Biology · Excretory Products and Their Elimination · NEET
The collecting duct reabsorbs large amounts of water from the filtrate as it passes through the salty medulla. This makes the urine concentrated and saves water for the body. NCERT states large amounts of water can be reabsorbed from this region to produce concentrated urine.
Mainly water. As the collecting duct runs from the cortex deep into the medulla, it passes through the high-osmolarity (300 to 1200 mOsmol/L) region. Because the surrounding medulla is very salty, water moves out of the duct by osmosis. It does not primarily reabsorb salt; salt handling is done more by the loop of Henle and DCT.
ADH (antidiuretic hormone, also called vasopressin) makes the walls of the collecting duct more permeable to water. When ADH is high (body low on water), more water is reabsorbed and urine is concentrated. When ADH is low (excess water), less water is reabsorbed and urine is dilute. So ADH controls how much water the collecting duct saves.
The collecting duct allows a small amount of urea to leak into the medullary interstitium. This added urea helps keep the medulla very concentrated (helps maintain the osmolarity gradient). That high osmolarity is exactly what pulls water out of the collecting duct, so this urea recycling supports concentrated-urine formation.
Technically the collecting duct is a shared tube: the DCTs of many nephrons open into one common collecting duct. So it is not the personal tubule of a single nephron, but for NEET it is studied as part of the urine-concentrating pathway. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids.
The DCT does conditional reabsorption of Na+ and water and reabsorbs HCO3-. The collecting duct is longer, extends from cortex deep into the medulla, does the major ADH-driven water reabsorption to concentrate urine, passes urea into the medulla, and secretes H+ and K+ for pH and ionic balance. NCERT groups DCT and collecting duct together for water/electrolyte reabsorption and H+/K+/NH3 secretion.
Which of the following factors is responsible for the formation of concentrated urine?
Which of the following statements are correct? A. Excessive loss of body fluid switches off osmoreceptors. B. ADH facilitates water reabsorption to prevent diuresis. C. ANF causes vasodilation. D. ADH causes increase in blood pressure. E. ADH is responsible for decrease in GFR.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) Reabsorb large amounts of water (ADH-controlled) to make concentrated urine. 2) Pass small amounts of urea into the medullary interstitium to maintain osmolarity. 3) Secrete H+ and K+ to help maintain blood pH and ionic balance.
It extends from the cortex of the kidney down into the inner parts of the medulla. Many collecting ducts converge and open into the renal pelvis through the medullary pyramids.
Without ADH the collecting duct becomes less permeable to water, so little water is reabsorbed and large volumes of dilute urine are passed. This is what happens in diabetes insipidus.
Yes. It selectively secretes H+ and K+ ions into the filtrate, which helps maintain the pH and ionic balance of the blood.
Together they concentrate the filtrate about four times, from about 300 mOsmol/L to about 1200 mOsmol/L, which is an excellent mechanism for conserving water.
