In this article, we will talk about the microscopic anatomy part of the kidney known as the collecting duct system. The collecting duct system consists of a series of tubules and ducts that connect the nephrons to a minor calyx or to the renal pelvis. This collecting duct system, as the name implies, is responsible for the reabsorption and excretion of different electrolytes and for fluid balance in our bodies “collecting”.
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Water drop

Image : “Old drops” by Lóránt Szabó
. License: CC BY-SA 2.0


Overview

The collecting duct system functions are regulated by the activity of aldosterone and vasopressin. Aldosterone is released from the adrenal glands whereas vasopressin which is also known as the antidiuretic hormone is released from the posterior pituitary. Vasopressin is synthesized in the hypothalamus. The absence of antidiuretic hormone (ADH) results in a high concentration of aquaporin 2 in the storage vesicles present in the cytoplasm of collecting duct epithelial cells. Aquaporins are proteins produced within the membranes of vesicles that bud from the Golgi apparatus. They permit diffusion of water via the plasma membrane.

The main components of the collecting duct system are:

  • Connecting tubules
  • Cortical collecting ducts
  • Medullary collecting ducts

There are two functional types of cells within the collecting duct system, namely, principal cells and intercalated cells.

Microscopic Structure of the Collecting Duct System

The terminal portion of the distal convoluted tubule empties through collecting tubules that joins into a straight collecting duct in the medullary ray. Collecting ducts are different from other renal tubules by their prominent lateral borders of the epithelial cells in their structure. The collecting duct system consists of different segments that have their own characteristics and functions.

The Connecting Tubule

The connecting tubule is the most proximal part of the collecting duct system. It is adjacent to the distal convoluted tubule. The connecting tubules from different adjacent nephrons merge together to form what is known as cortical collecting tubules. The cortical collecting tubules merge together to form cortical collecting ducts.

Renal tubule

Image : “Scheme of renal tubule and its vascular supply” by Henry Gray. License: Public Domain

From an embryogenic point of view, the connecting tubules are the only part of the collecting duct system that derives from the metanephric blastema. The rest of the collecting duct system derives from the ureteric bud. Because of this embryogenic difference, there is currently a debate on whether the connecting tubules are part of the nephron or the collecting duct system.

The cortical collecting ducts receive filtrate from multiple initial collecting tubules. They descend into the renal medulla to form what is known as medullary collecting ducts.

The main function of the connecting tubule is the regulation of water and electrolytes including sodium and water. The connecting tubule is sensitive to vasopressin; however, the cortical collecting ducts are more sensitive compared to the connecting tubule to this hormone.

Medullary Collecting Ducts

These ducts can be divided into outer and inner segments. The inner segments reach more deeply into the renal medulla. The absorption of water, sodium, potassium, hydrogen, and bicarbonate continues here. This is tightly regulated by the action of the different regulatory hormones such as vasopressin and aldosterone and the fluid-balance status of the person. Presence of vasopressin enables the collecting ducts to become more permeable to water. The high osmotic pressure in the medulla is generated by the counter-current multiplier system in the loop of Henle which draws out water from the renal tubules, back to vasa recta.

Papillary Collecting Ducts

These structures were previously known as ducts of Bellini. They represent the most distal portion of the collecting duct system. They receive renal filtrate from the medullary collecting ducts and empty into a minor calyx. Papillary ducts also work in water reabsorption and electrolyte balance.

The cells that comprise the upper portion of the papillary collecting duct are similar to the cells found in the rest of the collecting system. They can be divided into principal and intercalated cells.  The cells of the lower papillary ducts near the papillary duct junction with the minor calyx derive from urothelium.

The absorption of water, sodium, urea, and excretion of hydrogen and potassium is again regulated by aldosterone and vasopressin.

Cells of the Collecting Duct System

All segments of the collecting duct system have intercalated cells except for the distal portion of the papillary collecting ducts. In addition to the intercalated cells, each segment of the collecting duct system has segment-specific cells. The different types of cells found in each segment of the collecting duct system in addition to the intercalated cells are given below:

Connecting tubule cell  Connecting tubulus
 Principal cell
  • Cortical collecting ducts
  • Medullary collecting ducts
  • Papillary collecting ducts
 Inner medullary collecting duct cells  Medullary collecting ducts

Because the two most common types of cells in the collecting duct system are the principal cells and the intercalated cells, we will limit our discussion to these two.

The Principal Cells of the Collecting Duct System

These cells are responsible for the mediation of the collecting duct’s influence on sodium and potassium homeostasis via sodium and potassium channels. The channels are found on the apical membrane.

Aldosterone regulates the expression of sodium channels by the principal cells. Increases in the levels of the hormone aldosterone result in an increase in the expression of sodium channels. Na/K-ATPase pump expression is also regulated by aldosterone. Therefore, an elevated level of aldosterone would result in an increase in the absorption of sodium and secretion of potassium by the principal cells of the collecting duct system.

The principal cells also express aquaporin channels which allow for water to pass through the principal cell. These channels’ expression is regulated by vasopressin. Therefore, both aldosterone and vasopressin regulate water and electrolyte homeostasis by the principal cells.

The Intercalated Cells of the Collecting Duct System

The intercalated cells can be classified into alpha and beta. They are responsible for acid-base homeostasis. Therefore, to emphasize, the principal cells are responsible for water and electrolyte homeostasis, whereas, the intercalated cells are involved in acid-base homeostasis.

The alpha-intercalated cells have apical H-ATPase and H/K exchanger channels or pumps. These pumps are responsible for the secretion of hydrogen ions. They are involved when the patient develops a state of acidosis. The alpha-intercalated cells reabsorb bicarbonate via band 3 which is part of the Cl/HCO3 exchanger. Damage to the alpha-intercalated cells’ ability to secrete hydrogen ions result in distal renal tubular acidosis which is known as renal tubular acidosis type I or classical renal tubular acidosis.

The beta-intercalated cells, on the other hand, are responsible for the secretion of bicarbonate via a specialized apical Cl/HCO3 exchanger known as pendrin. They reabsorb hydrogen ions via basal H/ATPase. Clearly, these cells are responsible for acid-base homeostasis in case of alkalosis.

A balance in the function of both types of intercalated cells would result in a state of physiologic acid-base homeostasis.

Water Absorption in the Collecting Duct System

An important note about water absorption in the collecting duct system needs to be emphasized. The osmotic gradient created by the counter-current multiplier system in the loop of Henle provides the force responsible for water reabsorption through the collecting ducts. The rate of this osmotic movement is greatly determined by the permeability of the collecting duct to water and it is directly proportional to the number of aquaporins in the collecting duct epithelial cells.

Therefore, this gives ADH its main role in collecting ducts which is to facilitate transportation of aquaporin 2 from storage vesicles in the cytoplasm to the plasma membrane, achieved when the antidiuretic hormone binds to its membrane receptors on the collecting duct to stimulate the fusion of aquaporin 2 with the membrane. This leads to increased permeability of the collecting ducts and resulting increase in the number of aquaporin 2 channels in the epithelial cells.

Insufficient ADH leads to dissociation of water channels via endocytosis.

Responsibilities of the collecting duct system

  • Reabsorbed up to 24 % of filtered water in renal filtrate in case of severe dehydration
  • Impermeable to water without the presence of the antidiuretic hormone

Therefore, if the antidiuretic hormone is absent, the patient will develop diuresis.

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