Absorption in the Kidneys

The data in Table 1 represent the results of some blood tests for a variety of solutes normally found in the blood. As you know, when blood passes through the kidneys, some solutes are removed while others remain. The data apply when 1 L of urine is formed.

Table 1 Amounts of various compounds in blood and urine

Questions

1. For each substance, calculate the amount left in the urine.

Amount in urine = amount initially filtered out - amount returned to blood.

2. a) For each substance, calculate the percent reabsorbed into the blood.

Percent reabsorbed = Amount returned to bloodstream x 100

                                         amount initially filtered out

3. a) On a percentage basis, which of these five substances is most efficiently removed from the blood? (urea)

b) On a percentage basis, which two substances are returned to the blood with the highest efficiency? (glucose and water)

4. What do the answers to question 3 suggest about the importance of these substances?

5. a) How does the body benefit by reabsorbing so much water and so much glucose back into the bloodstream? (glucose is primary fuel so body does not want to waste it)

b) All glucose is normally returned by the kidneys to the bloodstream. Why, then, do you need to keep eating foods that are a source of new glucose? (glucose is used as energy source)

6. a) If you had data for someone on a low salt diet, explain the changes you would expect to see in the amount of reabsorption of certain substances. (less chloride and more water in the urine; this would benecessary to keep the [salt] constant)

b) If you had data for someone on a high protein diet, explain the changes you would expect to see in the amount of reabsorption of certain substances. (more urea and uric acid; these are both by-products ofprotein metabolism)

7. a) What might the presence of glucose in urine say about levels of glucose in the blood? (too high;kidneys are trying to maintain balance)

b) What disease could cause this condition? (diabetes)

Fluids from Bowman’s capsule, the glomerulus, the loop of Henle, and the collecting duct were collected and tested for the presence of several solutes. The data are displayed in Table 2. Unfortunately, some of the data were not recorded, and those absences are indicated on the table by “no data.”

Table 2 Amounts of various compounds in filtrate

Note: Quantities are recorded in g/100 mL.

Questions

8. Which of the solutes was/were not filtered into the nephron? Explain your answer. (protein andsubstance X show value of 0; glucose we dont know because no data)

9. Why is protein not found anywhere except the glomerulus? (too large to be filtered)

10. Unfortunately, the test for glucose was not completed for the sample taken from Bowman’s capsule. Predict whether or not glucose would be found in Bowman’s capsule. Provide reasons for your prediction. (should be because glucose is filtered and not reabsorbed until proximal tubule)

11. If the presence of amino acids had been checked, what would you expect to find? (you would not besurprised to find amino acids in the filtrate)

12. Why does the urea level increase after filtration occurs? (as water is reabsorbed urea becomes moreconcentrated)

13. Chloride ions (Cl^-) follow actively transported Na⁺ ions from the nephron into the blood. You might expect the [Cl^-] to decrease toward the collecting duct. Why is that not what we observe? (the job of thekidney is to concentrate the urine so as water is reabsorbed, the [Cl^-] increases)

14. Suggest a possible identity for Substance X. Support your suggestion. (hormone or other protein. Must be large because is not filtered)