Definition

Tubular reabsorption is the mechanism by which water and other substances are transferred back to the blood from the renal tubules. As the glomerular filtrate flows through the nephron tubular portion, both quantitative and qualitative changes occur. Significant volumes of water (more than 99 %), electrolytes, and other substances are reabsorbed by the tubular epithelial cells. Reabsorbed substances pass through the renal medulla interstitial fluid, and the substances move into the blood in peritubular capillaries from there.

Because the substances are brought back into the blood from the glomerular filtrate, the whole process is called tubular reabsorption. For many substances, the mechanism of glomerular filtration and tubular reabsorption is (quantitatively large compared to urinary excretion). This means that a relatively large change in urinary excretion can occur because of a small change in glomerular filtration or tubular reabsorption and, tubular reabsorption is (highly selective), unlike the glomerular filtration, which is relatively nonselective (essentially all solutes in the plasma are filtered except the plasma proteins or substances bound to them).

The urinary excretion rate of some substances is essentially zero, such as (glucose and amino acids), because they are almost entirely reabsorbed from the tubules. Many of the ions in the plasma, such as sodium, chloride, and bicarbonate, are also highly reabsorbed, but their reabsorption and urinary excretion rates differ depending on the body's needs. Waste materials, such as urea and creatinine, are, on the other hand, poorly reabsorbed from the tubules and excreted in relatively large quantities.

For a substance to be reabsorbed, it must first be transferred (1) through the tubular epithelial membrane into the renal interstitial fluid and then (2) through the peritubular capillary membrane back into the blood, the reabsorption of water and solutes includes a series of transport steps.

Mechanisms of reabsorption

The basic transport mechanisms involved in tubular reabsorption are two types:

1. Active reabsorption
2. Passive reabsorption

Reabsorption through the tubular epithelium into the interstitial fluid requires active or passive transport. For example, water and solutes can be transported through the cell membranes themselves (transcellular route) or through spaces between the cell junctions (paracellular route).

Then water and solutes are transferred through the peritubular capillary walls into the blood through ultrafiltration (bulk flow), After absorption through the tubular epithelial cells into the interstitial fluid. Mediated through hydrostatic and colloid osmotic forces. The peritubular capillaries function like the venous ends of most other capillaries since there is a net reabsorptive force that transfers fluids and solutes from the interstitium to the blood.

Active transport

Active transport is the movement of molecules against the electrochemical gradient and involves energy extracted from metabolism. Transport that is directly connected to an energy source, such as adenosine triphosphate hydrolysis (ATP), is referred to as primary active transport. A good example of this is the sodium-potassium ATPase pump, which functions throughout most parts of the renal tubule, other Substances reabsorbed actively from the renal tubule are calcium, phosphate, sulfates, bicarbonate, glucose, amino acids, ascorbic acid, uric acid, and ketone bodies.

Transport that is indirectly connected to an energy source, such as that due to an ion gradient, is referred to as secondary active transport. An example of secondary active transport is the reabsorption of glucose by the renal tubule. Although the solutes can be reabsorbed by tubule in active and/or in passive mechanisms. Water is always reabsorbed by osmosis which is a passive (non-active) physical mechanism.

Primary active transport

Solutes can be moved against their electrochemical gradient, because of the special ability of the primary active transport. The energy for this active transport comes from the hydrolysis of ATP via membrane-bound ATPase; ATPase is also a component of the carrier mechanism that binds and moves solutes across cell membranes. Sodium potassium ATPase, hydrogen ATPase, hydrogen potassium ATPase, and calcium ATPase are the primary active transporters in the kidneys that are known.

Secondary active transport

In secondary active transport, two or more substances interact with a particular membrane protein (a carrier molecule) and are transported together through the membrane. If one of the substances (e.g., sodium) diffuses down its electrochemical gradient, the energy released is used to force another substance (e.g., glucose) against its electrochemical gradient.

Consequently, secondary active transport does not require energy directly from ATP or from other high energy phosphate sources. Rather, the simultaneous facilitated diffusion of another transported substance down its own electrochemical gradient liberates the direct source of the energy.

Passive transport

The movement of molecules along the electrochemical gradient is referred to as Passive reabsorption. This process does not involve energy and occurs spontaneously by the following mechanisms:

1. Diffusion
2. Facilitated diffusion (coupled or carrier-mediated diffusion) depends on the interaction of the molecule or ion with a particular membrane carrier protein that facilitates its passage through the lipid bilayer of the cell membrane. In nearly all cases of carrier-mediated transport in the kidney, two or more ions or molecules share the carrier: one ion moves down its electrochemical gradient and the other ion moves against the gradient.
3. Solvent drag is the process in which the water moves across the tight junctions by osmosis and carry other solutes with it. Substances reabsorbed passively are (chloride, urea, and water).

Sites of reabsorption

Transport across different segments of the renal tubule

Reabsorption of the substances occurs in almost all the segments of the tubular portion of nephron.

Approximately 7/8 of the filtrate (about 88 %) is reabsorbed in the proximal convoluted tubule. The brush border of epithelial cells in proximal convoluted tubules increases the surface area and promotes reabsorption. Reabsorbed substances from proximal convoluted tubules are sodium, potassium, calcium, bicarbonate, chloride, phosphate, urea, uric acid (almost all glucose and amino acids), and water. This segment does not reabsorb inulin, creatinine sucrose, and mannitol.

About 20% of filtered Na+ and Cl−, 15% of filtered water and cations such as K+, Ca2+ and Mg2+ are reabsorbed in the loop of Henle. In different parts of the loop of Henle Reabsorption can occur:

Thin descending limb of the loop of Henle

Exclusively in this part of the loop of Henle, water absorption occurs passively (because of hypertonic interstitial fluid). It is achieved by the diffusion of sodium ions from the interstitial fluid into the tubular lumen.

Thin ascending limb of the loop of Henle

Limited passive reabsorption of Na+ and Cl− occurs in this water-impermeable limb. The fluid leaving this limb is hypotonic relative to plasma, because of impermeability to water.

Thick ascending limb of the loop of Henle

This limb is impermeable to water but is involved in the reabsorption of 25% of the filtered Na+, Cl−, and potassium, as well as large amounts of calcium, bicarbonate, and magnesium. This segment also secretes hydrogen ions into the tubular lumen. About half of the Na+ is reabsorbed actively and transcellularly, while the other half of the Na+ is reabsorbed passively by paracellular pathway along with other cations.

Sodium, calcium, bicarbonate, and water are reabsorbed from the distal convoluted tubule. Approximately 7% of the filtered NaCl and about 8–17% of water is reabsorbed by the Distal Convoluted Tubule, early distal tubule (the initial segment of distal tubule) reabsorbs Na+, Cl−, Ca2+, and is impermeable to water.

1. Substances reabsorbed from proximal convoluted tubule
2. Substances reabsorbed from Loop of Henle
3. Substances Reabsorbed from Distal Convoluted Tubule