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What Is the Proximal Tubule?

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  • Written By: Jo Dunaway
  • Edited By: S. Pike
  • Last Modified Date: 07 November 2016
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The proximal tubule is a part of the kidney that is responsible for filtering glucose, salts, amino acids, potassium, phosphates, vitamins and minerals, and urea in the blood, and reintroducing the blood back into the blood supply. It is made up of two parts: the pars convoluta and the pars recta. The pars convoluta is, as its name implies, convoluted, and it is the initial portion of the proximal tubule, reabsorbing water, salt and the above nutrients and by-products, and producing what are called organic cations and anions such as creatinine as well as hydrogen ions in a secretory process. The pars recta, a straight descending portion, also filters all of these as well as bicarbonate for pH regulation in the blood.

Clearing substances from the blood that are not to be reabsorbed back into the blood supply is another function of the structure. It works with other parts of the renal tubule system and is a member of what is known as the collecting duct system. The proximal tubule receives the nutrients from the Bowman’s capsule through tiny arteriole openings under pressure; these openings are too small for proteins to pass through. The structure can manufacture anhydrous ammonia to clean out protons, produce bicarbonate ions as needed to regulate pH, and is even a major contributor in regulating blood pressure.

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The proximal tubule mainly works as an absorption and re-absorption assembly line and manufactures ions such as bicarbonate ion and organic acids such as creatinine into the filtrate. Sodium transport carries the filtrate through the tubule, and the filtration is basically isotonic, where mechanisms that are energy dependent along the line absorb and reabsorb nutrients to achieve a proper balance of them in the blood. Most of the re-absorption takes place in the pars recta, where high concentrations of the nutrients gather and cause osmotic re-absorption of water.

When filtrate is received from Bowman’s capsule the epithelial cells within this structure are the transport for the movement and re-absorption of glucose and amino acids, as well as most of the water and some urea. Mitochondria also transport nutrients along where buffering phosphates can regulate the pH of the urine. Proton pumps then dispel protons into the urine for excretion.

The most common form of kidney cancer is renal cell carcinoma, which usually arises in the proximal tubule. In addition, nonmalignant acute tubular necrosis can occur when there are damages due to toxins or some antibiotics, pigments, and sepsis. Failures in absorption and re-absorption can be due to a disease known as Hartnup disease, which is a congenital gene maladaption that causes victims to be unable to absorb amino acids and thus fail to thrive. Other diseases are ischemia, which is a blood supply restriction causing necrosis of tissues and cells robbed of oxygen and nutrition from the blood, and interstitial nephritis, which is an inflammation of the tissues surrounding the proximal tubule, which, if unattended, can lead to kidney failure.

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