Question

Assertion: Counter-current mechanism is responsible for the concentration of urine. Reason: Counter-current mechanism helps to maintain the concentration gradient in the medullary interstitium.

Short answer

Yes, the counter-current mechanism is indeed responsible for the concentration of urine. By preserving the concentration gradient in the medullary interstitium, it helps facilitate water reabsorption from the urine, leading to a more concentrated urine.

Step-by-step solution

Understanding the counter-current mechanism

The counter-current mechanism is a physiological process in the nephrons of the kidneys. This mechanism involves the flow of fluids in opposite directions which allows the exchange and preservation of heat or solutes. In the context of urine concentration, it plays a crucial role in the reabsorption of water and solutes, thus influencing the concentration of urine.

Understanding the medullary interstitium and concentration gradient

The medullary interstitium is the tissue surrounding the loop of Henle in the nephrons of the kidneys. This area has a gradient where the concentration of solutes is higher towards the inner medulla compared to the outer medulla and cortex. This gradient is vital for reabsorption of water from the urine, which ultimately determines the concentration of urine.

Connecting the counter-current mechanism and concentration of urine

The counter-current mechanism, via the loop of Henle, helps preserve the concentration gradient in the medullary interstitium. The descending limb of the loop of Henle is permeable to water but not to solutes, leading to an increase in solute concentration (hypertonic urine). The ascending limb is impermeable to water, but permeable to solutes, thus the filtrate becomes dilute (hypotonic urine). Thus, counter-current mechanism contributes to the concentration of the urine by maintaining high solute concentration in the medullary interstitial fluid which promotes removal of water from the urine.

Key concepts

Urine Concentration

Urine concentration refers to the process the kidneys use to regulate the amount of water and solutes excreted in the urine. The kidneys adjust the amount and concentration of urine based on the body's hydration levels and salt balance, ensuring homeostasis is maintained. This regulation is crucial for:

• Maintaining proper blood pressure.
• Balancing electrolytes in the body.
• Removing waste products efficiently.

The key player in this process is the nephron, particularly the loop of Henle, which operates using the counter-current mechanism. This allows for high urine concentration when the body is dehydrated, conserving water. Conversely, when excess water is present, the urine produced is more dilute. The ability to change the urine concentration so drastically is a testament to the efficiency of the counter-current mechanism used by the kidneys, enabling them to respond promptly to changes in the body's needs.

Medullary Interstitium

The medullary interstitium is the tissue surrounding the loop of Henle within the kidney. It plays an essential role in the kidney's ability to concentrate urine through its unique gradient of solutes. Here's why it matters:

• The outer region has a lower solute concentration compared to the inner medulla, creating a concentration gradient.
• This gradient is critical for reabsorbing water from the filtrate within the nephron.
• The difference in solute concentration ensures that water can be effectively reabsorbed from the descending limb of the loop of Henle.

By maintaining a gradient where solute concentration increases deeper into the medulla, the kidney can adjust the amount of water reabsorbed to fit the body's needs. This capability is one of the reasons why humans can survive in varying hydration states, as it helps the kidneys either conserve water in times of scarcity or expel excess in times of plenty.

Loop of Henle

The loop of Henle, a critical component of the nephron, functions with the counter-current mechanism to effectively concentrate urine. It's structure includes:

• The descending limb which is highly permeable to water but not to solutes.
• The ascending limb which is impermeable to water but allows solutes to pass.

This arrangement ensures that as the filtrate travels down the descending limb, water is reabsorbed due to the high solute concentration in the medullary interstitium. This process results in a higher concentration of solutes in the urine.
In contrast, as the filtrate moves up the ascending limb, solutes are removed while water remains, leading to a more diluted filtrate by the time it reaches the distal convoluted tubule.
Through these dynamic changes in permeability and reabsorption, the loop of Henle not only contributes to creating a concentrated urine but also plays a vital role in maintaining the body's overall fluid balance and solute homeostasis.