The proximal tubule reabsorption: active and passive reabsorption

2021-05-03 10:27 PM

The proximal tubule has a large resorption capacity due to its cell special structure. Proximity tubular epithelial cells have a high metabolic capacity and a large number of mitochondria support strong active transport.

Normally, about 65% of the filtered sodium, water and less than chlorine are reabsorbed by the proximal tubule before the filtrate enters the Henle loop. This percentage can be increased or decreased under different physiological conditions, which will be discussed later.

The prox tubule has a large passive and active reabsorption capacity

The proximal tubule has a large resorption capacity due to its cell special structure. Proximity tubular epithelial cells have a high metabolic capacity and a large number of mitochondria support strong active transport. Furthermore, the proximal tubular epithelial cell has a brush frill system in the apical membrane, as well as the extensive basal channel labyrinth in the intercellular, all of which together create a large surface area at the apex and membrane. the bottom of the epithelial cell, which helps in the rapid transport of sodium ions and other solutes.

Figure. Characteristics of the primary cell and transport superstructure of the proximal tubule. The proximal tubules reabsorb about 65% of the filtered sodium, chloride, bicarbonate and potassium and basically all the filtered amino acids and glucose. The proximal tubules also secrete organic acids, bases, and hydrogen ions into the lumen.

The brush border surface of the epithelial cell is also attached to a carrier protein molecule to help transport large amounts of sodium ions through the apical membrane, combining the transport of many nutrients such as glucose and amino acids. The process of transporting sodium from the tubular lumen into the cell is supplemented by a reverse transport mechanism that reabsorbs frozen sodium while excreting other solutes into the lumen, especially hydrogen ions. Excretion of hydrogen ions into the renal tubule is an important step in removing bicarbonate ions from the renal tubule (by attaching H + to HCO3- to form H2CO3, which then dissociates into H2O and CO2).

Although the sodium-potassium ATPase pump is a major factor in the reabsorption of sodium, chlorine and water throughout the proximal tubule, there is a difference in the mechanism by which sodium and chloride are transported through the apical membranes of the top and bottom of the tubule. near the.

In the first half of the proximal tubule, sodium reabsorbed is transported in the same direction as glucose, amino acids, and other solutes. However, in the back half of the proximal tubule, less glucose and the remaining small amounts of amino acids are reabsorbed. Instead, sodium is reabsorbed primarily with chloride ions. The back half of the proximal tubule had a relatively high concentration of chlorine (about 140mEq / L) compared with the first half of the proximal tubule (about 105 mEq / L) because when sodium was reabsorbed, it preferred transport with glucose, bicarbonate and organic ions are in the proximal tubule, leaving a solution with a higher concentration of chlorine.

In the back half of the proximal tubule, the higher concentration of chlorine allows this ion to diffuse from the lumen of the tubule through the gasket around the interstitial space. A small amount of chlorine may also be reabsorbed through specific chlorine channels in the proximal tubular membrane.

Figure. Changes in the concentration of various substances in the tubular fluid along the proximal tubule are related to the concentrations of these substances in the plasma and in the glomerular filtrate. A value of 1.0 indicates that the concentration of the substance in the tubular fluid is the same as the concentration in the plasma. A value below 1.0 indicates that the substance is reabsorbed more than water, while a value above 1.0 indicates that the substance is reabsorbed at a lower level than water or secreted into a tube.

The concentration of solutes along the proximal tubule

The concentration changes of solutes vary widely along the proximal tubule. Although the amount of sodium in the tubular fluid decreased markedly along the proximal tubule, the sodium concentration (and total osmolality) remained relatively stable because the water permeability in the near tubule was very large to help the water reabsorb. Keep pace with sodium reabsorption. Some organic solutes such as glucose, amino acids and bicarbonate are reabsorbed more than water, and thus their concentration decreases markedly along the short tubule length. Other organic solutes have low permeability and are not actively reabsorbed such as creatinine, and their concentration increases along the proximal tubule. The total solute concentration, reflected by permeability, remains largely the same along the proximal tubule because water has very high permeability in this part of the nephron.

Excretion of organic acids and bases in the proximal tubule

The proximal tubule is also an important site for the excretion of organic acids and bases such as bile salts, oxalate, urate, and catecholamine. Many of these substances are end products of metabolism and must be quickly eliminated from the body. The excretion of these substances into the proximal tubule plus the filtration by the glomerular capillary enters the proximal tubule and is almost not reabsorbed, all pooling and excreted rapidly in the urine.

In addition to metabolic waste products, the kidneys excrete many dangerous drugs or toxins directly through the cells into the kidney tubules and quickly remove this substance from the bloodstream. In some drugs, such as penicillin and salicylates, rapid renal excretion poses a challenge in maintaining drug concentrations for effective treatment.

Another compound that is rapidly excreted in the proximal tubule is para-amino hippuric acid (PAH). PAH is excreted very quickly, on average about 90% of PAH can be removed from plasma flowing through the kidneys and can secrete urine. Hence, PAH clearance can be used to estimate renal plasma flow RPF.



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