Reabsorption is the process by which substances are transported from the glomerular filtrate into the blood of the peritubular capillary.

the process by which substances are transported out of the tubular fluid, through the epithelium of the renal tubule, and into the interstitial fluid. These substances then diffuse into the peritubular capillaries (fig. 20.19).

Tubular reabsorption returns substances to the internal environment. The term tubular is used because this process is controlled by the epithelial cells that make up the renal tubules. In tubular reabsorption, substances must first cross the cell membrane facing the inside of the tubule (mucosal surface) and then the cell membrane facing the interstitial fluid (serosal surface).

The basic rules for movements across cell membranes apply to tubular reabsorption. Substances moving down a concentration gradient must either be lipid soluble or there must be a carrier or channel for that substance. Active transport, requiring ATP, may move substances uphill against a concentration gradient. If active transport is involved at any step of the way, the process is considered active tubular reabsorption. In all other cases, the process is considered passive.

Reconnect to chapter 3, Movements Into

and Out of the Cell, pages 82-89

Peritubular capillary blood is under relatively low pressure because it has already passed through two arte-rioles. Also, the wall of the peritubular capillary is more permeable than that of other capillaries. Finally, the relatively high rate of glomerular filtration has increased the protein concentration, and thus the colloid osmotic pressure of the peritubular capillary plasma. All of these factors enhance the rate of fluid reabsorption from the renal tubule.

Although tubular reabsorption occurs throughout the renal tubule, most of it occurs in the proximal convoluted portion. The epithelial cells in this portion have many microvilli that form a "brush border" on their free surfaces facing the tubular lumen. These tiny extensions greatly increase the surface area exposed to the glomeru-lar filtrate and enhance reabsorption.

Segments of the renal tubule are adapted to reabsorb specific substances, using particular modes of transport. Glucose reabsorption, for example, occurs through the walls of the proximal convoluted tubule by active transport. Water also is rapidly reabsorbed through the epithelium of the proximal convoluted tubule by osmosis; however, portions of the distal convoluted tubule and collecting duct are almost impermeable to water. This characteristic of the distal convoluted tubule is important in the regulation of urine concentration and volume, as described in the section titled "Regulation of Urine Concentration and Volume."

Recall that active transport requires carrier proteins in a cell membrane. The molecule to be transported binds to the carrier; the carrier changes shape, releases the transported molecule on the other side of the cell membrane, and then returns to its original position and repeats the process. Such a mechanism has a limited transport capacity; that is, it can transport only a certain number of molecules in a given length of time because the number of carriers is limited.

Usually all of the glucose in the glomerular filtrate is reabsorbed because there are enough carrier molecules to transport it. When the plasma glucose concentration increases to a critical level, called the renal plasma threshold, more glucose molecules are in the filtrate than the active transport mechanism can handle. As a result, some glucose remains in the filtrate and is excreted in the urine. This explains why the elevated blood glucose of diabetes mellitus results in glucose in the urine.

Any increase in urine volume is called diuresis. Nonreabsorbed glucose in the tubular fluid draws water into the renal tubule by osmosis, thus increasing urine volume. Such an increase is called an osmotic diuresis.

Glucose in the urine is called glucosuria. It may follow intravenous administration of glucose, epinephrine, or eating a candy bar, or it may occur in a person with diabetes mellitus. In the case of type I diabetes, blood glucose concentration rises because of insufficient insulin secretion from the pancreas.

One in three people who have diabetes mellitus sustains kidney damage, as evidenced by protein in their urine. Following a low-protein diet can slow the loss of kidney function.

Amino acids enter the glomerular filtrate and are reabsorbed in the proximal convoluted tubule. Three different active transport mechanisms reabsorb different groups of amino acids, whose members have similar structures. As a result, normally only a trace of amino acids remains in the urine.

Although the glomerular filtrate is nearly free of protein, a number of smaller protein molecules, such as albumin, squeeze through the glomerular capillaries. These proteins are taken up by pinocytosis through the brush border of epithelial cells lining the proximal convoluted tubule. Once they are inside an epithelial cell, the proteins are degraded to amino acids and moved into the blood of the peritubular capillary.

The epithelium of the proximal convoluted tubule also reabsorbs creatine; lactic, citric, uric, and ascorbic (vitamin C) acids; and phosphate, sulfate, calcium, potassium, and sodium ions. Active transport mechanisms with limited transport capacities reabsorb all of these chemicals. Such substances begin to appear in the urine when their concentrations in the glomerular filtrate exceed their respective renal plasma thresholds. Clinical Application 20.3 discusses how the nephrotic syndrome causes plasma proteins to appear in the urine.

Sodium and Water Reabsorption

Water reabsorption occurs passively by osmosis, primarily in the proximal convoluted tubule, and is closely associated with the active reabsorption of sodium ions. In the proximal convoluted tubule, if sodium reabsorption increases, water reabsorption increases; if sodium reabsorption decreases, water reabsorption decreases also.

Much of the sodium ion reabsorption occurs in the proximal segment of the renal tubule by active transport (sodium pump mechanism). When the positively charged sodium ions (Na+) are moved through the tubular wall, negatively charged ions, including chloride ions (Cl-), phosphate ions (PO4-3), and bicarbonate ions (HCO3-), accompany them. This movement of negatively charged ions is due to the electrochemical attraction between particles of opposite electrical charge. Although it depends on active transport of sodium, this movement of negatively charged ions is considered a passive process because it does not require a direct expenditure of cellular energy. Some of these ions (such as HCO3- and PO4-3) are also reabsorbed by active transport.

As more and more sodium ions are reabsorbed into the peritubular capillary along with negatively charged ions, the concentration of solutes within the peritubular blood might be expected to increase. However, since water diffuses through cell membranes from regions of lesser solute concentration (hypotonic) toward regions of greater solute concentration (hypertonic), water moves by osmosis, following the ions from the renal tubule into the peritubular capillary.

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