COLLECTING TUBULE: CHARACTERISTICS, FUNCTIONS, HISTOLOGY - ANATOMY AND PHYSIOLOGY - 2025

The collecting tubule is one of the regions of the uriniferous tubule of the vertebrate kidneys. Filtered material (urine) from the nephrons is discharged into this tubule.

The collecting tubules are involved in the change in urine concentration and direct it towards the collecting duct that empties into the smaller renal calyx, marking the beginning of the excretory duct.

Source: Modified from Kidney Nephron.png on Wikimedia Commons by Holly Fischer

Collecting tubules are found in the cortex of the kidneys and in the cortical labyrinths, which are the regions between the medullary rays. In cortical labyrinths the tubules connect with the collecting ducts.

characteristics

The collecting tubules are considered the distal segments of the nephrons and connect the distal convoluted tubules of the nephrons with a collecting duct. Numerous collecting tubules of different nephrons can lead to the same collecting duct.

They can have different lengths and shapes, in some cases they are short and moderately straight, called connecting tubules, or they can be longer and curved, receiving the name of arcuate collecting tubules.

These tubules originate in the cortical labyrinth, presenting some of the aforementioned forms, and reach the medullary radius when they join the collecting ducts.

Features

There are several cell types organized interspersed in the collecting tubules. In the cortical collecting tubule, the reabsorption of water, thanks to the permeability conferred by the clear cells, increases the concentration of urea in the filtrate that passes through the tubules.

After urea passes into the medullary canal, its high concentration and the action of specific transporters allow it to flow into the interstitial fluid, passing to the loop of Henle and back to the convoluted tubule and collecting tubule.

This recycling of urea contributes to the formation of a hyperosmotic renal medulla and thus increases the reabsorption of water and solutes, concentrating the urine.

Sodium / potassium balance

The tubule is involved in the reabsorption and excretion of water and some solutes such as K + and Na +. This region is important for the regulation of the Na + balance.

Aldosterone, a hormone found in the clear cells of the collecting tubules, regulates the sodium channels found in this segment. When this hormone allows the channels to open, almost 100% sodium is reabsorbed.

The accumulation of sodium generates a negative charge in the lumen of the tubule. This allows easier secretion of potassium and hydrogen ions (H ⁺). This mechanism occurs by stimulating the Na ⁺ / K ⁺ pump on the basolateral side of the membrane, in addition to increasing sodium permeability on the luminal side of the membrane.

Pathologies caused by failures in sodium balance

Aldosterone acts under two important stimuli: the increase in the concentration of potassium in the extracellular space and the increase in angiotensin II, associated with conditions of sodium loss or low blood pressure.

The inability to maintain sodium balance produces, in the human species, conditions such as Addison's disease, where there is a loss of sodium and an accumulation of potassium in the interstitial fluid, due to the absence of aldosterone.

On the other hand, in Conn's syndrome or adrenal tumor there is a high accumulation of sodium and a loss of potassium, caused by the very eminent secretion of potassium in the kidneys.

Histology

In the collecting duct some portions are differentiated, depending on the position they occupy in the regions of the kidney. Thus, the cortical collecting duct (CBT), the external medullary collecting duct (MSCT) and the medullary collecting duct (IMCT) are differentiated.

The TCME region is divided according to whether they are in the outer band (TCMEe) or the inner band (TCMEi).

Like the collecting ducts, the tubules are made up of a simple epithelium, with flattened cells with a pave to cubic shape.

Cell composition

There are two very well defined cell types in the tubules which are the light cells and the dark cells.

Clear cells or collecting duct (DC) cells are the main cells of the urinary system. These cells are pale and contain basal folds that replace the processes with which the cells intertwine.

They have a primary cilium or monocilium, some short microvilli, and small spheroidal mitochondria.

CD cells have a large number of aqueous channels (aquaporin 2 or AQP-2), which are regulated by ADH (antidiuretic hormone). These aquaporins confer a high water permeability to the tubules, in addition to having aquaporin 3 and 4 (AQP-3, AQP-4) in the basolateral membranes of the cells.

Dark cells or intercalary cells (IC) are less abundant in these structures. They have a dense cytoplasm and abundant mitochondria. They present cytoplasmic micro-folds on the apical surface and microvilli, in addition to interdigitations with neighboring cells. The apical cytoplasm contains a large number of vesicles.

IC cells participate in the secretion of H + (intercalary cells α or A) or bicarbonate (intercalary cells β or B), depending on whether the kidneys must excrete acids or alkaloids.

Type A intercalated cells

Intercalated cells are found in the TCC, TCME regions. In IMCT, they are found to a lesser extent and progressively decrease as the tubule approaches the papillary collecting duct.

Type A cells are involved in the secretion of H ⁺ and ammonia and the reabsorption of bicarbonate. The protein composition of these cells differs from those of the convoluted tubules and the thick branches of the loop of Henle.

The H ⁺ -ATPase protein is found in the apical plasma membranes and is responsible for secreting H ⁺, in addition to having an important role in the maintenance of cell volume and the regulation of electronegativity, replacing the function of the Na ⁺ / K pump ⁺.

Another mechanism of H ⁺ secretion is electro-neutral, and it depends on the negativity existing in the lumen of the tubule due to sodium accumulation.

Type B intercalated cells

These cells are involved in the secretion of bicarbonate and reabsorption of Cl ^- towards the lumen of the tubule. It has a protein responsible for the exchange between Cl ^- and bicarbonate called pedrin.

They also present H + -ATPase in cell vesicles that is responsible for maintaining cell electronegativity, although these proteins are not found in the plasma membrane.

Cytoplasmic AQP-2 is found in type B intercalary cells, which is involved in the production of cytoplasmic H ⁺ and bicarbonate.

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