Proximal tubule

From Wikipedia, the free encyclopedia

You have new messages (last change).
Proximal tubule
Scheme of renal tubule and its vascular supply. (1st convoluted tubule labeled at center top.)
1 Glomerulus, 2 proximal tubule, 3 distal tubule
Latin tubulus contortus proximalis
Gray's subject #253 1223
Precursor Metanephric blastema
Dorlands/Elsevier t_22/12830072

The proximal tubule is the portion of the duct system of the nephron leading from Bowman's capsule to the loop of Henle.

Contents

[edit] Structure and appearance

The most distinctive characteristic of the proximal tubule is its brush border (or "striated border").

The luminal surface of the epithelial cells of this segment of the nephron is covered with densely packed microvilli forming a border readily visible under the light microscope.

The microvilli greatly increase the luminal surface area of the cells, presumably facilitating their resorptive function.

The cytoplasm of the cells is densely packed with mitochondria in keeping with the energetic requirements of the cells resorptive activity.

Agonal resorption of the contents of the proximal tubular contents after interruption of circulation in the 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 "dirty looking", and to contrast this with the "clean" appearance of distal tubules, which have quite different properties.

[edit] Divisions

The proximal tubule as a part of the nephron can be divided into two sections. Differences in cell outlines exist between these segments, and therefore presumably in function too.

[edit] Pars convoluta

The "Pars convoluta" is the initial convoluted portion.

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

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

[edit] Pars recta

The "Pars recta" is the following straight (descending) portion.

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.

As a logical extension of the nomenclature described above, this segment is sometimes designated as "S3".

[edit] Absorption

Fluid in the filtrate entering the proximal convoluted tubule is reabsorbed into the peritubular capillaries. This is driven by sodium transport from the lumen into the blood by the Na+/K+ ATPase in the basolateral membrane of the epithelial cells. Sodium reabsorption is primarily driven by this antiporter. This is the most important transport mechanism in the PCT.

Substance % of filtrate reabsorbed Comments
salt and water approximately two-thirds Much of the mass movement of water and solutes occurs in between the cells through the tight junctions via paracellular transport, which in this case are not selective. The solutes are absorbed 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% Glucose, amino acids, inorganic phosphate, and some other solutes are reabsorbed via secondary active transport through cotransport channels driven by the sodium gradient out of the nephron.
potassium approximately 65%
urea approximately 50%
phosphate approximately 80% Parathyroid hormone reduces reabsorption of phosphate in the proximal tubules, but because it also enhances the uptake of phosphate from the intestine and bones into the blood, the responses to PTH cancel each other out, and the serum concentration of phosphate remains approximately the same.

[edit] Pathophysiology in kidney disease

Proximal tubular epithelial cells (PTEC) have a pivotal role in kidney disease.

Acute tubular necrosis occurs when PTEC are directly damaged by toxins such as antibiotics (e.g. gentamicin), pigments (e.g. myoglobin and sepsis (e.g. mediated by lipopolysaccharide from gram negative bacteria). Renal tubular acidosis (proximal type) (Fanconi syndrome) occurs when the PTEC are unable to properly reabsorb glomerular filtrate so that there is increased loss of bicarbonate, glucose, amino acids and phosphate.

PTEC also participate in the progression of tubulointerstitial injury due to glomerulonephritis, ischemia, interstitial nephritis, vascular injury and diabetic nephropathy. In these situations PTEC may be directly affected by protein (e.g. proteinuria in glomerulonephritis), glucose (in diabetes mellitus), or cytokines (e.g. interferon-γ and tumor necrosis factor). There are several ways in which PTEC may respond - by producing cytokines, chemokines and collagen; undergoing epithelial mesenchymal transdifferentiation; necrosis or apoptosis.

[edit] See also

[edit] Additional images

Distribution of blood vessels in cortex of kidney.

Transverse section of pyramidal substance of kidney of pig, the bloodvessels of which are injected.

Nephron.

Glomerulus.

TEM of negatively stained proximal convoluted tubule of Rat kidney tissue at a magnification of ~55,000x and 80KV with Tight junction.

[edit] External links

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained herein may be outdated. Please edit the article if this is the case, and feel free to remove this notice when it is no longer relevant.

 This anatomy article is a stub. You can help Wikipedia by expanding it.
Retrieved from "http://en.wikipedia.org../../../p/r/o/Proximal_tubule.html"