Question

Counter current mechanism maintains the concentration gradient in the medullary interstitium. It helps in (a) Easy passage of water from PCT (b) Easy passage of water from DCT (c) Easy passage of water from HL (d) Easy passage of water from collecting duct

Short answer

The correct answer is (d) Easy passage of water from collecting duct.

Step-by-step solution

Understand the Counter Current Mechanism in the Kidney

The counter current mechanism in the kidney is a process by which the kidney absorbs salts and concentrates urine. This mechanism creates a concentration gradient in the medullary interstitium which is a key part in regulating water and electrolyte balance.

Identify the role of different parts of the Nephron

Proximal Convoluted Tubule (PCT): Major site for reabsorption of water, glucose, and ionic salts.\n Distal Convoluted Tubule (DCT): Functions in the selective secretion and absorption of ions and water based on hormonal signals.\n Henle's Loop (HL): Creates a solute concentration gradient in the medulla of the kidney.\n Collecting Duct: Responsible for the final concentration of urine. Water permeability is determined by the hormone ADH.

Match the Options with the Counter Current Mechanism

The mechanism promotes the reabsorption of water in the parts of the nephron where ADH-induced water channels are present. The high concentration in the medullary interstitium is crucial for water reabsorption in the collecting duct. The option aligning with this understanding is the 'easy passage of water from collecting duct'.

Key concepts

Nephron Function

Nephrons are the functional units of the kidney, and they play a crucial role in filtering blood and forming urine. Each nephron is composed of several parts, including the Proximal Convoluted Tubule (PCT), Distal Convoluted Tubule (DCT), Henle's Loop (HL), and the Collecting Duct.

• The PCT is the starting point where the majority of reabsorption occurs. Water, glucose, and ions such as sodium are reabsorbed here. This section helps reduce the filtrate volume significantly.
• The DCT is involved in the fine-tuning of electrolyte and pH balance. Hormones like Aldosterone and ADH influence this part to reabsorb or secrete various ions and water.
• Henle’s Loop plays a key role in establishing a concentration gradient with its descending and ascending limbs. It's vital for the counter current mechanism, which aids in concentrating urine.
• The Collecting Duct is the final part of the nephron that regulates water reabsorption based on the body’s needs, largely influenced by ADH.

Nephrons help regulate the body's water, electrolyte balance, and waste elimination, all crucial for homeostasis.

Medullary Interstitium

The medullary interstitium is the area in the kidney's medulla that surrounds Henle’s Loop and the Collecting Ducts. It plays a vital role in the counter current mechanism by maintaining a concentration gradient that is essential for water reabsorption.
This gradient is created by the differential permeability of the membranes of the nephron's various sections, especially Henle's Loop.

• The descending limb of Henle's Loop is permeable to water but not to ions, allowing water to move out and into the hypertonic medullary interstitium.
• The ascending limb, on the other hand, is impermeable to water but actively pumps ions into the medullary interstitium, further increasing its concentration.

This set-up ensures that the collecting ducts are surrounded by a hypertonic environment, which is crucial for the reabsorption of water and concentration of urine.

Water Reabsorption

Water reabsorption in the kidney is a finely regulated process that happens predominantly in the nephron. It is driven by the counter current mechanism and influenced by hormones like ADH.
In the Proximal Convoluted Tubule, about 70% of water is reabsorbed back into the bloodstream passively. The water moves along an osmotic gradient, driven by the active transport of sodium and other solutes.

• In the Loop of Henle, the descending limb allows more water to be reabsorbed, contributing to the creation of a concentrated medullary interstitium.
• As the filtrate travels up the ascending limb, ions are actively transported out, but water remains, as this limb is impermeable to it.

In the Collecting Duct, ADH regulates water permeability. When ADH is present, water channels called aquaporins open, allowing significant reabsorption due to the existing gradient. This leads to the production of more concentrated urine, conserving water.