Question

Which of the following statements is/are true? 1\. Urine is hypertonic in distal convoluted tubule. 2\. When the urine passes into the collecting tubule it becomes hypotonic. 3\. Urine is isotonic in proximal convoluted tubule. 4\. Urine becomes more and more hypotonic as it passes through the Henle's loop. (a) \(\mathrm{I}\) and 4 only (b) 1,2 and 3 only (c) 2 and 3 only (d) 3 only

Short answer

The correct answer is (d) - Statement 3 only is true.

Step-by-step solution

Analysing Statement 1

Statement 1 states 'Urine is hypertonic in distal convoluted tubule.' With the function of the distal convoluted tubule in mind, which includes reabsorption of sodium ions, chloride ions, and water, resulting in the concentration of the urine. Thus, statement 1 is True.

Analysing Statement 2

Statement 2 says 'When the urine passes into the collecting tubule it becomes hypotonic.' With respect to the function of the collecting tubule, we know that the final concentration of urine depends on the body's hydration level. The collecting tubule has the ability to both dilute and concentrate urine, but in most cases, the urine becomes hypertonic, not hypotonic, as a final urine production step. Thus, statement 2 is False.

Analysing Statement 3

Statement 3 claims 'Urine is isotonic in proximal convoluted tubule'. This matches our understanding of proximal convoluted tubule function, which allows both solute and water reabsorption at the same time, keeping the urine mostly isotonic. This means statement 3 is True.

Analysing Statement 4

Statement 4 states 'Urine becomes more and more hypotonic as it passes through the Henle's loop.' The primary purpose of Henle's loop is to create a concentration gradient. The descending limb of the loop of Henle is permeable to water but not solutes, resulting in increased concentration (hypertonic solution), not decreased. Thus, statement 4 is False.

Identifying the Correct Answer

Looking at the options given, the correct answer is the one which includes the true statements. So, option (d) including statement 3 only, is the correct answer.

Key concepts

Distal Convoluted Tubule

The distal convoluted tubule (DCT) plays a crucial role in the kidney's ability to concentrate urine. At this stage, reabsorption of sodium ions, chloride ions, and water takes place. During this process, the liquid, which will eventually become urine, is concentrated. This happens because the DCT has various ion channels and transporters that facilitate the movement of ions back into the blood. This not only helps in balancing the body's electrolyte levels but also ensures that water follows these ions osmotically. Consequently, the urine becomes hypertonic at this stage before moving on to the next sections of the nephron.

Proximal Convoluted Tubule

The proximal convoluted tubule (PCT) is the first segment of the renal tubule. Here, a substantial reabsorption of filtered substances occurs. Approximately two-thirds of the filtered salt and water, along with nutrients such as glucose and amino acids, are reabsorbed. The reabsorption process is isosmotic, meaning both solutes and water leave together at the same rate. This characteristic of the PCT maintains the isotonic nature of the fluid, making the urine here neither more nor less concentrated than the blood plasma at this stage.

Henle's Loop

Henle's Loop is a critical structure in the nephron, responsible for creating a concentration gradient in the kidney's medulla. This loop has two limbs: descending and ascending. The descending limb is permeable to water but not to solutes, allowing water to exit, making the fluid inside more concentrated or hypertonic. Conversely, the ascending limb is impermeable to water but transports sodium, potassium, and chloride ions from the tubular fluid to the medullary interstitium. This unique arrangement helps establish an osmotic gradient essential for water reabsorption elsewhere in the nephron. Hence, the concept that urine becomes more hypotonic in Henle’s Loop is incorrect.

Urine Concentration

Urine concentration is a dynamic process influenced by various elements of the nephron, particularly the collecting duct system. The body regulates urine concentration based on hydration levels and hormonal signals, such as antidiuretic hormone (ADH). ADH increase permeability of water in the collecting ducts, resulting in reabsorption of water and the concentration of urine to conserve body water. Conversely, when the body is well-hydrated, ADH levels drop, leading to the production of dilute urine. Ultimately, this process ensures that the balance of water and electrolytes in the body remains stable.

Collecting Tubule

The collecting tubule, often referred to as the collecting duct, is the final segment in the nephron where the composition of urine can be either concentrated or diluted further. This variability is dependent upon the body's needs at any given time, particularly its hydration status. The collecting tubule is responsive to hormones like ADH, which increases its permeability to water, promoting water reabsorption, hence concentrating the urine if the body needs to conserve water. Alternatively, if the body needs to release excess water, the collecting tubule can help produce a more diluted urine. The versatility of the collecting tubule thus plays a key role in fluid and electrolyte homeostasis.