Question

In lab rats studied in a state of antidiuresis, the urea concentration and osmotic pressure in the inner-medullary collecting-duct fluid were about the same as the urea concentration and osmotic pressure in the inner-medullary interstitial fluid. However, the concentration of \(\mathrm{Na}^{4}\) in the collecting-duct fluid was only about one-eighth of the \(\mathrm{Na}^{*}\) concentration in the interstitial fluid, and the collectingduct fluid was rich in \(\mathrm{K}^{+}\)even though the interstitial fluid contained hardly any \(\mathrm{K}^{+}\). How are these results to be explained by the difference in concentrating mechanisms for urea and for inorganic ions?

Short answer

The urea is reabsorbed in the collecting duct through passive diffusion, which results in similar concentrations in the duct and the interstitial fluid. In contrast, \(\mathrm{Na}^{+}\) is actively reabsorbed, which accounts for its lower concentration in the collecting duct. The high \(\mathrm{K}^{+}\) concentration in the collecting duct is due to secretion of \(\mathrm{K}^{+}\) from the interstitial fluid into the collecting duct.

Step-by-step solution

Understanding the given information

Firstly, identify all the given data. In this case, the given data points are the concentrations of urea, \(\mathrm{Na}^{+}\), and \(\mathrm{K}^{+}\) in both the inner-medullary collecting-duct fluid and the interstitial fluid.

Identify and understand the body's ion concentration mechanisms

The kidney used a variety of mechanisms to adjust the concentration of different substances. For urea, it is reabsorbed by the inner-medullary collecting-duct fluid through passive diffusion. Therefore, the concentrations in the collecting duct and the interstitial fluid would be similar, which aligns with the given information.

Contrast the concentration mechanisms between urea and Na+

On the other hand, the concentration of \(\mathrm{Na}^{+}\) in the collecting duct is much lower than that in the interstitial fluid. This is because \(\mathrm{Na}^{+}\) is reabsorbed actively, not passively. Therefore, it requires energy in the form of ATP to be reabsorbed, and this process is regulated by different hormonal and molecular signals.

Explain the presence of K+

As for \(\mathrm{K}^{+}\), it is secreted into the collecting duct fluid from the interstitial fluid, which explains its increased concentration in the collecting duct fluid despite its low concentration in the interstitial fluid.

Key concepts

Kidney Ion Concentration Mechanisms

Understanding how the kidneys regulate ion concentration is essential for comprehending how the body maintains homeostasis. In the renal system, the kidneys filter blood, reabsorb needed substances, and secrete waste to form urine.

The primary site of this action is the nephron, of which there are approximately one million in each human kidney. The concentration of ions is meticulously controlled through various parts of the nephron, including the glomerulus, proximal tubule, loop of Henle, distal convoluted tubule, and the collecting duct.

In the case of the lab rats studied, despite a high concentration of urea in both the inner-medullary collecting duct fluid and interstitial fluid, the sodium concentration in the collecting duct fluid is much lower compared to the interstitial fluid. This discrepancy highlights the selective nature of the kidney's ion concentration mechanisms, which favor certain ions over others based on the body's immediate needs.

The kidneys can conserve water by producing concentrated urine. This involves the reabsorption of a large amount of water from the filtrate, which is facilitated by osmotic gradients created by the active and passive transport of ions like sodium and chloride, particularly in the loop of Henle. These gradients help to achieve an environment where water can be reabsorbed effectively, concentrating urea and other waste products in the process.

Urea Reabsorption

Urea is a waste product formed from the metabolism of proteins and is excreted in the urine. The reabsorption of urea is a passive process; it occurs without the expenditure of energy because urea moves along its concentration gradient.

In the inner-medullary collecting duct, the lab rats exhibited similar urea concentrations in the duct fluid and interstitial fluid due to urea's reabsorption. As urea is reabsorbed along with water, it helps in creating the hyperosmotic conditions necessary for water reabsorption in adjacent segments of the nephron, in this case, the inner medullary region.

Urea reabsorption is particularly prominent when the body is in a state of antidiuresis, which refers to decreased urine production. This state is stimulated by the antidiuretic hormone (ADH), which increases the permeability of the collecting ducts to water and urea, allowing more of these substances to be reabsorbed and thus retaining water to maintain blood pressure and volume.

Sodium and Potassium Regulation in Kidneys

Regulation of sodium (a+) and potassium (a+) ions in the kidneys is a finely tuned process and crucial for maintaining electrolyte balance and blood pressure.

Sodium reabsorption is an active process; it requires energy, which comes from ATP hydrolysis. The presence of pumps in the nephron, like the Na+/K+ ATPase pump, actively transports Na+ out of the nephron and into the blood, which in turn facilitates the reabsorption of other substances by secondary active transport.

However, the potassium ion (a+) has a different story. In the collecting duct, particularly in the lab rats' case, potassium is secreted into the urine. This can serve to eliminate excess a+ from the body or to maintain acid-base balance. Dietary intake, the body's overall potassium load, and the hormone aldosterone influence the rate of a+ secretion.

Overall, sodium and potassium regulation involves several hormones, such as aldosterone, which increases sodium reabsorption and potassium excretion, and ADH, which can influence the permeability of the collecting ducts and help the body retain water as needed. Understanding these mechanisms is vital for treating conditions like hypertension and electrolyte imbalances.