Question

Substance X has a clearance greater than zero but less than that of inulin. What can you conclude about substance X? a. It is not filtered. b. It is filtered, but neither reabsorbed nor secreted. c. It is filtered and partially reabsorbed. d. It is filtered and secreted.

Short answer

Substance X can be concluded to be filtered and partially reabsorbed. This is because its clearance is greater than zero (meaning it is filtered) but less than that of inulin, which indicates some reabsorption.

Step-by-step solution

Understand the clearance of substances

Clearance is the volume of plasma from which a substance is completely removed per unit time, usually expressed in mL/min. In the context of kidney function, clearance is used as a measure of the efficiency with which the kidneys remove a substance from the blood. Inulin is a substance that is only filtered by the kidneys and not reabsorbed or secreted, making it a good marker to measure glomerular filtration rate (GFR). Since Substance X has a clearance greater than zero but less than that of inulin, we can compare its clearance with inulin's clearance to determine its behavior within the kidney.

Analyze the given information

Based on the given information: 1. Substance X has a clearance greater than zero, which means that it is not completely retained in plasma and is filtered by the kidneys to some extent. 2. Substance X has a clearance less than that of inulin, which implies some difference in the behavior compared to inulin.

Compare the clearance of Substance X with the options

We will now compare the given information about Substance X's clearance with the four options provided: a. Substance X is not filtered: This option contradicts the information that Substance X has clearance greater than zero. b. Substance X is filtered, but neither reabsorbed nor secreted: This option suggests that Substance X behaves similarly to inulin. However, we know that the clearance of Substance X is less than that of inulin, so this option is also incorrect. c. Substance X is filtered and partially reabsorbed: If Substance X was filtered and partially reabsorbed by the kidneys, its clearance would indeed be less than that of inulin, which is only filtered. This is consistent with the given information. d. Substance X is filtered and secreted: If Substance X was filtered and secreted, it would imply that its clearance would be greater than that of inulin, as more of the substance would be removed from plasma than just through filtration. However, we know that Substance X has a clearance less than that of inulin, so this option is not correct.

Conclude the behavior of Substance X

Based on the comparison with the given information, we can conclude that option c (Substance X is filtered and partially reabsorbed) is consistent with the given details about Substance X's clearance.

Key concepts

Glomerular Filtration Rate

The glomerular filtration rate (GFR) is a crucial indicator of kidney function. It reflects how well the kidneys filter blood. GFR is the amount of blood filtered by the glomeruli—the tiny filters in the kidney—each minute.
In simple terms, if GFR is high, the kidneys are filtering blood efficiently. A low GFR indicates potential issues with kidney function.

GFR is measured in milliliters per minute (mL/min) and can be estimated using markers like inulin. Inulin is ideal because it is neither reabsorbed nor secreted after filtration, so its clearance equals the GFR.

When a substance has a clearance rate lower than inulin, it suggests that the kidney is either reabsorbing some of it back into circulation or handling it differently. Understanding GFR helps in diagnosing and monitoring kidney diseases.

Kidney Function

Kidney function refers to the nephrons' ability within the kidneys to filter blood and maintain chemical balance. This filtering process removes waste products and excess substances, keeping vital nutrients in the body.

Key aspects of kidney function include:

• Filtration: The process by which blood plasma is filtered through the glomeruli.
• Reabsorption: The return of necessary substances from the filtrate back into the bloodstream.
• Secretion: The transfer of additional waste products from the blood into the filtrate.

Kidneys play a critical role in homeostasis by regulating blood pressure, electrolyte balance, and toxin clearance. When kidney function declines, it can lead to health issues like hypertension and toxin build-up.
Testing for substances like creatinine and using markers such as inulin help evaluate how well kidneys perform their filtration task.

Substance Reabsorption

Substance reabsorption is a key process in which the kidneys recover essential molecules from the filtered fluid. This process prevents the loss of valuable nutrients and maintains electrolyte balance in the body.

Reabsorption happens mostly in the renal tubules after the initial filtration in the glomeruli. Key components typically reabsorbed include:

• Water: Helps maintain body fluid balance.
• Glucose: Necessary for energy.
• Electrolytes: Such as sodium and potassium, crucial for various body functions.

If a substance is filtered by kidneys but shows a clearance rate lower than inulin, it suggests partial reabsorption. This means that some of the substance is taken back into the bloodstream after filtration.

Ultimately, understanding reabsorption aids in assessing how kidneys manage and conserve essential nutrients, indirectly affecting overall kidney health.