The proximal convoluted tubule (PCT), also called the proximal convoluted tubule, is a small tubular structure found in the nephrons of the kidney. The TCP is part of the proximal tubule, which connects Bowman's capsule to the loop of Henle. TCP, as part of the tubular system of the kidney, has an important role in the formation of urine and therefore in the homeostasis of the organism.
Description and histological structure
Each kidney contains about 4 million functional units, the nephrons. In each nephron, a blood filter is made from which urine is formed and which serves to eliminate waste substances. Roughly, each nephron is composed of the renal glomerulus, Bowman's capsule, the proximal tubule, the loop of Hanle, the distal tubule, and the collecting ducts.
In the proximal tubule, a straight part can be differentiated, called the proximal straight tubule or pars straightand a convoluted part, that corresponding to the proximal convoluted tubule or pars convolute (hence also known as the proximal convoluted tubule). The TCP is the portion of the proximal tubule closest to Bowman's capsule and connects Bowman's capsule to the proximal straight tubule, which in turn connects to the loop of Henle. On a larger scale, the TCP is located in the renal cortex while the straight part descends along the medullary rays towards the medulla.
From a functional point of view, the proximal tubule is divided by some authors into three portions called S1, S2 and S3. The TCP would correspond to the S1 portion, which constitutes approximately two thirds of the TCP, and the first part of the S2.
The epithelial tissue of the TCP is of the cubic type and the luminal surface is covered with numerous and compact microvilli that give the "brush edge" appearance characteristic of the proximal tubule and that is easily observable under the light microscope. TCP cells show complex interconnections with each other, allowing paracellular ion transport, which will play an important role in their function. The cytoplasm is acidophilic, abundant and has a high number of mitochondria.
In the S2 portion, the brush border is less developed, presenting fewer and shorter villi. The interconnections become less complex and the number of mitochondria is smaller. The transition from the S1 portion to the S2 portion is gradual.
It must be taken into account that some authors call the pars straight of the proximal tubule as the thick descending limb of the loop of Henle and use the term proximal tubule as a synonym for proximal convoluted tubule.
Function
The proximal convoluted tubule has a fundamental role in the homeostasis of the organism playing an active role in the reabsorption of water, electrolytes and various organic compounds. This reabsorption is carried out by active transport and passive transport through numerous protein channels on the epithelial surface. It occurs in all portions of the proximal tubule but is especially important in the PCT. TCP also secretes a variety of substances, including some organic acids, nitrogenous bases, and some waste substances that are not filtered at the glomerulus, such as some drugs.
Absorption
The glomerular filtrate is collected in Bowman's capsule and passes directly to the TCP, where between 40 and 60% of the filtrate is reabsorbed. Substances in the filtrate with greater nutritional importance, especially glucose and amino acids, are practically completely reabsorbed by active co-transporters. Potassium is reabsrove by 65%, urea by 50% and phosphate and citrate by around 80%.
Water and salt (ClNa) are reabsorbed in the proximal convoluted tubule in a variable way, around 60%, depending on the conditions in which the organism is found, since water and ClNa are key in the regulation of body water volume. Sodium is reabsorbed primarily in the proximal convoluted tubule, while chloride is reabsorbed primarily in the proximal convoluted tubule. pars straightespecially in the S3 segment, by passive diffusion.
Secretion
Most of the ammonia that is excreted in the urine is secreted in the TCP. Various unfiltered waste substances are also secreted here in the glomerulus. For example, creatinine (the breakdown product of creatine) and other nitrogenous bases, toxins, and many drugs are eliminated by secretion in the TCP.
medical implications
The epithelial cells of the proximal tubule are implicated in many kidney diseases, some very serious, such as renal cell carcinomathe most common type of kidney cancer and whose origin is frequently located in the convoluted tubules3.
The acute tubular necrosis it can occur in TCP as a consequence of direct tissue damage caused by toxins, infectious processes or some drugs (for example, gentamicin). Damage to the TCP can also be caused by many other causes, such as glomerulonephritis, ischemia, trauma or diabetic nephropathy, thus participating in the progress and development of these diseases.
The Fanconi syndrome (not to be confused with Fanconi anemia) is a disease with a possible genetic origin that is characterized by alteration of the proximal tubules whose function is diminished. As a consequence, abnormally high amounts of glucose, amino acids, phosphates, and bicarbonate are excreted, causing symptoms similar to those of tubular acidosis.
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