Question

Read the given statements and select the correct option. Statement 1 : The final reabsorption of water from the urine into the blood occurs through the collecting duct of a mammalian nephron resulting in the production of hyperosmotic urine. Statement \(2:\) The loop of Henle creates a sodium gradient in the interstitial fluid. (a) Both statements 1 and 2 are correct and statement 2 is the correct explanation of statement 1 . (b) Both statements 1 and 2 are correct but statement 2 is not the correct explanation of statement 1 . (c) Statement 1 is correct and statement 2 is incorrect. (d) Both statements 1 and 2 are incorrect.

Short answer

Both statements 1 and 2 are correct, and statement 2 is the correct explanation of statement 1, making option (a) the correct choice.

Step-by-step solution

Review Statement 1

Assess the accuracy of Statement 1: The final reabsorption of water from the urine into the blood does indeed occur mainly in the collecting duct of a mammalian nephron due to the action of antidiuretic hormone (ADH). This process allows for the concentration of the urine and the production of hyperosmotic urine if necessary, to retain water in the body.

Review Statement 2

Examine the accuracy of Statement 2: The loop of Henle, particularly its descending and ascending limbs, plays a crucial role in the creation of a concentrated medullary interstitial fluid, which includes a sodium gradient. The descending limb is permeable to water but not to solutes, whereas the ascending limb actively pumps out sodium ions (Na+) but is impermeable to water. This action contributes to the increasing osmolarity of the interstitial fluid as you move deeper into the medulla.

Examine the Relationship Between the Statements

Determine the connection between Statements 1 and 2: Statement 2 does indeed help to explain Statement 1 because the sodium gradient created by the loop of Henle in the interstitial fluid of the kidney medulla is what allows water to be reabsorbed into the bloodstream in the collecting duct. The interstitial fluid's hyperosmotic nature draws water out of the collecting duct, concentrating the urine.

Select the Correct Option

Based on the analysis of the two statements and their relationship, we can conclude that both statements are correct, and statement 2 provides the correct explanation for statement 1. Therefore, the correct option is (a).

Key concepts

Collecting Duct

The collecting duct plays a crucial role in the regulation of water and electrolyte balance within the body. It functions as the last segment of the nephron's urine processing system, where the final reabsorption of water occurs. This process is heavily influenced by the antidiuretic hormone (ADH).

In response to high blood osmolarity, ADH is released from the pituitary gland, making the walls of the collecting duct more permeable to water. This increased permeability allows water to be reabsorbed from the urine back into the bloodstream, thereby reducing urine volume and increasing its concentration. The ability of the kidneys to produce hyperosmotic urine through this mechanism is fundamental for maintaining water homeostasis, particularly in environments where water intake is limited or in periods of dehydration.

Moreover, urea recycling within the collecting duct also contributes to the urine concentration process, as it enhances the medullary osmotic gradient that draws water out of the collecting duct. This intricate balance ensures our bodies conserve water effectively, highlighting the significance of the collecting duct in kidney function.

Loop of Henle

The loop of Henle serves as an integral part of the nephron, specifically designed to create a high osmolarity environment in the renal medulla, which is essential for urine concentration. It is typically divided into two limbs: the descending limb, which is permeable to water but not to solutes, and the ascending limb, which actively transports sodium out while being impermeable to water.

As filtrate descends into the deeper parts of the medulla via the descending limb, water is drawn out into the surrounding hyperosmotic interstitial space, concentrating the filtrate. In contrast, the ascending limb pumps sodium ions (Na+) into the interstitial space, which not only contributes to the osmotic gradient but also sets the stage for further water reabsorption downstream in the nephron. This function is so critical that it creates a counter-current multiplier system that amplifies the osmotic gradient in the medullary interstitium, ensuring efficient water reabsorption and urine concentration.

Urine Concentration

Urine concentration is the process by which the kidneys adjust the volume and osmolality of urine. This process is vital for conserving water and maintaining a stable internal environment within the body. It is achieved through the coordinated actions of various parts of the nephron, including the collecting duct and the loop of Henle.

When the body needs to conserve water, the collecting duct reabsorbs more water back into the bloodstream, which leads to the production of concentrated urine. The loop of Henle plays a foundational role in this process by establishing an osmotic gradient that allows the collecting duct to reclaim water efficiently. Factors like ADH and the sodium-potassium pumps in the nephron's tubules enable this delicate balance, aiding in the production of urine that is hyperosmotic to the plasma when necessary.

Sodium Gradient

The sodium gradient in the kidney is strategically used to drive the reabsorption of water and other solutes. Generated by the cells of the ascending limb of the loop of Henle, it is a crucial component of the countercurrent multiplier system that concentrates urine.

Sodium and chloride ions are actively transported out of the ascending limb into the interstitial space, raising its osmolarity. The descending limb of the loop of Henle, being permeable to water but not to solutes, responds to this gradient by allowing water to exit the filtrate, thus concentrating the remaining solute. This interplay between the limbs of the loop of Henle establishes a strong osmotic gradient, where the concentration of solutes is greatest in the deepest part of the medulla. The existence of this gradient enables the kidney to vary urine concentration remarkably, ensuring that the body can conserve water under different conditions of hydration and dietary intake.