If the plasma concentration of a freely filterable substance is 2 mg/mL, GFR is 100 mL/min, urine concentration of the substance is 10 mg/mL, and urine flow rate is 5 mL/min, we can conclude that the kidney tubules
**Question:** If the plasma concentration of a freely filterable substance is 2 mg/mL, GFR is 100 mL/min, urine concentration of the substance is 10 mg/mL, and urine flow rate is 5 mL/min, we can conclude that the kidney tubules
A. reabsorb more than the substance is secreted
B. reabsorb less than the substance is secreted
C. reabsorb the same amount of the substance as it is secreted
D. reabsorb more than the substance is secreted
**Correct Answer:** C. reabsorb the same amount of the substance as it is secreted
**Core Concept:** Glomerular Filtration Rate (GFR) and Tubular Function
The correct answer is C, as the kidney tubules reabsorb the same amount of the freely filterable substance as it is secreted. This is a crucial aspect of renal tubular function.
**Why the Correct Answer is Right:**
1. **GFR (Glomerular Filtration Rate):** GFR is the rate at which the substance enters the tubules from the glomerulus. In this scenario, we have a GFR of 100 mL/min, meaning 100 mL of the substance enters the tubules per minute.
2. **Urine Concentration:** Urine concentration is the ratio of the substance concentration in urine to plasma concentration. Here, the urine concentration is 10 mg/mL, which is higher than the plasma concentration of 2 mg/mL. This indicates that the kidney tubules reabsorb the substance from the filtrate, thus concentrating the urine.
3. **Urine Flow Rate:** Urine flow rate is the rate at which urine is formed in milliliters per minute. Given as 5 mL/min, the urine flow rate is lower than the substance secretion rate (100 mL/min). This implies that the substance is secreted into the tubules, which leads to an increase in urine concentration.
4. **Tubular Secretion:** Tubular secretion is the excretion of substances from the filtrate into the tubule. In this case, the substance is secreted into the tubule at a rate of 100 mg/min.
5. **Tubular Reabsorption:** Tubular reabsorption is the process of substances being taken back from the tubule into the bloodstream to maintain homeostasis. In this scenario, the substance is reabsorbed at a rate of 100 mg/min, which is equal to the rate of tubular secretion.
**Why Each Wrong Answer is Incorrect:**
A. **Option A:** If the tubules reabsorbed more substance (secretory rate reabsorptive rate), urine concentration would be lower (not higher) than plasma concentration. This is also not the case in this scenario.
C. **Option C:** Plasma concentration (2 mg