Problem 5 - Campbell Biology Concepts & Connections

Question

As filtrate passes through the loop of Henle, salt is reabsorbed and concentrated in the interstitial fluid of the medulla. This high solute concentration in the medulla enables nephrons to a. excrete the maximum amount of salt. b. neutralize toxins that might be found in the kidney. c. excrete a large amount of water. d. reabsorb water from the filtrate very efficiently.

Step-by-Step Solution

Answer

d. reabsorb water from the filtrate very efficiently.

Step 1 – Understanding the Loop of Henle

The loop of Henle in the nephron plays a key role in concentrating urine by reabsorbing salt (sodium and chloride ions). This process creates a high solute concentration in the medullary interstitial fluid.

Step 2 – Role of High Solute Concentration

The high solute concentration in the medullary interstitial fluid creates an osmotic gradient that draws water out of the filtrate back into the interstitial fluid as the filtrate moves through the collecting duct.

Step 3 – Identifying the Correct Function

The efficient reabsorption of water from the filtrate minimizes water loss, making urine more concentrated. This process does not primarily focus on excreting maximum salt, neutralizing toxins, or excreting large amounts of water.

Step 4 – Choosing the Correct Answer

Given the functions and the role of the loop of Henle in water reabsorption, the correct answer is: d. reabsorb water from the filtrate very efficiently.

Key Concepts

Water Reabsorption

Water reabsorption is a fundamental function of the kidneys which helps maintain the body's fluid balance.
The loop of Henle, a part of the nephron within the kidney, plays a crucial role in this process. As the filtrate (the fluid filtered from the blood) moves through the loop of Henle, it passes through different segments that facilitate water reabsorption.
The descending limb of the loop of Henle is permeable to water but not to salt. Water moves out of the filtrate into the interstitial fluid due to the osmotic gradient created by the high concentration of solutes in the medulla. This movement is driven by osmosis, where water follows the high solute concentration.
By the time the filtrate reaches the ascending limb of the loop, it becomes less concentrated because this segment is impermeable to water but actively reabsorbs salt.

• Descending limb: Water reabsorption occurs
• Ascending limb: Salt reabsorption occurs

The ultimate effect is the concentration of the urine, regulated by the nephron's reabsorption capacity and the body's fluid requirements.

Urine Concentration

Urine concentration is the final stage of the urine formation process, ensuring that the body maintains its water and electrolyte balance.
When the filtrate reaches the distal tubule and collecting duct, its concentration is fine-tuned to meet the body’s needs.
Urine concentration is heavily influenced by hormones like antidiuretic hormone (ADH). ADH increases the permeability of the collecting ducts to water, allowing more water to be reabsorbed into the bloodstream, and resulting in concentrated urine.
The process within the collecting ducts is controlled by the osmotic gradient established by the loop of Henle:

• High solute concentration in the medulla attracts water out of the filtrate.
• ADH increases water reabsorption to produce concentrated urine.

Concentrated urine ensures that the body does not lose excess water, which is vital for maintaining hydration and proper physiological functions.

Osmotic Gradient

The osmotic gradient is a key factor that drives water reabsorption in the kidneys.
This gradient is created by the loop of Henle, which sets up a high solute concentration in the medullary interstitial fluid.
The descending and ascending limbs of the loop of Henle work together to create this gradient:

• The descending limb allows water to leave the filtrate, increasing the solute concentration of the filtrate.
• The ascending limb actively pumps out salts but prevents water from leaving, contributing to a high solute concentration in the surrounding interstitial fluid.

This difference in solute concentration draws water out of the filtrate in the collecting ducts, led by the hyperosmolar environment of the medulla.
This process ensures that the kidneys can reabsorb water efficiently and regulate urine concentration under different hydration states of the body.
Thus, the osmotic gradient is essential for water conservation and proper kidney function.