtered from the blood

It has been estimated that an amount of water equal to all the water of a person's blood (two or three liters) is filtered into the tubules in only 20 to 30 minutes.

Using the method of withdrawing samples of fluid with small glass tubes, Alfred Richards and Arthur Walter studied the fluids at different levels of the frog kidney tubule. When they analyzed the chemicals of these fluids, they found that urine in the bladder was unlike the fluid found in the capsule. They hypothesized that as fluid moves through the tubule of the nephron, useful substances are returned to the blood by the single layer of cells forming the tubule wall, in a process called reabsorption.

Many other experiments have confirmed the hypothesis that most of the water is returned to the blood. Numerous other chemicals are also reabsorbed from the tubule and passed back into the bloodstream by way of the capillaries surrounding the tubules. Experimental evidence indicates that reabsorption of useful materials by tubule cells is accomplished by active transport. The work of active transport requires energy. As you recall, mitochondria are the structures in cells where energy release takes place. The mitochondria of the nephron cells release energy for the work of active transport.

Ninety-nine per cent of the water that filters into the tubules is reabsorbed. Glucose, salts, amino acids, and many other substances are reabsorbed along with the water. After all the substances useful to the cells have been returned to the bloodstream, the fluid urine remains. So much water is reabsorbed that only one-half to one liter of urine is formed each day from approximately 180 liters of fluid that filter through the nephrons

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