Proximal tubule

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Proximal tubular
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Proximal tubule
Metanephric blastema
Identifiers
Latintubulus proximalis, pars tubuli proximalis
MeSHD007687
Anatomical terminology]

The proximal tubule is the segment of the nephron in kidneys which begins from the renal pole of the Bowman's capsule to the beginning of loop of Henle. At this location, the glomerular parietal epithelial cells (PECs) lining bowman’s capsule abruptly transition to proximal tubule epithelial cells (PTECs). The proximal tubule can be further classified into the proximal convoluted tubule (PCT) and the proximal straight tubule (PST).

Structure

The most distinctive characteristic of the proximal tubule is its luminal brush border.[citation needed]

Brush border cell

The luminal surface of the

light microscope giving the brush border cell its name. The microvilli greatly increase the luminal surface area of the cells, presumably facilitating their reabsorptive function as well as putative flow sensing within the lumen.[1] The microvilli are composed of actin filament bundles that have been visualized using confocal microscopy.[2]

The

mitochondria, which are largely found in the basal region within the infoldings of the basal plasma membrane. The high quantity of mitochondria gives the cells an acidophilic appearance
. The mitochondria are needed in order to supply the energy for the active transport of sodium ions out of the cells to create a concentration gradient which allows more sodium ions to enter the cell from the luminal side. Water passively follows the sodium out of the cell along its concentration gradient.

Cuboidal epithelial cells lining the proximal tubule have extensive lateral interdigitations between neighboring cells, which lend an appearance of having no discrete cell margins when viewed with a light microscope.

capillaries
surrounding the tubule often leads to disturbance of the cellular morphology of the proximal tubule cells, including the ejection of cell nuclei into the tubule lumen.

This has led some observers to describe the lumen of proximal tubules as occluded or "dirty-looking", in contrast to the "clean" appearance of

distal tubules
, which have quite different properties.

Divisions

Based on its appearance at low magnification, the proximal tubule can be divided into two sections: the proximal convoluted tubule (PCT), and the proximal straight tubule (PST).[3] Differences in cell outlines exist between these segments, and therefore presumably in function too.[citation needed]

Based on ultrastructure, it can be divided into three segments, S1, S2, and S3.

Segment Gross divisions Ultrastructure divisions Description
Proximal tubule convoluted S1[4] Higher cell complexity[4]
S2[4]
straight
S3[4] Lower cell complexity[4]
Proximal tubule cell showing pumps involved in acid base balance, left is the lumen of tubule

Proximal convoluted tubule (pars convoluta)

The pars convoluta (Latin "convoluted part") is the initial convoluted portion.[citation needed][5]

In relation to the morphology of the kidney as a whole, the convoluted segments of the proximal tubules are confined entirely to the renal cortex.[citation needed]

Some investigators on the basis of particular functional differences have divided the convoluted part into two segments designated S1 and S2.[citation needed]

Proximal straight tubule (pars recta)

The pars recta (Latin "straight part") is the following straight (descending) portion.[citation needed][5]

Straight segments descend into the outer medulla. They terminate at a remarkably uniform level and it is their line of termination that establishes the boundary between the inner and outer stripes of the outer zone of the renal medulla.[citation needed]

As a logical extension of the nomenclature described above, this segment is sometimes designated as S3.[citation needed]

Functions

Absorption

The proximal tubule efficiently regulates the pH of the filtrate by secreting hydrogen ions (acid) into the tubule and reabsorbing approximately 80% of the filtered bicarbonate.[6]

Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the

basolateral membrane of the epithelial cells.[6]

Sodium reabsorption is primarily driven by this

electrodiffusion. Active transport is mainly through the sodium/hydrogen antiporter NHE3.[6][7] Paracellular transport increases transport efficiency, as determined by oxygen consumed per unit of Na+ reabsorbed, thus playing a part in maintaining renal oxygen homeostasis.[8]

Substance % of filtrate reabsorbed Comments
water approximately two-thirds Mass movement of water and occurs both through the cells and between them,
paracellular
).
sodium approximately two-thirds Mass movement of sodium occurs through the cells, by secondary active transport on the apical membrane, followed by active resorption across the basolateral membrane via the
isotonically
, in that the osmotic potential of the fluid leaving the proximal tubule is the same as that of the initial glomerular filtrate.
organic solutes (primarily glucose and amino acids) 100%
co-transporters driven by the sodium gradient
out of the nephron.
potassium approximately 65% Most of the filtered potassium is resorbed by two paracellular mechanisms – solvent drag and simple diffusion.[11]
urea approximately 50% Paracellular fluid reabsorption sweeps some urea with it via solvent drag. As water leaves the lumen, the concentration of urea increases, which facilitates diffusion in the late proximal tubule.[11][page needed]
phosphate approximately 80%
intestine and bones
into the blood, the responses to PTH cancel each other out, and the serum concentration of phosphate remains approximately the same.
citrate
70%–90%[12] Acidosis increases absorption. Alkalosis decreases absorption.

Secretion

Many types of

medications are secreted in the proximal tubule. Further reading: Drugs secreted in the kidney

Most of the

glutamate and the conversion of glutamate to alpha-ketoglutarate.[13] The alpha-ketoglutarate generated in this process is then further broken down to form two bicarbonate anions,[13]
which are pumped out of the basolateral portion of the tubule cell by co-transport with sodium ions.

Clinical significance

CD10

Proximal tubular epithelial cells (PTECs) have a pivotal role in kidney disease. Two mammalian

cell lines are commonly used as models of the proximal tubule: porcine LLC-PK1 cells and marsupial OK cells.[14]

Cancer

Most renal cell carcinoma, the most common form of kidney cancer, arises from the convoluted tubules.[15]

Other

amino acids, and phosphate.[citation needed
]

PTECs also participate in the progression of tubulointerstitial injury due to

chemokines, and collagen; undergoing epithelial mesenchymal trans-differentiation; necrosis or apoptosis.[citation needed
]

See also

Additional images

  • Distribution of blood vessels in cortex of kidney.
    Distribution of blood vessels in cortex of kidney.
  • Glomerulus.
    Glomerulus.
  • TEM of negatively stained proximal convoluted tubule of Rat kidney tissue at a magnification of ~55,000x and 80KV with Tight junction.
    TEM of negatively stained proximal convoluted tubule of Rat kidney tissue at a magnification of ~55,000x and 80KV with Tight junction.
  • Renal corpuscle
    Renal corpuscle
  • Diagram outlining movement of ions in nephron.
    Diagram outlining movement of ions in nephron.

References

Public domain This article incorporates text in the public domain from page 1223 of the 20th edition of Gray's Anatomy (1918)

External links